Rochester Institute of Technology



[pic]

Preliminary Design Review

November 12, 2004

Team 05512

Brian Gonzales

Naanzem Hoomkwap

William Lambert

Surat Teerakapibal

Department of Electrical Engineering

Kate Gleason College of Engineering

Rochester Institute of Technology

76 Lomb Memorial Drive

Rochester, NY 14623-5604

Executive Summary

Modern cars frequently come equipped with remote keyless entry systems. These systems allow the automobile owner to perform a variety of tasks, including unlocking doors, opening doors, opening trunks, arming alarms, and setting off panic alarms from a maximum distance of 15 to 100 feet. Secondary uses for these systems have evolved as their presence has become ever more ubiquitous. One such application is in locating a vehicle lost in a parking lot – the automobile owner presses the “lock” or “panic” button, causing the car horn to emit a beep. The user is then directed by sound to his or her car.

Some of these RKE applications benefit from a range which exceeds many of the standard systems. The purpose of this project is to implement a non-intrusive method of extending existing RKE systems ranges. In order to accomplish this, several versions of repeaters are proposed. The advantages and disadvantages of each approach are discussed, and a design for the circuitry is developed.

After feasibility assessments, an approach which could repeat RKE systems for a wide variety of cars was developed. A receiver is implemented with high performance components. A microcontroller then detects the incoming digital data at a much higher rate than is being sent. The data is filtered then stored. When the end of the transmission is detected, the microcontroller activates a small transmitter which repeats the signal to the car.

The system developed in this paper will function for all RKE systems operating in the 315 MHz band using ASK modulation (covering most cars in the US). The system is compact, battery powered, and requires no connections to the automobile that will contain it.

1. Introduction 7

1.1 Background 8

1.1.1 RKE Systems 8

1.1.2 Repeater Systems 9

1.2 Project Description 10

1.3 Project Objective 10

1.4 Project Scope 11

1.5 Funding 11

2. Specifications 12

2.1 Antenna Specifications 12

2.2 Receiver Specifications 12

2.3 Transmitter Specifications 13

2.4 Control Specifications 13

2.5 Housing Specifications 14

2.6 Legal Specifications 15

3. Concept Development 16

3.1 Antenna 16

3.1.1 Antenna Theory 16

3.1.2 Antenna Simulation 19

3.2 Receiver 23

3.2.1 Receiver Theory 23

3.2.2 Receiver Possibilities 27

3.3 Repeater 28

3.4 Transmitter 32

3.5 Filters 34

3.5.1 Preselector 34

3.5.2 Intermediate Frequency Filter 35

3.5.3 Transmitter Output Filter 35

3.5.4 Filter Designs 35

3.5.5 Active Filters 36

3.5.6 Passive Filters 36

3.5.7 SAW Filters 40

3.6 System Control 41

3.7 Housing 42

4. Feasibility Assessment 43

4.1 Antenna 43

4.2 Receiver 43

4.3 Repeater 47

4.4 Transmitter 49

4.5 Filters 51

4.5.1 Preselector 51

4.5.2 Intermediate Frequency Filter 52

4.5.3 Transmitter Output Filter 53

4.5 Controller 54

4.6 Repeater Housing 54

5. Analysis and Design 56

5.1 Antenna Design 57

5.2 Receiver Design 59

5.2.2 Low Noise Amplifier 60

5.2.3 Mixer and IF Preamp 60

5.2.4 IF Limiting Amplifier with RSSI 61

5.3 Transmitter 63

5.4 Filters 68

5.4.1 Preselector 68

5.4.2 Intermediate Frequency Filter 69

5.4.3 Transmitter Output Filter 70

5.5 T/R switch 71

5.6 Housing Design 72

5.7 Control System Design 75

5.8 Impedance Matching Network 92

6. Work Completed 93

Bibliography 97

Appendix A – FCC Regulations 99

Appendix B – Bill of Materials 101

Appendix B – Bill of Materials 101

Appendix C – Complete Circuit Schematic 102

Table of Figures

Table 2.1: Controller Responsibilities 14

Figure 3.1: MININEC Program used for antenna modeling 20

Figure 3.2: Radiation pattern of ¼ wave dipole 21

Figure 3.3: Directive Gain pattern of the Quarter wave antenna 22

Figure 3.4: Current distribution of the quarter wave antenna 23

Table 3.1: Electrical characteristics for a quarter wave antenna 23

Table 3.2: Important Mixer Parameters [RF Design Guides] 26

Table 3.3: Receiver IC Possibilities 27

Figure 3.6: Digital waveform (above) and carrier (below) which comprise the signal for ASK 30

Figure 3.7: Example of ASK modulation of the sequence “1 0 1 0 1 1 0 0” 30

Figure 3.8: Signal Corrupted by Gaussian Noise 31

Figure 3.9: Signal after going through the envelope detector 32

Figure 3.10: Transmitter block diagram 33

Table 3.4: Transmitter IC Possibilities 34

Figure 3.11: A passive bandpass filter 37

Figure 3.12: Frequency response of a passive bandpass filter 37

Figure 3.13: Some lowpass IF Filters 38

Figure 3.14: Frequency response of various IF lowpass filters 38

Figure 3.15: Some Transmitter PA Low Pass Filters for different cutoff frequencies 39

Figure 3.16: Frequency response of the transmitter low pass filters 39

Figure 3.17: SAW Filter 40

Figure 3.18: SAW Filter Impedance Matching 41

Table 4.1: Weighted antenna feasibility analysis 43

Table 4.2: Feasibility analysis of the receiver 47

Table 4.3: Feasibility assessment for the repeater 49

Table 4.4: Feasibility assessment for the Transmitter 51

Table 5.1: Estimated power draw for circuitry 56

Table 5.2: Link Budget Analysis 57

Figure 5.1: Directive Gain pattern of the Quarter wave antenna 58

Figure 5.2: Current distribution of the quarter wave antenna 58

Table 5.3: Electrical characteristics for a quarter wave antenna 59

Figure 5.3: The final antenna design 59

Figure 5.4: rfRD0420 Pin Diagram 60

Figure 5.5: Full Receiver Schematic 62

Figure 5.6: Transmitter Schematic 63

Figure 5.7: Transmitter Output Circuitry 64

Figure 5.8: Simulation of Transmitter Output Circuitry 64

Table 5.2: MAX1472 Pin Description 66

Figure 5.9: Complete Transmitter Schematic 67

Figure 5.11: SAW Filter Frequency Response 69

Figure 5.12: Intermediate Frequency Low Pass Filter 69

Figure 5.13: IF Filter Frequency Response 70

Figure 5.14: Transmitter Output Filter 70

Figure 5.15: Transmitter Filter Frequency Response 71

Figure 5.16: Transmitter/Receiver Switch 72

Table 5.4: ADG918 Pin Description 72

Figure 5.17: Specifications for the housing 74

Table 5.5: Control System Specs [Microchip] 76

Table 5.6: Connections Needed for Microcontroller 76

Table 5.7: Pin connections to the Microcontroller [Microchip] 77

Figure 5.18: PIC16F87 Microcontroller Schematic Diagram 78

Figure 5.19: High Level Overview of Control Systems 79

Table 5.8: Pin States at power on 80

Figure 5.20: Power On Sequence 81

Figure 5.21: Interrupt handling for change on RB4 82

Figure 5.22: Routine to check for valid received data and enter receive mode. 84

Figure 5.23: Rate independent detection of the ASK signal 86

Figure 5.24: Routine for receiving valid data 87

Figure 5.25: Example of corrupted data 88

Figure 5.26: The binary filtering subroutine. 89

Figure 5.27: The transmit routine 91

Figure 5.28: Impedance matching network 92

Figure 6.2: Tank Filter 94

Figure 6.3: Tank Filter Frequency Response 94

Figure 6.4: First Generation Receiver Schematic 95

1. Introduction

A large percentage of automobiles manufactured in the last 15 years have come equipped with Remote Keyless Entry (RKE) systems. These systems allow the owner of the automobile to perform basic functions such as locking the automobile, unlocking the automobile, opening the trunk, starting the car, opening doors, setting alarms, or even setting off a panic alarm by pressing a button of a remote control. The remote control is often a small device attached to the keychain called a “key fob”. Having the ability to perform these functions remotely allows the user to open a car quickly if needed, open a trunk if his hands are full, unlock doors for everyone so they do not have to wait around in inclement weather, and so on. Because of the usefulness of these features, RKE systems have become nearly ubiquitous.

In addition to their intended functions, RKE systems have become commonly used as locator devices for automobiles in parking lots. If an owner leaves his or her car at the mall and can remember the general location of the car, he or she can press the “lock” or “panic” functions, causing the automobile to sound the horn. This can greatly aid the user in finding his or her car.

RKE systems are often implemented in lowest cost and least obtrusive manner possible. Because of this, the systems on some automobiles are restricted to a very short range, down to 15 feet, while others will function up to and over 100 feet. Sometimes, the short range of the systems can be an unfortunate limiter to their utility. For instance, someone looking for his or her car might want to be able to sound the horn from a distance away. People with car alarms which go off frequently might want to be able to turn them off without walking all the way out to their car.

Such applications make extending the range of the repeater desirable. There are obvious ways of doing this, for instance modifying the car to have a better antenna for the RKE device. However, the average user does not necessarily want to modify his or her car to improve RKE performance. Instead, a non-intrusive way of extending an RKE systems range is desired. The best solution for to this problem was determined to be an RKE repeater. The repeater would have a high performance antenna and receiver capable of substantial gain over the system included in the automobile. The repeater would listen for a transmission, store it, and then retransmit it to the car, allowing the car to perform the requested operation. In this way, the range of the system could be transparently extended with no modifications to the automobile required.

By using careful design, a cost effective, marketable product could be produced that could ultimately be sold at an electronics store. If the range extension is significant, the product would be useful to a wide range of consumers looking for the added advantages of a more robust RKE system.

1 Background

1.1.1 RKE Systems

RKE systems generally communicate in an unlicensed portion of the spectrum reserved for intermittent transmission of control data. This frequency is typically 315.0 MHz for US systems and 433.92 MHz for European systems. They key fob is generally an extremely low power device, on the order of 1 mW.

When a key is pressed on the key fob, it translates it into a digital code of a few bits in length. This code is combined with a pseudo-random hopping code (for security purposes). The whole code is then modulated into a radio signal, most often using amplitude shift keying (ASK) modulation. Upon receipt of a transmission, the vehicle verifies the security code and then executes the command.

The code that is sent has a random part and a fixed part. The fixed part of the code tells the vehicle what function to implement. The random part is used by the vehicle to verify that the signal it is receiving is from the owner and not from someone else’s key fob. The key fob and the vehicle both have random number generators that are seeded the same. The vehicle stores the next 256 possible numbers and checks to see of the signal sent has one or those numbers. If it does, then the vehicle will respond to the signal. If it does not, then receiver ignores the signal. The random code ensures that vehicle does not respond to anything other then the owner’s key fob.

1.1.2 Repeater Systems

A repeater is a device used in communication systems to extend the range of an existing communication system. Repeaters are used often in today’s wide cell phone networks and radio networks. Repeater stations and towers are common throughout the United States. Because of this there are a wide range of standard repeater designs. These designs all have the same basic process - a signal is received through an antenna, the frequency of the signal is shifted, and the signal is retransmitted through the same antenna.

If the signal in a standard repeater system is not shifted it would be impossible to simultaneously receive and then retransmit the signal through the same antenna because the power out of the transmitter would overload the receiver. The receiver for the repeater and the transmitter for the receiver must be isolated. Both the receiver and the transmitter will have a narrow bandpass filter in front of them (called a duplexer). As long as the output signal is shifted enough to be attenuated by the receivers filter, then there will be sufficient isolation between the receiver and the transmitter. Without sufficient isolation there will be positive feedback in the repeater.

If retransmission is on the same frequency, two approaches may be used. First, two highly isolated antennas may be used. In such a system, the gain of the repeater must not exceed the isolation between the antennas. In the second system, the signal is recorded until the end is reached. Once the end is reached, the signal is transmitted again. This is called a “Parrot” repeater. According to FCC regulations, such a device is not a repeater.

1.2 Project Description

Existing RKE systems work at ranges between 15 and 100 feet. This purpose of this project is extend the range of the existing RKE system by designing a repeater that will receive the signal from the key fob and retransmit it to the vehicle’s receiver. The range of RKE systems should be extended to greater than 200 feet. To do this, the repeater must be able to receive the signal from the key fob at a greater range then vehicle and then be able to rapidly retransmit that signal to the vehicle.

1.3 Project Objective

An affordable RKE repeater capable of extending the range of existing RKE systems with out modification to the automobile will be constructed. The repeater will be independent of the vehicle and the existing RKE system, including the power supply. The device will be small, inexpensive, easily transferable between automobiles, and work with most existing systems.

1.4 Project Scope

The scope of the project will be limited to systems functioning on 315 MHz using ASK modulation. Other types of RKE systems will not be covered by the repeater.

1.5 Funding

According to the financial constraint specified by the sponsor, the RF repeater is expected to have a competitive price in its market, determined by the developer to be $30-50. A total of $200.00 has been committed by the sponsor for the development of the product. More funds are available if needed.

For the prototype, the antenna will require approximately $20 since the only connectors would need to be purchased. Mixers, filters and housing for the receivers were purchased for roughly $80 in order to conduct preliminary test circuits. These parts will also be used when building the prototypes. The controller part of the prototype should cost about $10 if a programmer does not need to be purchased. The rest of the funds would be allocated towards the receiver and transmitter circuitry.

2. Specifications

2.1 Antenna Specifications

The antenna required for this project needs to be omni-directional (radiating equally well in all directions in one plane), based on the assumption that the user is not going to be using the device from an elevation significantly above or below the vehicle. The design frequency for this project is assumed to be 315MHz, which means that the wavelength is 0.952m. For this wavelength, many different antennas can be considered, such as the quarter or half wave dipole, the helix antenna, and the loop antenna among others.

2.2 Receiver Specifications

The chief requirement for the RKE repeater is that it be able to receive and retransmit all RKE devices. RKE devices on cars in the United States operate primarily in two bands: at 315MHz (American/Japanese cars) and 433MHz (European cars). The repeater will only be able to function at one of these frequencies at a time, so the receiver will be designed accordingly.

In order for the receiver to function for all the different available RKE systems, it must be capable of receiving and retransmitting different modulation types at different data rates. The two modulation types that are likely to be encountered are ASK and FSK. These two modulations types are very different – because of this the receiver must be limited to recovering one or the other. Research indicates that the vast majority of RKE systems use ASK, therefore the receiver will be designed to detect ASK.

2.3 Transmitter Specifications

Because of the proximity of the repeater to the car’s receiver, very few requirements are placed on the transmitter. It is merely required to transmit an ASK signal over a range for a couple of feet to the automobile’s receiver. The transmitter must be a low power device and capable of transmission at sufficient speed.

2.4 Control Specifications

For all repeater designs in which the repeater does not continuously retransmit constantly, some sort of control unit is necessary. The controller will be responsible for activating and deactivating the different portions of the circuit. When no signal is incoming, the controller must keep the receiver turned on and listening while keeping itself in a minimum power draw state.

When a valid transmission is being received, the controller must ensure that the data from the transmission is being captured and stored in memory. After this, the controller must activate the transmitter and send the data.

The power management done by the controller is crucial in making the final product marketable – if the power consumption is too high for AA batteries, the repeater will either have an unnecessarily short lifetime or will require more sizeable batteries.

|Controller Responsibilities |

|1. Power Management |

|2. T/R Switch operation |

|3. Received data detection or A/D conversion |

|4. Data storage |

|5. Transmitter/Receiver operation |

Table 2.1: Controller Responsibilities

2.5 Housing Specifications

In order for the RKE to be an effective consumer product, some amount of thought must go into its physical form. It should be very small (consumers are unlikely to purchase something that would prove unsightly in their expensive cars), easy to insert and remove, and look attractive. Because a high quality antenna will be one of the best ways of improving the range of the system, it is necessary that an attractive enclosure that still facilitates the antenna to be designed.

The design specifies that the unit must be completely independent of the automobile it will be used in. It must require no installation other than placing it appropriately in the car and it must require no connections to the automobile. These requirements lead to the following list of specifications for the housing:

1. Must contain room for batteries (preferably a standard size, such as AA or AAA).

2. Must not exceed 6” in length or width, excepting the antenna.

The housing should also facilitate its final placement. The only known specification for placement of the unit at this time is that it should be above the window level.

2.6 Legal Specifications

The operation of the repeater within the United States is subject to FCC regulations, Title 47, Chapter 1, Part 15, Section 231 – Periodic Operation including the band of the repeaters operation. It specifies, “The provisions of this section are restricted to periodic operation within the band 40.66-40.70 MHz and above 70 MHz. Except as shown in paragraph (e) of this section, the intentional radiator is restricted to the transmission of a control signal such as those used with alarm systems, door openers, remote switches, etc.” [47CFR15.231].

The use of a repeater is not specifically granted by the FCC regulations. However, so long as adequate circuitry is included in the design to ensure that the repeater does not continuously transmit noise, no rules are being broken.

The rules govern maximum transmitted field strength. The transmission, however, will only be going from the repeater to the receiver in a car, a distance that will, in the worst case scenario of a minivan, not exceed a couple of meters. In order to best conserve power the transmitter will operate at an extremely low voltage so the field strength limitations will not be a concern.

It is likely that the device would require FCC approval in order to go to market. This step should be taken in tandem with the rest of the design process in order to ensure that no legal issues are encountered. Should legal issues be encountered, no part of the current design would be usable for the system.

3. Concept Development

3.1 Antenna

Available antenna designs include the loop, the quarter wave antenna, and the half wave antenna. In the case of the loop, the input impedance is in the order of a few thousand ohms; unfortunately a loop doesn't offer a good impedance match to a coaxial transmission line. Using two identical loops side by side with a few inches spacing between them reduces the impedance. Space does not permit this though. The directional pattern becomes asymmetrical and the nulls off the side may be only a few dB down from the peak of the radiation pattern. An unbalanced, unshielded loop can also pick up conducted interference from the feed line. The half wave antenna is similar to the quarter-wave and even though the half-wave antenna's impact on installation is minimal, it is taller than a quarter-wave antenna cut for the same frequency.

3.1.1 Antenna Theory

For theoretical purposes a finite length dipole will be analyzed to find the radiation characteristics. It will be assumed that the dipole has a negligible diameter smaller than the operating wavelength. Hence the current distribution for this dipole can be described by the following equations:

[pic] (3.1.1)

Using the far field approximations given by the equation below,

[pic] (3.1.2)

Where,

dz’ = length of an infinitesimal dipole

The electric field can be obtained by integration.

[pic] (3.1.3)

The resulting expression for the electric field takes the form of

[pic] (3.1.4)

Using the relationship between E( and H(, H( can be found and can be written as

[pic] (3.1.5)

Since a quarter wave dipole is being examined l can be replaced by (/4 and k= 2(/( in equation (3.1.4).

The quarter wave antenna was simulated using EXPERT MININEC, an engineering tool for the design and analysis of wire antennas. MININEC’s solution is based on the numerical solution of an integral equation representation of electric fields given in equation (3.1.4) above. MININEC assumes that the wire radius is very small with respect to the wavelength and the wire length and the wire must be subdivided into short segments so the radius is assumed small with respect to segment lengths. MININEC makes use of the boundary condition on tangential electric field at the surface of a perfect conductor, namely that the electric field must be zero. Based on the initial assumptions that the wires must be thin, the total axial electric field on the wire is forced to zero. The three sources of the tangential electric field on the wire are:

• Currents and charges on the wires and on nearby wires.

• Incoming waves from distance or nearby radiators.

• Local sources of electric field on the wire.

Voltage sources or current sources are local sources that connect to the wires.

MININEC uses the moment method (MM) solution, which is a numerical procedure for solving electric field integral equation. An important step in the MM is the choice of basis functions; basis functions are chosen to represent the unknown currents. The triangular basis function also known as the piecewise linear function is chosen in this case. The piecewise linear function is defined by

[pic] (3.1.6)

Testing functions are also chosen to enforce the integral equation on the surface of the wire [Antenna Theory]. A typical but not unique inner product is given by

[pic] (3.1.7)

Where the weighting testing function is represented by w in the equation above and S is the surface of the structure to be analyzed. With the choice of basis and testing functions, a matrix approximating the integral is defined. To achieve the matrix a set of N testing functions {wm} = w1, w2…wN are defined in the domain of the operator. If this matrix is inverted and multiplied by the local sources of electric field, the complex magnitudes of the current basis functions are derived.[Antenna Theory]

3.1.2 Antenna Simulation

To run MININEC for a complete analysis of the current, impedance and radiation patterns of the quarter wave antenna some parameters have to be defined.

[pic]

Figure 3.1: MININEC Program used for antenna modeling

The antenna is placed on the z-axis as shown in the figure above and fed at the end of the antenna at z =0. The antenna is simulated on a ground plane, because it is going to be mounted on the metal top of the housing unit. The ground plane is also used to limit the downward radiation of the antenna.

Given that the repeater is transmitting and receiving at 315MHz the wavelength can be obtained as thus

[pic] (3.1.8)

The length therefore is quarter of the wavelength resulting in l = 0.238m. After a few iterations the length had to be changed to get the optimal impedance and gain, the optimum length was found to be 0.226m. Two geometry points are then defined as (x1, y1, z1)=(0, 0, 0) and (x2, y2, z2)=(0, 0, 0.226). The method of moments requires that the wire be broken into segments, the greater the number of segments the more accurate the result. The number of segments for this antenna was set to 40; the points at which the different segments of the wire are connected are identified as current nodes. The program was then run to obtain the following results.

[pic]

Figure 3.2: Radiation pattern of ¼ wave dipole

The Radiation pattern, which is a “mathematical function or a graphical representation in this case of the radiation properties of the antenna as a function of space coordinates”. The radiation pattern seen in figure 2 above sweeps from 0( ((( and from 0( ((( because the antenna is on a ground plane half of the radiation pattern is not shown.

There are two measurements of gain, namely directive gain and power gain.   Directive gain is the ratio of the power density radiated in a particular direction to the power density radiated to the same point by a reference antenna, assuming both antennas are radiating the same amount of power. The power gain is the same as directive gain except that antenna efficiency is taken in to account and the total power fed to the antenna is used in the calculations. It is assumed that the antenna and the reference have the same input power and the reference is lossless [Antenna Theory]. The power gain is equal to the directive gain if an antenna is lossless (it radiates 100% of the input power). The gain of an antenna is often used as a figure of merit. For the quarter-wave antenna the as obtained from the simulation is seen the figure below, with the maximum at 5.15dB.

[pic]

Figure 3.3: Directive Gain pattern of the Quarter wave antenna

[pic]

Figure 3.4: Current distribution of the quarter wave antenna

|Freq |Resistance |Reactance |Impedance |Phase |VSWR |S11 |S12 |

|(MHz) |(() |(() |(() |(Deg) |dB |dB |dB |

|315 |35.789 |-.79302 |35.798 |-1.27 |1.3978 |-15.603 |-1.5756 |

Table 3.1: Electrical characteristics for a quarter wave antenna

Technically, antenna impedance is the ratio at any given point in the antenna of voltage to current at that point. Depending upon height above ground, the influence of surrounding objects and other factors, a quarter-wave antenna with perfect ground exhibits a nominal input impedance of around 36 ohms [A.R.R.L Antenna Book], which is pretty close to the value seen in the table above.

3.2 Receiver

3.2.1 Receiver Theory

After the signal is received from the antenna, it is passed to the first filter. This particular filter is known as the preselector. The preselector is designed to limit the bandwidth of spectrum reaching the RF amplifier and mixer to minimize distortion. The receiver spurious responses can also be attenuated using the preselector. The preselector must also be able to suppress local oscillator energy originating in the receiver. A possibility of the RF preselector filter is a highly selective, cavity tuned filter, cascaded with a low-pass filter. As this filter will encounter the highest RF levels, it should possess a high intercept point.

The RF amplifier is required to mainly isolate filter 1 and filter 2 from each other in order to maintain the overall selectivity. Due to this fact, a high reverse isolation amplifier is necessary. Other characteristics of the RF amplifier such as the noise figure, gain and intercept point are determined by the receiver performance requirements.

The received signal is then passed to another filter, Filter 2. It is usually called the image filter because of its nature to rejects image noise. This filter attenuates the receiver spurious response frequencies, direct IF frequency pickup and noise at the image frequency caused by the RF amplifier. The second harmonic occurred in the RF amplifier and local oscillator energy leaking back to the antenna can also be suppressed by this filter. Moreover, due to the fact that the mixer usually has very little rejection for odd harmonics of the receive frequency that may leak to the system, it is extremely important for this filter to not have any return responses at high frequencies.

To further maximize the intercept performance, a diplexer network can be added in order to reject any signals that would reflect back into the mixer. This network has the ability to suppress the local oscillator harmonics that might disturb the functionality of the mixer.

Due to the fact that the LO signal has to have a relatively high amplitude, the mixer will generate its own harmonics as it operates. As a result, double balanced mixer should be used since they are internally balanced and so would not cause this particular problem. To improve the mixer performance through optimizing the second-order intercept point, externally generated second harmonics of the LO signal should be suppressed using the injection filter.

The receiver’s channel selectivity is determined by the single-sideband (SSB) phase noise of the first local oscillator. It can also be affected by the wideband noise which is measured at the frequency offsets that are greater than the SSB phase noise. In addition, it is very crucial to have a slow spurious signals in the LO signal to prevent the corresponding receiver spurious responses. A LO synthesizer can be used to limit the circuit block for frequency change lock time. As the LO signal is very significant to the system performance as a whole, it must be able to oscillate regardless of temperature and power supply variations.

After the signal is mixed, it is then passed to the first IF stage. The function of this filter is to protect the following stages from close-in IM signals. It also has to provide adjacent channel selectivity and attenuates the second image. Although two different characteristics of the filter must be met, the number of poles required is determined by the required second-image selectivity. This is due to the fact that the requirement on the second-image selectivity is much more stringent than that of the adjacent channel selectivity. The equivalent noise bandwidth of the IF chain is also a very important receiver property as it determines the level of noise that reach the detector and the modulation bandwidth that can be received. Group delay must also be compensated by either software or hardware in order to minimize the group delay distortion. To improve mixer’s IM performance, the maximum impedance presented to the high impedance mixer on the filter skirts must be limited. This can be done by isolating the filter from the mixer by an impedance inverter network. It is very crucial to select the first IF crystal with a good IM.

Another high gain stage is then followed. This IF amplifier should possess a high intercept point. However, if the earlier mentioned IF filter stage is present, the required intercept point does not necessary have to be as high. [RF Design Guides]

|Mixer Parameter |Affected Receiver Specification |

|Conversion loss |Receiver sensitivity |

|Third-order intercept point |Intermodulation distortion |

|Second-order intercept point |Half IF spurious response rejection |

|Higher-order intercept point |High-order spurious rejection |

|Noise balance |Receiver sensitivity, AM noise rejection |

|LO to RF isolation |Conducted LO energy propagating toward antenna |

|RF to IF isolation |Susceptibility to direct IF frequency pickup |

Table 3.2: Important Mixer Parameters [RF Design Guides]

Receiver Design Procedures

1. Allocate approximate gains and losses as needed to meet the required receiver sensitivity specification and IM distortion requirements.

2. Select the first IF frequency.

3. Select the first LO injection side.

4. Investigate the mixer.

5. Based on mixer performance, design the injection filter and select LO technology.

6. Investigate filter topologies

7. Design the RF amplifier

[pic]

Figure 3.5: Basic Receiver Design

3.2.2 Receiver Possibilities

The receiver can be built using discrete parts that meet the specifications as noted in an earlier section. Although discrete parts were used in order to support the preliminary assumption, existing receiver integrated circuit chips would reduce the cost in the final prototype assembly. Further detailed discussions will be made in the Feasibility section of this report.

| |rfRXD0420 |MAX7033 |RXM-315-LC-P |

|Manufacturer |Microchip |Maxim |Linx |

|Frequency Range |300-450 MHz |300-450 MHz |FSK/ASK |

|Power Consumption |8.2mA |5.2mA |5mA |

|IF Frequency |455kHz to 21.4MHz |10.7MHz |Not Available |

|Modulation Mode |ASK/FSK/FM |ASK |ASK |

|Price |$2.79 |$4.53 |$17 |

Table 3.3: Receiver IC Possibilities

3.3 Repeater

All of the RKE systems that will work with the repeater are designed for a single frequency at 315MHz, which means that in order for the car to open, a 315MHz signal must be received and retransmitted on the same frequency.

This leads to three basic repeater designs:

1. Simultaneous Retransmission - The received signal is simultaneously retransmitted on a separate, highly isolated antenna. This is the simplest system, requiring essentially no control circuitry. It requires, however, directional antennas which are highly isolated from one another if feedback is to be avoided.

2. Waveform Storage – The incoming waveform is mixed down to a low IF and then sampled with an A/D converter. The resulting data is stored till the end of the transmission is detected. The waveform is then mixed back to the RF frequency and retransmitted. This technique would retransmit ASK and FSK, but would offer little gain advantage and frequently transmit extra noise. High performance signal processing capability along with a large memory would be required.

3. Demodulating the signal – The incoming signal is demodulated and stored. After the end of the transmission is detected, the data is output to an ASK transmitter and sent again as before. This approach is limited to ASK and would require novel circuitry if it is to work with more than a very small variety of automobiles. This could be implemented using either a microcontroller or a DSP.

3.3.1 ASK Theory

Amplitude Shift Keying (ASK) is the simplest form of bandpass digital communication. The premise is simple – a carrier at the frequency of transmission is adjusted to different levels in order to represent different sequences of digital data.

The binary form of ASK (OOK, or on/off keying) is even simpler – the carrier is turned on and off. When the carrier is on, a “1” is being sent. When the carrier is off, a zero is being sent. In the example below, a simple sample sequence “1 0 1 0 1 1 0 0” is modulated. A 2 Hz sine wave is sent. It is then multiplied by bit value being sent for each bit in the sequence. Because the data in the example is being sent at 1 bit/s, the sinusoid is multiplied by each bit for 1 second. In figure 3.6, an example baseband digital signal is seen along with a carrier generated at the given frequency. By multiplying the carrier by the baseband waveform, a bandpass digital signal is generated. [Digital Communications]

Mathematically, this can be expressed as:

[pic] (3.3.1)

[pic]

Figure 3.6: Digital waveform (above) and carrier (below) which comprise the signal for ASK

[pic]

Figure 3.7: Example of ASK modulation of the sequence “1 0 1 0 1 1 0 0”

Obviously the modulated signal is not band limited. Therefore, filtering is performed before the signal is finally transmitted. This filtering leads to distortion, but at slow data rates with relatively wide bandwidth, like those encountered in RKE systems, the effect on communication is negligible.

Once the signal has been transmitted, it is corrupted by noise. The simplest noise model is white, Gaussian noise.

[pic] (3.3.2)

For example:

[pic]

Figure 3.8: Signal Corrupted by Gaussian Noise

The signal is then filtered heavily. Finally, an envelope detector filters and rectifies the signal:

[pic]

Figure 3.9: Signal after going through the envelope detector

In spite of what figure (3.9) may look like, the data can be successfully demodulated and stored.

3.4 Transmitter

The final stage of this repeater system will have to be a transmitter. The primary function of the transmitter is to amplify the power in the signal so that it will be able to reach the vehicles existing antenna. There does not have to be a lot of power in the signal being retransmitted from the repeater system to the vehicle’s receiver. This is because the signal does not have to travel very far. The signal only has to be able to go from the repeater system in vehicle, most likely in the back seat of the vehicle, to the receiver in the vehicle, normally placed under the dashboard. In fact, a less powerful transmitter is desired since it would be a smaller current draw.

There are two basic design concepts for the transmitter. It can be built using discrete components or the transmitter can be bought from a company that manufactures RF transmitters.

The transmitter for this project does not have to be very complicated since the total power amplification requirements are small. Therefore, it is possible to build a transmitter from discrete components. The basic design for a transmitter is shown below in figure 8.

[pic]

Figure 3.10: Transmitter block diagram

This design includes a buffer, a power amplifier, and an oscillator. To design this system discretely means that each one of these components would be designed separately and then put together to make the transmitter. The design of the individual components would most likely mean buying the components from RF manufactures. To design these components from transistors up is beyond the scope of this project.

There are large numbers of existing RF transmitters that have been designed for RKE systems. It would be possible for this system to take an existing transmitter, designed for RKE systems and use as our transmitter. The only design involved would be picking the transmitter that best fit the specifications for this project. Some the possibilities are as follows:

|Part # |ADF7012 |MAX1472 |TH7107 |rfHCS362F |

|Manufacture |Analog Decives |Maxim |Melexis |Microchip |

|Modulation Mode |FSK/ASK |ASK |FSK/ASK |ASK, FSK |

|Frequency Range |50-1000 MHz |300-450 MHz |315/433 MHz |310-480 MHz |

|Maximum Data Rate |150kbs |100kbs |40kbs |3334bps |

|Output Power |-16dBm to +13dBm |+10dBm |-12dBm to +2dBm |-12dBm to +2dBm |

|Power Consumption |21mA |5.3mA |4.8 to 11.5 mA |4.8 to 11.5 mA |

|Price: |$1.89 |$3.74(free |$6.04 |Samples Available |

| | |Samples) | | |

Table 3.4: Transmitter IC Possibilities

3.5 Filters

One of the most important design considerations for this project is the amount of noise interfering with the system. If large enough, this noise can completely distort the signal and prevent the repeater from communicating the signal to the vehicle’s receiver. To keep the noise from dominated the system analog filters must be incorporated into the design. There are three important filters needed in the system. These are the preselector, the intermediate frequency (IF) filter, and the transmitter output filter. These can be implemented using active filters, passive filters, or SAW filters.

3.5.1 Preselector

The first filter needed on the receiver is called the preselector or front end filter. This should be a bandpass filter centered at the operation frequency of the system, in this case 315MHz. The purpose of this filter is to reduce the amount of noise coming into the system by only allowing a narrow band around the given center frequency into the system.

The preselector must be a bandpass filter with a center frequency at 315MHz. It must have a bandwidth of less then 600 kHz. It has to have a fairly good attenuation outside of the bandwidth. Finally, it must match the 50Ω load on the input and a 50Ω load on the output since the T/R switch will have 50Ω impendence and the input the receiver circuit will be designed to have 50Ω impedance.

3.5.2 Intermediate Frequency Filter

The second filter need on the receiver is on the output of the receiver. This is should be low pass intermediate frequency filter (IF). The purpose of this filter is to reduce prevent the image frequencies from continuing to detection. The image frequencies are a result of the signal being mixed down from an RF signal to an IF signal.

3.5.3 Transmitter Output Filter

The final filter required for this system is a low pass filer. This time it is on the output of the transmitter. It is there to prevent the image frequencies from being transmitted. The image frequencies are a result of the signal being mixed up from an IF signal to an RF signal.

3.5.4 Filter Designs

To implement the different filters needed for this project there are several different filter designs that can be used, the active filter, the passive filter or the SAW filter. For each of these types of designs, there is a board range of designs for each filter specifications.

3.5.5 Active Filters

Active filters are filters that are designed using a mathematical approximation to meet the desired specifications. These filters then implement the approximation using operational amplifiers, resistors, and capacitors. To implement an active filter requires some input power to the operational amplifiers. These filters can be designed to include a gain. Examples of some mathematical approximations for the filters are:

Butterworth Approximation: [pic] (3.5.1)

Chebyshev Approximation:[pic] (3.5.2)

For these approximations α is the attenuation in decibels at some frequency ω. The designer uses these to find n which is the order of the filter.

3.5.6 Passive Filters

Passive filters are implemented using only resistors, capacitors, and inductors. These are components that do not require any power to operate. These filters are designed using the same mathematical approximations as the active filter. After these filters have been designed as an active filter a transformation can be preformed on them to make them into a passive filter. This is called a lossless-ladder transformation, since the combination of passive components is called a ladder and to make it lossless is important. Some examples for passive filter are:

[pic]

Figure 3.11: A passive bandpass filter

[pic]

Figure 3.12: Frequency response of a passive bandpass filter

[pic]

Figure 3.13: Some lowpass IF Filters

[pic]

Figure 3.14: Frequency response of various IF lowpass filters

[pic]

Figure 3.15: Some Transmitter PA Low Pass Filters for different cutoff frequencies

[pic]

Figure 3.16: Frequency response of the transmitter low pass filters

3.5.7 SAW Filters

A surface acoustic wave (SAW) filter is a passive filter which does not use normal discreet elements such as resistors, capacitors, or inductors. It is a thin metal film structure deposited on top of a piezoelectric crystal substrate, definition found at . An example of a simple SAW filter is shown below:

[pic]

Figure 3.17: SAW Filter

The filter only resonates at a specific frequency. Because it only resonates at a given frequency, it acts as a very narrow band filter centered at that frequency. This type of filter is advantageous because it has a very narrow pass band and does not require any power to implement. The disadvantage is that it is expensive. To implement a SAW filter, an impedance match must be designed. Below is an example of a SAW filter implemented with impendence matching to 50 Ohm:

[pic]

Figure 3.18: SAW Filter Impedance Matching

3.6 System Control

There are several important functions that require control circuitry. These include enabling the transmitter and receiver and controlling the T/R switch. A microcontroller is required to perform these operations. The microcontroller will also need to store the signal received. The controller must operate fast enough to decode the signal, have sufficient memory to store the decoded data, and have enough outputs to control all of the other circuitry.

The microcontroller will control the enable for the transmitter; the transmitter only needs to be on when the signal is transmitting so that power is not wasted. It also controls the enable for the receiver. The receiver will only turn off when the transmitter is transmitting; this will increase the isolation and prevent feedback. The microcontroller will control the T/R switch. The switch needs to be set to receiver when the receiver is enabled and set to the transmitter when the transmitter is enabled. Finally, it will send the stored data to the transmitter during retransmission.

The microcontroller needs to operate fast enough to decode the input data. If the data rate is too fast for the microcontroller then it will store an incorrect signal and repeater will not function. It also needs to have a stable time reference. This time reference will most likely be set by an external crystal - if the time base varies then the microcontroller’s internal clock will be disrupted, preventing a successful storing and retransmission of the signal.

3.7 Housing

The housing will be as small and attractive as possible. It should not occupy any excess space and should be able to be attached to the parts of the car as appropriate. Further development of the housing solution cannot be completed until the antenna and circuit board have been completed.

4. Feasibility Assessment

4.1 Antenna

The size and housing unit of this project limits the length of the antenna design. Since space does not permit for a non-wire antenna, the quarter-wave antenna was the most feasible for this project because of its length. Since the user should be able to use the key fob from all directions it is required that the antenna be omni directional as opposed to directional antenna. Using the weighted method of feasibility the quarter-wave antenna was the most feasible. The quarter wave antenna is also an optimum length because the gain diminishes with a decrease in antenna length.

|Evaluate each additional concept against the baseline, score |Half-wave |Quarter-Wa|Loop |  |Relative|

|each attribute as: 1 = much worse than baseline concept 2 = |Antenna |ve Antenna|Antenna | |Weight |

|worse than baseline 3 = same as baseline 4 = better than | | | | | |

|baseline 5= much better than baseline | | | | | |

|Sufficient Student Skills? |5 |5 |5 |  |11% |

|Sufficient Lab Analysis Equipment? |4 |4 |4 |  |11% |

|Cost of Materials? |5 |5 |5 |  |3% |

|Cost of Purchased Components? |5 |5 |5 |  |6% |

|Complete within 2 quarters? |4 |4 |4 |  |11% |

|Complete by 1 student? |5 |5 |5 |  |17% |

|Has a similar technology been used before? |5 |5 |5 |  |0% |

|Is it theoretically possible? |4 |4 |4 |  |19% |

|Size? |3 |5 |4 |  |22% |

|  |  |  |  |  |0% |

|  |  |  |  |  | |

|Weighted Score |4.1 |4.6 |4.4 | | |

| | | | | | |

|Normalized Score |90.3% |100.0% |95.2% | | |

Table 4.1: Weighted antenna feasibility analysis

4.2 Receiver

An estimation of the required gains, losses, and intercept points must be calculated in order to show the feasibility of the system. In general, the RF amplifier gain should not exceed 20 dB as it would create many problems. These issues include unavailability of a single device, instability of the system and the unachievable required amplifier intercept point. A filter insertion loss of 3 dB or less could also be implemented without any problems.

The selection of IF frequency is also a very crucial process because it would determine the location of the image and the half IF spurious response frequencies. The choice of IF frequency, however, is not totally flexible as crystals and IF filters are only manufactured in certain standard center frequencies. The IF frequency must also be different from harmonics of the other discrete frequencies such as the digital clock operating frequency and reference frequency.

When selecting the first LO injection side, a few considerations must be thought of. High-order spurious responses and self-quieting frequencies may favor on or the other injection side, once the IF frequency has been chosen. Also, higher-frequency oscillators typically have worse SSB phase noise but the required voltage controlled oscillator (VCO) tuning range (in percent) for synthesized sources is less for high-side injection than for low-side injection. The chosen mixer may have a limited frequency of operation, forcing low-side injection. A lower-frequency LO that is multiplied up in frequency may sometimes offer advantages over a high-frequency LO without frequency multiplication as well.

There are a few differences between passive and active mixers. It is one of the most important selection that would significantly affect the receiver overall performance. While passive mixers possess better IM performance, it requires much higher local oscillator power and do not provide conversion gain. Active mixers are directly opposite. Active mixers require less local oscillator power and still do not have a much better noise figure. The second-order intercept point of the mixer will determine the necessity of the RF filtering for the half IF spurious response. Also, with higher amplification of the VCO signal, the wide band noise will be higher. As a result, an injection filter may be needed in order to suppress image noise to achieve better sensitivity. This particular filter does not necessary have to be highly complicated. A simple low-pass filter would be adequate to suppress the second harmonic from the LO signal and help balancing the mixer by improving the mixer second intercept point.

The LO technology selection is probably one of the most flexible part of the receiver which relies heavily on the receiver application. For a single-frequency receiver, such as this project, a simply crystal oscillator can be used. Although in many other systems, a frequency synthesizer or a LC discrete inductor-capacitor oscillator circuit could be possible candidates.

The RF filter must be chosen to correspond with the determined IF frequency and the first LO injection side. Then, a filter topology that rejects the appropriate signal must be selected. For this specific application, a high-side injection must be used in order to reject the high-frequency noise that is coupled with the signal. Since the selectivity is a trade off for insertion loss, the selected filter’s selectivity must be sacrificed as the input to the RF amplifier must have low insertion loss.

The RF amplifier is the last block of the receiver circuitry. It is there to fine-tuned the signal properties after all of the other earlier mentioned parts are determined. It is much more feasible to shape the signal in this stage than in other stages.

As this particular receiver design is aimed to have low power consumption, there are more constraints on its operation. With high tendency of the receiver to overload and a possibility of IM distortion, it is critical to design the receiver to have as narrowband as possible. This type of receiver usually alternately switches itself on and off to conserve battery power. [RF Design Guides]

When the feasibility assessment was carried out to compare the benefits between receiver construction from discrete components and existing receiver IC, it would be cost and time effective to purchase the IC. Moreover, all of the necessary components of a receiver could be found in existing receiver IC. In the final design, rfRXD0420 will be used. This IC will results in a receiver system that will match the need of this project at a reasonable price. However, this existing receiver IC does not include the filters that are necessary. As a result, additional discrete SAW and low pass filters must be purchased. Information on filters could be found in the filter section of this report.

|Evaluate each additional concept against the baseline, score each |Discrete |Existing |  |  |Relative |

|attribute as: 1 = much worse than baseline concept 2 = worse than |Parts |Parts | | |Weight |

|baseline 3 = same as baseline 4 = better than baseline 5= much better | | | | | |

|than baseline | | | | | |

|Sufficient Student Skills? |4 |4 |  |  |11% |

|Sufficient Lab Analysis Equipment? |4 |4 |  |  |3% |

|Cost of Materials? |2 |4 |  |  |6% |

|Cost of Purchased Components? |2 |4 |  |  |8% |

|Complete within 2 quarters? |3 |4 |  |  |14% |

|Complete by a student? |3 |4 |  |  |17% |

|Has a similar technology been used before? |4 |5 |  |  |0% |

|Is it theoretically possible? |4 |5 |  |  |19% |

|Does it use the spectrum well? |4 |5 |  |  |22% |

|  |  |  |  |  |0% |

|  |  |  |  |  | |

|Weighted Score |3.4 |4.4 |  | | |

| | | | | | |

|Normalized Score |77.4% |100.0% |  | | |

Table 4.2: Feasibility analysis of the receiver

4.3 Repeater

Three approaches for repeater design were identified. Simultaneous retransmission was almost immediately ruled out due to the omnidirectional antenna requirement. Because of it, the isolation between the transmit and receive antennas would be minimal – the gain would be far greater than the isolation. This would create feedback, making the system useless.

The IF waveform storage approach was initially the most promising. It appeared relatively easy to implement while being extremely robust (independent of modulation type). However, many problems existed. First, the key fobs do not have a particularly stable frequency source. Thus, the bandwidth which would need to be stored would be very large, requiring an extremely high performance A/D converter. The circuitry required to implement such a device would consume a large amount of power. Furthermore, since no form of data detection is implemented, the repeater would be repeating noise frequently, which would be an irresponsible use of the spectrum.

Demodulating the data was the most restrictive method investigated but also the most feasible. It limits the scope to only ASK type transmitters. It has the advantage of requiring much lower performance parts than the IF approach as well as the ability to detect if real data is being received. A microcontroller was decided upon over a DSP for this approach because of power considerations. The weighted analysis agreed well with the reasoning, as seen below:

|Evaluate each additional concept against the |Sample Signal |Demodulation |Demodulation DSP |Simaltanous |  |Relative |

|baseline, score each attribute as: 1 = much worse | |Microcontroller| |Retransmissi| |Weight |

|than baseline concept 2 = worse than baseline 3 = | | | |on | | |

|same as baseline 4 = better than baseline 5= much | | | | | | |

|better than baseline | | | | | | |

| | | | | | | |

|Normalized Score |94.2% |100.0% |55.2% |86.6% | | |

Table 4.3: Feasibility assessment for the repeater

4.4 Transmitter

There were two basic ideas for transmitter design presented in section 3.4. These were to either design a transmitter from discrete components or to buy a transmitter chip that has been designed for RKE systems. Both of these ideas are feasible from a technical point of view. To design the transmitter discretely would simply require finding the correct parts and making sure that they operated together properly to get the desired output. To buy a transmitter chip would only require finding the chip that best suited the desired output.

There are two key factors for feasibility other then the technical factor. These are the price of the design and the power consumption of the design. To design the transmitter from discrete parts would cost more. To get a good power amplifier cost almost as much as the entire transmitter chip. The mixer and the demodulator will also add to the cost. The cost of the discrete parts is going to be significantly greater, therefore, than the cost of the transmitter chip. The power consumption for the discrete parts may or may not be less then the transmitter chip. That depends on which chip is used and what parts are used for the discrete design. To get less power consumption in the discrete parts will drive up the cost for the discrete parts.

Building the transmitter out of discrete parts will result in a better transmitter since each of the parts can be higher quality. However, it will cost more and may have higher power consumption. Since the transmitter for this project does not have to be very good, but it does have to be cheap and low in power, the transmitter chip idea is more feasibly to the design.

|Evaluate each additional concept against the baseline, score each |Discrete |Transmitter |  |  |Relative|

|attribute as: 1 = much worse than baseline concept 2 = worse than |Components | | | |Weight |

|baseline 3 = same as baseline 4 = better than baseline 5= much better | | | | | |

|than baseline | | | | | |

|Sufficient Student Skills? |4 |4 |  |  |11% |

|Sufficient Lab Analysis Equipment? |4 |4 |  |  |3% |

|Cost of Materials? |2 |4 |  |  |6% |

|Cost of Purchased Components? |2 |4 |  |  |8% |

|Complete within 2 quarters? |3 |3 |  |  |14% |

|Complete by a student? |3 |4 |  |  |17% |

|Has a similar technology been used before? |4 |5 |  |  |0% |

|Is it theoretically possible? |4 |5 |  |  |19% |

|Power Consumption |1 |5 |  |  |22% |

|Does it use the spectrum well? |4 |5 |  |  |22% |

|  |  |  |  |  | |

|Weighted Score |3.6 |5.4 |0.0 | | |

| | | | | | |

|Normalized Score |67.6% |100.0% |# | | |

Table 4.4: Feasibility assessment for the Transmitter

4.5 Filters

4.5.1 Preselector

There are three different design concepts for the preselector. These are an active filter, a passive filter, or a SAW filter, as discussed in section 3.5. The key factors in the feasibility for the preselector are the pass band, the cost of the filter, the physical capability to implement the filter, and the power consumption of the filter. Due to the fact that power consumption is such a big concern for this project the active filter is ruled out.

For the preselector a passive filter will be very hard to actually implement. This is because at it will be very hard to get the desired bandwidth it passive components. The problem with bandwidth of the filter is that it relies completely on the load. The load for this circuit is very small, only 50 Ω. This makes the values of the inductor and capacitors very small and therefore more expensive and less reliable. SAW filters give amazingly good response with very little loss and no input power. The bandwidth for SAW filter is very narrow and therefore meets a key specification for the preselector.

Overall the SAW filter is best option for the preselector. This is because it is easy to implement, does not require any input power and has a very narrow bandwidth.

4.5.2 Intermediate Frequency Filter

There are only two different design concepts for the IF filter. The SAW filter can not be used for the IF filter since this filter should be a lowpass filter rather then a bandpass filter. Therefore, the two design concepts are either using a passive filter or an active filter. The active filter is much easier to design and would give a gain rather then a loss. However, the active filter is going consume power and since power consumption is an important design consideration, passive filters will better meet the design specifications for this project.

Figure (3.13) displays all of the different passive filter designs that were considered for this project. Figure (3.14) shows the result from the simulation of these different filters. The simulation in this case is an important deciding factor. For the low pass filter there is a significantly better response due to the addition of an inductor and capacitor. The single capacitor filter has a much slower fall time. Therefore the two capacitor and inductor filter will be used for this project

Due to power considerations a passive filter will be used to the IF filter. To get a sharper cut off in the filter a second order, two capacitor and inductor, filter will be used.

4.5.3 Transmitter Output Filter

There are only two different design concepts for the transmitter output filter. The SAW filter cannot be used for this filter since this filter should be a lowpass filter rather then a bandpass filter. Therefore, the two design concepts are either using a passive filter or an active filter. The active filter is much easier to design and would give a gain rather then a loss. However, the active filter is going consume power and since power consumption is an important design consideration, passive filters will better meet the design specifications for this project.

Figure (3.15) in displays all of the different passive filter designs that were considered for this project. Figure (3.16) in the same section shows the result from the simulation of these different filters. The simulation in this case is an important deciding factor. For the low pass filter there is a significantly better response due to the addition of an inductor and capacitor. The single capacitor filter has a much slower fall time. Therefore the second order, two capacitor and inductor, filter will be used for this project. There are different designs for the second order filter. Each of these designs is based upon a different cutoff frequency. For this project the magnitude at 315 MHz should be large and then cutoff sharp after that. The modified 315 MHz design therefore works best. This design is based upon a Butterworth approximation that was modified in the simulation to move the frequency slightly.

Due to power considerations a passive filter will be used to the transmitter output filter. To get a sharper cut off in the filter a second order, two capacitor and inductor, filter will be used. To get a good response at 315 MHz and a good attenuation after that the modified 315 MHz filter will be used.

4.5 Controller

There are a numerous controllers from various companies capable of handing the processing necessary for the repeater. The specifications will be determined by the rest of the system. After that, it is merely a matter of choosing an appropriate microcontroller that has low current consumption and meets the other design needs.

4.6 Repeater Housing

Commercial viability considerations dominate the feasibility analysis of the RKE repeater. Optimal performance might well be achieved by placing a large antenna array on the top of the car. This, however, would lead to increased price, system complexity, and most importantly to the end-user, an unsightly mess on the top of the car.

Placing the antenna on top of the car would lead to optimal system performance. With the antenna mounted in such a location, it would be free from the reflections and diffraction that it will likely experience if placed inside of the car. This, however, would lead to a significantly more complicated housing design, as it would have to withstand the harsh environment outside of a car (including high winds, water, temperature extremes, collisions with bugs and other debris, and a variety of other events). The car top location of the antenna would likely be considered unsightly by most consumers as it would need to be placed on the roof of the car and would likely not match the paint color of the car or the styling of the car. Additionally, some method of affixing the device to the top of the car would be required (for example, permanent magnets). These would likely add to the cost of the device and make installation more cumbersome, particularly for short consumers or people with vans or sport utility vehicles.

Making the antenna a permanent part of the unit and then placing the unit in a specified portion of the car would seem to be the most commercially viable option. As long as the repeater is sufficiently small, the final assembly could be quite unobtrusive. It could be little more than a small box strategically placed in the car that user does not see or think about except to change batteries every so often.

5. Analysis and Design

From a system overview perspective, one of the most important considerations is power consumption. Since the device should have a long lifetime on only a couple of batteries, careful attention is paid to power consumption. Based on the design presented, the following power budget is calculated:

|Component |Description |Power On Current |Standby Current |

|Sources: | | | |

| AA Bateries |Energizer AA Bateries1 | |2850 mAH |

| | | | |

|Sinks: | | | |

| rfRXD042 |Microchip |8.2mA | ................
................

In order to avoid copyright disputes, this page is only a partial summary.

Google Online Preview   Download