EE-458 LAB REPORT - Eastern Mediterranean University



EXPERIMENT #4

FREQUENCY MODULATION

Purpose:

The objectives of this laboratory are:

1. To investigate frequency modulation characteristics in the frequency domain.

2. To implement a classical double-tuned FM demodulator and measure its characteristics.

3. To implement a modern PLL FM demodulator and measure its characteristics.

4. To investigate the effect of FM signal bandwidth on the detected signal-to-noise ratio.

Equipment List

1. PC with Matlab and Simulink

Frequency Modulation

FM results when the time derivative of the phase of the carrier is varied linearly with the message signal m(t). The frequency deviation is proportional to the derivative of the phase deviation. Thus, the instantaneous frequency of the output of the FM modulator is maximum when the message signal m(t) is maximum and minimum when m(t) is minimum.

Carson’s Rule:

Carson’s Rule is used to determine the bandwidth of the FM wave. According to Carson’s Rule, the bandwidth is given by:

BW = 2((+1)fm Hertz.

Laboratory Procedure

Determining Constants:

Before proceeding to perform the experiment, the following steps were performed:

1. Calibrate the multiplier and determine the multiplier constant.

2. Determine the VCO conversion constant Ko

3. Set the VCO’s frequency for 5 kHz.

4. Verify the outputs of the 1st order Low Pass Filter.

1. Multiplier Constant, Km

With a 1V p-p sinusoidal voltage at both inputs of the multiplier, the output was observed and the multiplier constant was calculated to be 0.206.

2. VCO Conversion Constant, Ko

An external voltage may control the output frequency of the VCO. The change in the output frequency per change in the dc input voltage was measured.

Ko = (f / (v

Ko = 1 / 0.5 = 2 kHz/sec/volt

(Ko = 4(103 = 12566 rad/sec/volt

FM Transmission:

The following schematic was implemented.

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Figure 4 A (a) FM detector

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Figure 4 A (b) FM Input signal

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Figure 4 A (c) PSD of message signal

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Figure 4 A (d) Limter - Unmasked

The output of the function generator is set to 1 kHz (modulation frequency fm = 1kHz) and no output level.

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Figure 4 A (e)Band pass block parameters

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Figure 4 A (f)Output of Limiter

A 5kHz carrier “delta” function was observed on the signal analyzer. We increased the output level of the function generator by pressing the Delta Level key on the generator and selecting delta of 0.1 volt. Setting the Vpp to 0 volts and incrementing the Vpp by 0.1 V increments, we increased the level until a of ( = 0.5 was achieved.

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Figure 4 A (g)Output of 4k Band pass filter

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Figure 4 A (h)Output of 8k Band pass filter

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Figure 4 A (i)Block parameters of envelope detector

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Figure 4 A (j)Output of difference block

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Figure 4 A (k)Output of Cheby filter

( = Ko A./ (2(fm); A = Vpp /2

The above procedure is repeated for ( = 1 and ( = 2.

We observed that the carrier disappears at A = 1.05 V.

Setting the frequency axis on the spectrum analyzer to a linear scale, the approximate bandwidth for different values of ( was observed. The results are tabulated below:

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Figure 4 A (l) Received signal

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Figure 4 A (m) Recovered signal

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Figure 4 A (n) Recovered signal

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Figure 4 A (o) Input signal

The output is read from the frequency counter. By varying the frequency, and observing the output, we see that the discriminator output follows the following characteristic. The signal is also monitored on the oscilloscope.

PLL Detection

In this part of the experiment, we detect the Fm signal using a PLL. Using VCO #1, we made an FM signal by setting the center frequency of the VCO in open loop to 5 kHz and putting the function generator’s signal (1kHz 0 Vpp) and putting the function generator’s signal (1kHz 0Vpp) into the input of the VCO #1.

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figure 4 B (a) FM PLL

The PLL circuit is built according to the schematic shown below. The LPF with 1kHz cutoff frequency is to remove high frequency components from the detected signal. It is not a part of the PLL. In the open loop, the VCO#2 is set for 5kHz.

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figure 4 B (b) Block parameters of carrier VCO – 5kHz

VCO#1 is part of the FM transmitter, VCO #2 is part of the PLL detector. With the function generator putting out no signal, the VCO #1’s output frequency is varied. We note the dc voltages for the corresponding input frequencies. We observe that the discriminator curve generated using the PLL is more linear than the Double Tuned Detector.

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figure 4 B (c) PSD of message signal

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figure 4 B (d) Input signal

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figure 4 B (e) vco output

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figure 4 B (f) Limiter

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figure 4 B (g)VCO spectrum

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figure 4 B (h) Recovered Signal spectrum

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Appendix

Pre – Lab

Prelab Questions

Consider a carrier signal (cos ωct) being frequency modulated by a sinusoidal signal (A cos ωmt).

The result can be expressed as a series of Bessel functions:

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where

Jn(β) are Bessel functions of nth order

β = 2πkoA / ωm = koA / fm = modulation index

ko = frequency deviation constant

1. for β = .5, 2, and 2, sketch the positive frequency domain representation (magnitude only).

For β = 0.5

J0(β) = 0.9385

J1(β) = 0.2423

J2(β) = 0.0306

J3(β) = 0.0026

For β = 1

J0(β) = 0.7652

J1(β) = 0.4401

J2(β) = 0.1149

J3(β) = 0.0196

J4(β) = 0.0025

For β = 2

J0(β) = 0.2239

J1(β) = 0.5767

J2(β) = 0.3528

J3(β) = 0.1289

J4(β) = 0.0340

J5(β) = 0.0070

J6(β) = 0.0012

2. If fc = 5,000 Hz, fm = 1,000 Hz and ko = 2,000 Hz/V, find RMS value of the modulating signal for β = 0.5, 1, and 2

For β = 0.5

Then A = β/2

For β = 0.5

A = β/2 = 0.25

P = (0.24232 + 0.93852 + 0.24232 )*A2 /2 = 0.0312 J

RMS value = P1/2 = 0.02441/2 = 0.1766 V

For β = 1

A = β/2 ’ 0.5

P = ( 0.76522 + 0.44012 + 0.44012 + 0.11492 + 0.11492 )*A2 /2 = 0.1249 J

RMS value = P1/2 = 0.12491/2 = 0.3534 V

For β = 2

A = β/2 ’ 1

P = ( 0.22392 + 0.57672 + 0.57672 + 0.35282 + 0.35282 + 0.12892 + 0.12892 )*A2 /2

= 0.4987 J

RMS value = P1/2 = 0.49871/2 = 0.7062

3. Using Carson’s rule, what is the approximate bandwidth occupied by s(t) for β = 1 and 2

For β = 1

Bandwidth = 2(β + 1)fm = 2(1 + 1)*1000 = 4000 Hz

For β = 2

Bandwidth = 2(β + 1)fm = 2(2 + 1)*1000 = 6000 Hz

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