ECE 341 lab 1- INTRODUCTION TO THE SPECTRUM ANALYZER



ECE 341 lab 1- INTRODUCTION TO THE SPECTRUM ANALYZER

PREFACE – READ BEFORE LAB

SOME SPECTRUM ANALYZER (SA) PRINCIPLES

The SA is the most common way to watch a spectrum in real time. One use: every electrical device must satisfy FCC rules before it can be sold, and the FCC specifies certain measurements using a SA.

The SA effectively sweeps a narrow-band filter (with bandwidth being the “RESOLUTION BANDWIDTH”) repeatedly over the incoming signal, covering a range of frequencies designated by the “SPAN”. A big resolution bandwidth (RB) covers the span faster, but a smaller resolution bandwidth gives a better view of fine details in the spectrum.

As the filter scans the signal’s spectrum, the filter response varies with that spectrum. Meanwhile, the SA has a display much like that of an oscilloscope and sweeps to the right (synchronized with the increasing filter frequency) while the vertical position is controlled by the filter response. Thus a display of the spectrum is obtained repeatedly and appears continuous.

If you don’t know what resolution bandwidth to use, try different values until you find one with these properties: if you decrease the RB further, the spectrum does not change significantly (you are seeing all the fine details of the spectrum and narrower RB doesn’t help). However if you increase the RB, the finest details in the spectrum start to get smoothed out, or blurred.

Hence the display of the SA is the magnitude of the Fourier transform, assuming the spectrum does not change during a scan. Another way to observe the spectrum is to store a sample of the signal in an oscilloscope and calculate with a Fast Fourier Transform (FFT), which is built into some oscilloscopes.

A major convenience of using dB for the spectrum magnitude is that multiplying a signal by a constant (like the gain of an amplifier) becomes adding dB when using dB. Another is that very wide ranges of values can be represented by smaller ranges of dB. (Other examples: scale for sound volume, Richter scale for earthquakes.)

Although we pretended above that the SA scans a filter over the signal, the SA actually moves the signal’s spectrum over a fixed narrow-band filter. Scanning either filter or signal over the other gives the same display, and moving the signal’s spectrum is easier than moving the filter.

The SA has plenty of bells and whistles you can explore, and they will make sense when you think about the basic operation of the SA.

SOME DIGITAL TV (DTV) PRINCIPLES

This semester the TV spectra we view are new. In June 2009 all TV in the U.S. became digital. The spectrum of a digital signal is difficult to describe, but we can view it and check the frequency range.

Main advantages of DTV are that a given quality can be achieved with less bandwidth, and noise either causes no problem or kills the signal entirely (so if you get a picture it will be good). Decreased bandwidth let the FCC give the old channels 52-83 to other users. It also let each DTV station use the bandwidth to provide high definition TV, and/or surround sound, and/or more than one program on its allowed frequency range (by interleaving digital pulses of different programs).

The frequency range of each channel in the old analog system is given on the next page. Those channel numbers are still used today, but a station calling itself by one channel number (to keep its identity with the public) might be broadcasting now on a different channel! Knoxville’s channel 6 now really transmits on channel 26, probably because channel 6 frequencies are in the range of motor noise and digital signals often perform poorly in that environment.

|Channel |Frequency |

|Designation |Band (MHz) |

|2 |54-60 |

|3 |60-66 |

|4 |66-72 |

|5 |76-82 |

|6 |82-88 |

|7 |174-180 |

|8 |180-186 |

|9 |186-192 |

|10 |192-198 |

|11 |198-204 |

|12 |204-210 |

|13 |210-216 |

|14 |470-476 |

|15 |476-482 |

|16 |482-488 |

|17 |488-494 |

|18 |494-500 |

|19 |500-506 |

|20 |506-512 |

|21 |512-518 |

|22 |518-524 |

|23 |524-530 |

|24 |530-536 |

|25 |536-542 |

|26 |542-548 |

|27 |548-554 |

|28 |554-560 |

|29 |560-566 |

|30 |566-572 |

|31 |572-578 |

|32 |578-584 |

|33 |584-590 |

|34 |590-596 |

|35 |596-602 |

|36 |602-608 |

|37 |608-614 |

|38 |614-620 |

|39 |620-626 |

|40 |626-632 |

|41 |632-638 |

|42 |638-644 |

|43 |644-650 |

|44 |650-656 |

|45 |656-662 |

|46 |662-668 |

|47 |668-674 |

|48 |674-680 |

|49 |680-686 |

|50 |686-692 |

|51 |692-698 |

In the transition period to DTV, stations began temporarily transmitting DTV on new channels and kept analog on their old channels. In June 2009, all analog ceased (or went to low power in some cases) and basically all TV was DTV. After that, many stations stayed on the temporary channel, some went back to the old channel, and some went to still another channel! Each station kept saying it was the old channel, but often broadcast on another channel. If a channel puts more than one program on its channel, you see two channels on your tuner; e.g., you see channels 10.1 and 10.2; your receiver tunes to channel 10 and sorts out the digital pulses to give you 10.1 or 10.2.

The unchanging public name for the station is the “virtual channel”, and the channel number of the frequency actually used is the “actual channel” or “RF channel”. The Knoxville stations did the following:

|Virtual channel |DTV |DTV |

|(the name) |actual ( or RF) channel |actual |

| |during transition |(or RF) channel |

| | |now |

|6 |26 |26 |

|7 |7 |7? |

|8 |30 |30 |

|10 |31 |10 |

|15 |17 |17 |

|43 |34 |34 |

So in the lab you might find channels 7, 10, 17, 26, 30 and 34. However most transmitters are on the other side of Ferris Hall from our lab windows so you might not find all those. To increase chances of finding a signal on the spectrum analyzer, set the frequency range so the desired signal’s frequency range covers most of the screen horizontally, use very small reference level (high sensitivity), narrow resolution bandwidth (better resolution and lower noise) and average many (16-64) scans (makes signal stand out from noise).

End Preface.

................
................

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

Google Online Preview   Download