fm

FM Signals Equipment: Spectrum analyzer RF generator Communications system analyzer, Motorola R-2001C Function generator Note: Since there is only one communications system analyzer, each group will be assigned a time slot for the use of the analyzer during the lab session, in order to complete part II. It is not necessary to complete the parts in order. Part I: Single tone modulation 1. Connect the output of the RF generator to the spectrum analyzer and connect the function generator output to the external modulation input of the RF generator. Set the RF generator to give an output of -20 dBm at a frequency of 240 MHz. 2. Using a modulating frequency of 10 kHz, adjust the deviation to obtain each of the following modulation indices in turn: 0.5, 1, 2.4, 5, 8.65. For each, sketch the spectrum obtained and calculate the Bessel function coefficients for each sideband with an amplitude greater than 5% of the unmodulated carrier amplitude. Compare with theoretical values. 3. Adjust the deviation to 100 kHz and use each of the following modulating frequencies in turn: 10, 20, 30, 40, 50 kHz. For each frequency, measure the bandwidth ignoring all components with amplitude less than 5% of the unmodulated carrier amplitude, and compare with the bandwidth predicted by Carson's rule. Part II: FM Broadcast signal 1. Connect a cable FM/TV line to the Motorola communications system analyzer. (If cable is not available, the short antenna supplied with the analyzer can be used.) 2. Set controls as follows: Frequency: any local FM broadcast station RF scan: 0 Function: monitor/FM RF level: 0 if antenna used; if cable used, start at max. attenuation and reduce until satisfactory operation is obtained. Squelch: off Volume: adjust for convenient listening level BW: wide 3. Note the carrier frequency error and use it to calculate the station's exact carrier frequency. (What assumption is made here?) 4. Observe the deviation. Experiment with different settings of the deviation alarm: try 60, 70, and 75 kHz. (75 kHz is the legal maximum for FM broadcast stations.) 5. Note the minimum level for deviation during "dead air" (no audio signal). This deviation is due to the 19 kHz pilot carrier. 6. Switch BW (bandwidth) to narrow (12 kHz) and back to wide (200 kHz) and note the audible effect. 7. Change display setting to "modulation" to view the baseband signal in the time domain. Find the pilot carrier and verify its frequency. 8. Change display setting to "spectrum analyzer". Set dispr/swp to 1 MHz to view the station's sidebands. Find the approximate bandwidth of the signal. Then increase the span with the dispr/swp control to view other FM stations. 9. Change display setting to "IF". Note the presence of noise and the absence of an envelope. (If you switch the BW to "narrow" momentarily, you will see an envelope. Explain why this is to be expected.)