Experiment 9A. FM Receiver

ECE 460 LAB Experiment 9A. FM Receiver PROCEDURE: 1) Construct the circuit shown in the attached schematic. 2) Set the signal generator to generate an unmodulated 85 kHz signal at about 500 mV p-p. Attach the generator to the receiver by clipping the generator output to the INSULATED part of the antenna wire. This will induce enough voltage to drive the receiver. 3) Attach your scope probes to the circuit as follows: a. CH1 to MIXER MOD IN b. CH2 to MIXER CARRIER IN 4) Vary the carrier frequency from 83 kHz to 87 kHz in 250 Hz steps and note the phase difference of the two signals at each frequency. Note: one of the signals is a square wave. Be sure to measure the phase by comparing the zero crossings of the two signals or by using the scope’s phase measuring feature. 5) Plot the data above as phase in degrees vs. frequency. On a second plot, plot the cosine of the phase vs. frequency. 6) Set the generator frequency to 85 kHz. Turn on the FM modulation as done in the previous experiment. Set the modulation frequency to 1 kHz and the deviation to 250 Hz. 7) Attach your scope probe to the junction of R2 and C3 to observe the demodulated output of your receiver. Measure and record the peak to peak amplitude of the demodulated waveform. Note, you may have to use averaging to remove any residual carrier. 8) Increase the deviation of the generated FM signal in 250 Hz steps to 5 kHz, while measuring and recording the peak to peak voltage at the output of the generator. Note: the demodulated signal may become distorted at some point, with the top and bottom of the waveform being clipped. Measure the peak to peak output voltage even if the signal is distorted. 9) Plot the demodulated peak to peak voltage output from the previous step vs. the frequency deviation. This is the transfer function of your receiver. Over what region can you consider the transfer function linear? 10) Set up the spectrum analyzer in the scope to observe the spectrum of the demodulated output. Reduce the frequency deviation of the generator to 250 Hz. Increase the deviation slowly until the second harmonic of the modulating signal (2 kHz) is 40 dB below the fundamental. Record this deviation. 11) Keeping the frequency deviation from the previous step, vary the amplitude of the modulated carrier, starting at the lowest setting of 20 mV. Note the demodulated signal output level. Turn off the modulation and note any noise (you will have to estimate its amplitude). If necessary, use averaging to determine the amplitude of the demodulated signal. Turn the modulation back on and increase the carrier amplitude by doubling and record the amplitude of the demodulated signal. At each carrier level, turn off the modulation and record the noise level. Plot the noise level and the demodulated signal level on the same plot against the carrier level in decibels referenced to 1 volt. 12) Remove and TURN OFF the generator. Turn up your audio amplifier and listen to the received signal. Try moving the antenna around on your bench. How is the signal affected? 13) The instructor will vary the signal strength by turning the transmitter power up and down. How does this affect the demodulated signal? Observe and comment on the volume of the voice/music you hear and that of any noise and/or interference. 0 3 2 + - U1 6 LF411 E1 ANTENNA 12" Wire R3 2.2M From Mixer Output C1 R5 R2 1K C4 68K 0.033uF 10uF 0 3 2 + - R4 1N914 1N914 R1 POT 0.033uF C8 R6 470pF C2 C6 10uF 0 10K Mixer Mod In On all LF411's: Pin 4 = -10V pin 7 = +10V U2 6 LF411 D2 D1 100K C3 0.001uF 0 3 2 L1 1mH - + 7 1 U3 6 LF411 Mixer Carrier in Mixer Ground +10V LM384 OUT 8 0 470uF C7 Ground LM384 Pins 2,3,4,5,10,11, and 12 U4 +IN -IN BY PASS VS C5 0.1uF 14 1 2 6 4 5 7 1 4 5 7 1 4 5 SPEAKER SPEAKER RTN Antenna Buffer Amplifier/Limiter X Phase Shift Network Phase = 90⁰ + K*Δf Low Pass Filter DEMOD OUT