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