University of Jordan
Electrical Engineering Department
EE 429
Communications Lab
EXPERIMENT 3
AM RECEPTION
Lab Supervisor: Dr. Mohammed Hawa
Dr. Ahmad Mustafa
Lab Engineer: Eng. Reem Al-Debs
Prepared By: Dr. Mohammed Hawa
EXPERIMENT 3
AM RECEPTION
When you have completed this exercise, you will be able to describe the different components
of an AM receiver including the RF filter, mixer, IF filter and envelope detector.
DISCUSSION
On the ANALOG COMMUNICATIONS kit, the transmitted AM signal is sent by a direct
connection to the receiver, not through the airwaves as in regular radio transmission (see
Figure 3-3). A 1 MΩ resistor (R8) in this connection reduces the AM signal power, thus
simulating the attenuation in an RF radio channel.
The basic sections of the AM receiver are shown in Figure 3-14. Refer to this figure as necessary
to follow along with the general path of the received AM signal.
NOTE: The IF AMPLIFIER is not included on the ANALOG COMMUNICATIONS board.
The first stage in the AM RECEIVER circuit block contains an RF FILTER and an RF
AMPLIFIER (collectively called the RF stage). The RF FILTER schematic is shown in Figure 3-8
and consists of a variable inductor and a fixed capacitor in parallel, representing an LC
bandpass filter circuit.
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The 1 MΩ resistor (R8) connects to a tap in inductor L4 that is positioned to give a 7.7 turns
ratio. Because of this ratio, the RF filter's input impedance is about 50Ω when the filter is
running at 1000 kHz. A voltage divider is formed by R8 (1 MΩ) and the 50Ω antenna input
impedance of the inductor. Because of the voltage divider, the voltage of the signal to R8 will
be reduced by a ratio of 50 to 1,000,050 at the inductor tap (simulating losses on the
transmission channel).
When the input signal frequency equals the resonant frequency (fr) of the RF FILTER, L4
impedance equals that of C7, and the circuit becomes purely resistive and the output signal
passes with minimum attenuation. All frequencies far from the resonant frequency (i.e.,
outside the RF FILTER bandwidth) are rejected.
The RF FILTER bandwidth is designed such that it is wide enough to pass the upper sideband
(USB) and lower sideband (LSB) frequencies of the AM signal, which are on each side of the
1000 kHz carrier frequency (fc) (see Figure 1-17). For this to happen, the filter's resonant
frequency must be tuned to fc. Remember that the USB = fc + fm and the LSB = fc - fm. With a
1000 kHz carrier signal (fc) that carriers a 2 kHz message signal (fm), the sideband frequencies
are 1002 kHz and 998 kHz.
AM Signal Spectrum
RF FILTER transfer function
The RF FILTER variable inductor (L4) is already adjusted for you so that the center frequency
of the RF FILTER is set to 1000 kHz.
The RF AMPLIFIER schematic is shown in Figure 3-5, which is a single-ended differential
amplifier composed of two transistors: Q2 and Q3. The input connects to the bases of Q2; the
base of Q3 is grounded for ac signals. The emitters of Q2 and Q3 connect to the collector of Q4,
which functions as a constant-current source. Q3 is a common base amplifier.
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The Q3 collector is coupled to the base of Q5, which functions as an emitter-follower buffer.
The RF amplifier output is at C10 in the Q5 emitter circuit. The power gain of the RF
AMPLIFIER is very high so that the amplifier can greatly increase the power level of the AM
signal selected by the RF FILTER. The RF AMPLIFIER increases the AM signal’s power level
by about 72 dB (power gain of about 16,000,000).
The Q3 collector is connected to an RLC network, which contains a variable inductor (L5). The
variable inductor L5 is adjusted for you so that the RLC network is tuned to give maximum
gain at 1000kHz frequency. When the input frequency is 1000kHz, the inductive and capacitive
reactances in the RLC circuit cancel, and the circuit becomes purely resistive impedance.
The role of the MIXER (see Figure 3-14) is to join the RF stage to the IF stage. The MIXER
combines the 1000 kHz AM signal from the RF stage with a 1455 kHz local oscillator signal to
produce a 455 kHz difference signal for the IF stage.
The same type of IC (MC1496) used for the balanced modulator is used for the mixer. There
are two inputs to the mixer: M (message) and C (carrier). The MIXER'S (M) input is from the
RF AMPLIFIER output. The mixer's local oscillator input (C) is a 1455 kHz signal from the
VCO-HI circuit block.
The mixer converts (or translates) the AM signal frequency (1000 kHz) to the fixed IF signal
frequency (455 kHz) by the process of multiplication. This will also generate, in addition to the
455 kHz difference frequencies, the sum frequencies (around 2455 kHz) and the local oscillator
frequency (1455 kHz) (see Figure 3-22). You will later balance the mixer to suppress the
1455kHz local oscillator frequency in the mixer's output.
The IF FILTER in the IF stage is a ceramic filter, which has a higher attenuation outside of the
filter's bandwidth than LC filters do (see Figure 3-16). The IF FILTER is designed to pass a
455kHz signal with a bandwidth that includes the AM sidebands: the LSB at 453 kHz and the
USB at 457 kHz. The bandwidth of the IF FILTER is 20 kHz. Frequencies outside of this range
are greatly attenuated.
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Difference
frequencies
(an AM signal)
Sum
frequencies
(an AM signal)
Local oscillator
frequency
As shown in Figure 3-17, the message detector used here is the envelope detector formed by a
diode (CR1) in series with the parallel RC network. The circuit is designed to have a fast charge
time and a slow discharge time, with the resistor (R12) controlling the discharge time constant
(see Figure 3-18). There is an optimum value for the RC circuit discharge time constant
depending on the carrier frequency and the modulating signal frequency. A rule of thumb is:
R12C10 
1
fc
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When an AM signal is applied to the envelope detector circuit input, the diode conducts only
during the positive portion of the AM signal. The charging and discharging output signal of
the envelope detector RC circuit closely follows the positive envelope of the AM signal, thus
recovering the original message signal (see Figure 3-19).
On the positive half-cycles of the input signal, the capacitor charges to the peak input voltage.
Therefore, the voltage across R12 and C10 will equal that of the input signal (minus the diode
voltage drop) because the diode is forward biased. When the input signal drops below this
value, the diode turns off, and the capacitor (C10) starts to slowly discharge through the
resistor (R12) at the rate determined by the RC time constant.
On the next positive half-cycle of the input signal, the diode turns on, and C10 again charges to
the new value determined by the input signal. The process repeats itself.
A major drawback of the envelope detector is that approximately 0.4V to 0.6V voltage drop
must exist across the diode before the diode conducts. This drawback is more pronounced for
weak signals or 100% modulated signals. The effect of the diode's forward voltage drop is
shown in Figure 3-31.
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PROCEDURE A - RF FILTER AND RF AMPLIFIER
1. Connect the AM transmitter circuit as shown in Figure 3-6.
2. On the VCO-LO circuit block, insert the two-post connector in the 1000 kHz position.
3. Set switches S1 and S2 OFF. Set S3 ON.
4. Connect the oscilloscope channel 1 probe to the MODULATOR carrier signal input (C), and
trigger on CH1.
5. While observing the signal on channel 1, set the carrier signal amplitude to 300 mVpk-pk, and
its frequency to 1000 kHz.
6. Connect the oscilloscope channel 2 probe to the MODULATOR message signal input (M),
and trigger on CH2.
7. While observing the signal on oscilloscope channel 2, adjust the function generator for a
100 mVpk-pk, 2 kHz sine wave signal.
8. Connect the channel 1 oscilloscope probe to the output of the antenna (R5). Set the sweep to
0.1 ms/DIV, and trigger on channel 2. Adjust the MODULATOR potentiometer knob so that
the AM waveform is 100% modulated, as shown in Figure 3-7.
9. With a two-post connector, connect the TRANSMITTER to the 1MΩ resistor (R8) at the
AM/SSB RECEIVER circuit block input (see Figure 3-9). Connect the channel 1 oscilloscope
probe to the R8 input.
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10. If the receiver is properly configured to receive your AM signal, what is the resonant
frequency (fr) of the RF FILTER?
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11. With the 1000 kHz carrier signal and a 2 kHz message signal, what is the frequency of the
LSB that the RF filter has to pass?
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12. With a 1000 kHz carrier signal and a 2 kHz message signal, what is the RF FILTER'S
minimum bandwidth (BW) necessary to pass the received AM signal?
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PROCEDURE B - MIXER, IF FILTER, AND ENVELOPE DETECTOR
14. Connect the oscilloscope external probe to the (M) input of the MODULATOR block in the
AM Transmitter circuit, and trigger on EXT. Set the oscilloscope sweep to 0.2 ms/DIV.
15. As shown in Figure 3-23, connect the output of the 1455 kHz VCO-HI circuit block to the
local oscillator (C) input of the MIXER. Set the VCO-HI potentiometer knob fully CW.
CONNECT the MIXER to the IF FILTER with a two-post connector.
16. Connect the oscilloscope channel 1 probe to the MIXER'S output. Adjust the MIXER’S
balance potentiometer knob until the MIXER’S output signal appears as shown in Figure 3-25.
This adjustment suppresses the 1455 kHz local oscillator frequency in the mixed signal.
NOTE: Two AM signals and a suppressed carrier are shown in the MIXER’S output signal in
Figure 3-25.
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17. Connect the oscilloscope channel 2 probe to the MIXER'S output, and connect the channel 1
probe to the MIXER'S M input. Adjust the MIXER'S potentiometer one last time for a clear,
sharp output signal. Compare the MIXER'S output signal on channel 2 with the MIXER'S input
signal on channel 1. Is there another modulation envelope within the AM signal at the
MIXER’S output (similar to what is shown in Figure 3-25)?
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18. What are the frequencies present in the signal at the MIXER'S input?
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19. What are the frequencies present in MIXER'S output signal? Hint: The two envelopes
present in the MIXER'S output signal (see Figure 3-25) are two AM modulated signals.
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20. Connect the oscilloscope channel 2 probe to the output of the IF FILTER. While observing
the IF FILTER output, adjust the 1455 kHz VCO-HI frequency using the POSITIVE SUPPLY
knob on the base unit you receive maximum IF FILTER peak-to-peak signal. If the VCO-HI
frequency is not exact, the AM signal will not appear at the output of the IF FILTER.
21. Connect the channel 1 oscilloscope probe to the IF FILTER output. Channel 2 should be
connected to the IF FILTER input. Trigger on external.
22. Compare the IF FILTER output on channel 1 with the IF FILTER input on channel 2. How
many modulation envelopes are there in the IF FILTER output?
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23. While observing the IF FILTER output signal on channel 1, slightly vary the frequency of
the 1455 kHz signal to the MIXER by adjusting the POSITIVE SUPPLY fine tune knob on the
base unit. Why does the IF FILTER output signal disappear when the frequency of the 1455
kHz signal to the MIXER is slightly increased or decreased?
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24. While observing the IF FILTER output signal on channel 1, slightly vary the frequency of
the 1455 kHz signal to the MIXER by adjusting the POSITIVE SUPPLY fine tune knob on the
base unit to get the signal back at the IF FILTER output.
25. Connect the oscilloscope channel 1 probe to the MODULATOR message signal input on the
AM/SSB TRANSMITTER circuit block, and connect the channel 2 probe to the ENVELOPE
DETECTOR output. Trigger on channel 1. Observe the ripple on the ENVELOPE DETECTOR
output.
26. Does the signal at the ENVELOPE DETECTOR output have the same frequency as the
message signal?
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27. At the function generator, vary the frequency of the message signal. Did the ENVELOPE
DETECTOR output frequency vary with the frequency of the message signal?
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28. The distorted flattened negative peaks of the message signal are caused by the 100%
modulated signal at the ENVELOPE DETECTOR. Reduce the modulation index of the
transmitted AM signal by slowly turning the potentiometer knob on the MODULATOR in
the CCW direction, while observing the negative peaks of the recovered message signal
become normal. Explain your observations.
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29. Explain how does changing the value of the RC discharge time constant of the envelope
detector affects the ripple in the recovered message signal?
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PROCEDURE C - DESIGN A RECEIVER
30. Another method for recovering an AM signal is using a PRODUCT DETECTOR instead of
an envelope detector. A PRODUCT DETECTOR is simply a multiplying device (i.e., a mixer).
Show the connections you need to perform in the following figure to be able to recover the
message signal from an AM modulated signal.
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31. Test your design on the ANALOG COMMUNICATIONS circuit board and see the
resulting signals using the oscilloscope. Did you obtain the recovered signal from the output of
the PRODUCT DETECTOR or the output of the AUDIO FILTER (which is a LPF)? and why?
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32. Change the function generator’s output amplitude and frequency (i.e., the message signal).
How does that affect the recovered signal?
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