ECEN 1400 - Introduction to Digital and Analog Electronics

Peter Mathys, Spring 2014


Lab 4: Simple AM Radio Receiver, Part 1

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Goals of this Lab


Prelab

P1. How Does AM Radio Work? Find one or more sources on the internet that explain how AM radio works. An example is the beginning of this video (which also explains how FM radio works). In your own words, explain how an AM signal is generated and how it is recovered at the receiver. Sketch the waveform of an AM signal and explain what envelope detection means.

P2. A Simple Envelope Detector for AM radio. Build the simple transistor amplifier shown below in Multisim. This is essentially the same circuit you built in problem P1 last week.

Simple transistor amplifier with sinusoidal input signal

If the amplitude of the input signal is set to the value indicated in the schematic, then the amplifier is overdriven and the output signal is clipped as shown on the oscilloscope below.

Simple transistor amplifier with clipped output signal on scope

At which value of the AC source voltage does clipping start for 0.1 MHz sinusoids?

The simplest type of envelope detector for demodulating AM signals requires taking the magnitude of the AM signal (e.g., by essentially cutting off all negative voltages and keeping only the positive ones). If the simple amplifier in the above schematic is overdriven such that it clips off the negative side of the input signal, then it effectively acts as a AM demodulator (not perfect, but working reasonably well). In the schematic below the sinusoidal voltage source has been replaced by an AM voltage source (under "Signal Voltage Sources" in Multisim).

Simple transistor amplifier with AM signal input

Set the properties of the AM voltage source as shown next.

AM Voltage Properties

Now you should see the following signals on the oscilloscope.

Simple transistor amplifier with clipped AM signal output on scope

The upper trace is the AM signal that is generated by the AM source. The lower trace shows the clipped signal at the output of the overdiven transistor amplifier.

The next schematic shows a slightly improved version of the AM demodulator. A diode was added at the output to clip off some more of the negative voltages. The RC circuit consisting of R4, C4, and load resistor R3 was added to suppress the carrier frequency of the AM signal at the output of the demodulator.

Simple transistor amplifier with AM signal input

Determine the system function H(f)=Vout/Vin in terms of R3, R4, and C4 (not the numerical values of these elements). Note that Vout and Vin are phasors and that you have to use impedances to compute H(f), see the Frequency Response of RC Circuits notes. Determine the -3dB frequency of this RC filter. What is the gain G = |Vout|/|Vin| of this filter at f = 100 kHz and 1 MHz?

RC filter at output of demodulator

The oscilloscope screen shot below shows the AM signal at the input of the demodulator (upper trace) and the demodulated output (lower trace).

Simple transistor amplifier with demodulated AM signal output on scope

By varying the carrier amplitude, determine what the minimum input voltage is for which the AM demodulator produces a good demodulated signal when the carrier frequency is 0.1 MHz.

Next, insert a resistance (R5) between the AM source and the demodulator circuit as shown below.

Measuring input resistance of simple transistor amplifier with AM signal input

Determine the input resistance of the AM demodulator at 0.1 MHz by increasing R5 until the demodulated output signal is at one half of the value that is obtained when R5 is set to zero.

P3. AM Radio Receiver Demodulator and Audio Amplifier. Now we return to the LM386 audio amplifier that you used in the previous lab. A slightly modified schematic is shown below.

Schematic of audio amplifier with 1 kHz input

Note the combination of R2 and C5. Is this a lowpass or a highpass filter? what is its cutoff (-3 dB) frequency? What do you think is the function of this RC filter? Also note the variable resistor (potentiometer) R1 that can be used to adjust the volume of the amplifier. Check that your audio amplifier works by displaying the input and output waveforms for the amplitude and frequency values specified for the AC source.

Next, replace the AC source with the AM source as shown below.

Schematic of audio amplifier with AM signal input

Set the parameters of the AC source to the values shown below.

AM Voltage Properties

You should see the following waveforms on the oscilloscope. What do you think you would hear at the output of the amplifier?

Audio amplifier with AM signal input, scope waveforms

Finally, combine the AM demodulator and the audio amplifier as shown in the next schematic.

Schematic of AM demodulator and audio amplifier

The waveforms you should see on the oscilloscope at the input and the output of this combined circuit are shown below.

AM demodulator and audio amplifier waveforms

By varying the carrier amplitude of the AM source, determine the range for which the combination of AM demodulator and audio amplifier works as intended at a carrier frequency of 0.1 MHz. What happens if you change the carrier frequency to 1 MHz. Is the useful range of amplitudes te same or does it change? If so, why?


Lab Experiments

E1. There is no experiment E1.

E2. A Simple Envelope Detector for AM radio. Build the envelope detector described in P2 on your breadboard. The datasheet for the 2N2222 transistor can be found here (e.g., to look up the E,B,C pin identification). Repeat the measurements you made for P2 in Multisim on the real circuit. Note and explain any differences between the simulations and the measurements you make in the lab.

E3. The AM Radio Receiver Demodulator and Audio Amplifier. Build the audio amplifier together with the envelope detector described in P3 on your breadboard. The data sheet for the LM386 can be found here (e.g., for the identification of the pins). Note that the enevelope detector is the same circuit that you built in E2. Repeat the measurements you made for P3 in Multisim on the real circuit. Perform your measurements first with 8 ohm resistor as output load for the LM386. Then connect the speaker instead of the 8 ohm resistor. Verify that you cannot hear any signal if you connect the AM signal directly to the audio amplifier. Then verify that you can hear the audio signal if the AM signal is first demodulated by the envelope detector. Change the frequency of the modulating signal on the waveform generator to hear a corresponding change in the demodulated signal. What is the smallest carrier amplitude setting on the waveform generator for which you obtain a good demodulated signal?

Keep this circuit on your breadboard for the completion of the AM radio next week.