Lab 4 - AM Detector

The purpose of this lab was become familiar with the analysis and design of several AM detector circuits. I built and tested a biased diode detector circuit and a complementary feedback pair (CFP), and then proceeded to add the my chosen detector to my radio's audio amplifier.

I began the lab by constructing the biased diode detector shown in Figure 1. A simple diode detector without biasing works well as long as the signal is strong enough to turn on the 1N4148 diode shown below. However, if the input signal is too strong, then it tends to distort the output. Usually, the signal coming in to the detector is weak, and the output is suffers because the diode does not turn on. The remedy for this is to bias the diode detector as shown in Figure 1. This detector worked fairly well for a suitable input signal. However, when I increased the input voltage, the lower peak of the output waveform was clipped. A screenshot of the oscilloscope showing the input and output waveform corresponding the this configuration is given in Figure 3.

Figure 1: Biased Diode AM Detector Circuit

Figure 2: Breadboarded Biased Diode AM Detector (bottom left)

Figure 3: Input/Output Waveform for Biased Diode AM Detector

Next, I proceeded to build my AM Detector of choice: the CFP detector. I constructed the circuit as shown in Figure 5 and examined the resulting input/output waveforms shown in Figure 6. This circuit is a modified common-collector amplifier that functions as an AM detector. The CFP has the ability to achieve better detection for weaker AM signals and its operation is just like that of a detector diode. When the input is high, the transistor is on and capacitor Ce1 charges. When the input is low, the voltage at Ce1 prevents the emitter voltage from dropping low enough to turn the transistor on. Ce1 discharges through Re1 with a time constant too slow to follow the carrier but fast enough to follow the desired audio signal.

Figure 4: Complementary Feedback Pair (CFP) Detector

Figure 5: Breadboarded CFP Detector

Figure 6: Input/Output Waveform of CFP Detector

The CFP configuration was a bit noisy for me, but I tested it with the LM386 amplifier shown in Figure 7 to see how it sounded. The output at the speaker was loud and clear. Thus, the CFP/LM386 combination is the setup I will integrate into my radio.

Figure 7: LM386 amplifier with speaker as output

For the guided design assignment associated with this lab, it was asked that we improve the design of our detector. My goal was to get the CFP optimized for the 5mV carrier. I altered the time constant by raising the value of Ce1 to 100uF and Re1 to 1k. Then, I added a -9V voltage source at the bottom node of the circuit. I came to this setup mostly by trial and error and I had trouble figuring out what a good target range would be for my AM signal level in terms of carrier frequency amplitude. My design and simulation is given in Figure 8. Testing this in lab, I had a little trouble getting my circuit to function properly, and noticed little improvement over the original design.

Figure 8: CFP detector for guided design

In conclusion, this lab was pretty straightforward. It was very interesting to learn how to design and analyze AM detector circuits. I have always been interested in learning how a radio operates, and I am starting to see everything come together as progress toward the construction of a working radio. I am looking forward to seeing what effects my choice of amplifier and detector will have on my completed radio.