AM Detectors
This week we are learning about AM Detector circuit. The purpose of these circuits is to extract the intelligence signal out of the modulated signal. Over the course of this lab students will test and learn about three different AM detector circuits, and at then end choose the one the like best and test it with their audio amplifier.
Simple Diode Detector
Simple Diode Detector Schematic (1)
The first circuit we learned about was the simple diode detector circuit. This circuit uses a diode and an RC time constant that is fast enough to follow the intelligence signal, but too slow to pick up the carrier. This means that only the audio signal can pass through, successfully demodulating the signal.
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The student is asked to construct a simple diode detector circuit with R = 1K ohms and C = 0.1 uF. Then they are to feed an AM signal to the circuit and see how the output looks.
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Simple Diode Detector Output
We can see that we are getting some extraction from the signal, but it is no perfect. The circuit struggles to extract the signal at lower voltages, and this is because the diode is not letting these lower voltages pass through. This is why we see clipping at the bottom of the output. The next circuit we will look at aims to remedy this issue.
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Biased Diode Detector
Biased Diode Detector Schematic (1)
As we saw above, the simple detector circuit struggled to extract the lower voltage portions of the inteligence signal from the modulated signal. The biased diode detector circuit remedies this issues by forcing the diode into an on state, allowing these lower voltage portions to pass through. The student is now asked to construct the biased detector circuit and observe its output.
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Biased Diode Detector Output
This output looks much better than the simple diode circuit's output. The biased circuit has no problem extracting the lower portions of the signal and we get a nice sine wave as our output, no clipping.
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CFP Detector
CFP Detector Schematic (1)
Now we move on to the CFP or complementary feedback pair detector circuit. How this signal works is that when the signal is high, the transistors are on, and capacitor Ce1 is charged. When the signal is low both of the transistors switch off and Ce1 discharges through resistor Re1. The reason for the pnp transistor is to provide additional current allowing for the capacitor to charge more quickly. This circuit works very well at extracting weak signals. Once again, the student is asked to build this circuit and examine its output when feed an AM signal.
CFP Detector Output
As you can see, I had some issues with the oscilloscope during this step, and accidentally took my screenshot with the message on the screen, my apologies for this mistake. I also had some struggles getting the input signal to appear cleanly on the screen due to the lower voltages I used compared to the voltages used for the simple and biased circuits. Despite my short comings with taking a proper screenshot you can still see the extracted intelligence signal at the top, and it looks quite good. I was impressed at how clean this extraction was at the lower voltage input.
Detector + Audio Amplifier Test
The last thing we were asked to do in this lab was to feed an input AM signal to a detector circuit of out choosing, and then feed the detector output to out audio amplifier with a speaker connected and see how it sounded.
In my case, I chose to use the biased diode detector in combination with a CE CC amplifier circuit. I actually got a really nice output on my speaker, the intelligence went through very well, and it was decently loud. I was happy enough with the performance of this that I plan on using this detector amplifier combination in my final radio build.
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Overall, this was a pretty easy going and interesting lab in my opinion. The material in the lab manuals was well written and made understanding how these various detector circuits work easy to understand.
Citations
1.) Stuart M. Wentworth, 2018