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The goals of this lab are to understand a simple diode detector, a biased diode detector circuit, a complementary feedback pair (CFP) detector and analyze them all in spice. Then breadboard and test each on in lab and finally, add your chosen detector circuit to your radio’s audio amplifier and test.
I came to the understanding that a simple diode detector is simply a diode rectifier circuit that has a fast enough time constant to follow the audio signal, but to slow to follow the carrier frequency. Choosing the correct time constant is important because if it is too fast, the signal will flat line at zero for a time because the capacitor discharges too fast. This can also happen if voltage is to low and the capacitor cannot be charged with sufficient voltage. If it is too slow, the detector will behave more like an AC-DC converter because it will not have time to discharge enough before being recharged.
In lab, I was unable to get the simple AM detector to turn on. The voltage was likely to low .
to turn on the diode. However, In the Spice simulation, I was able to see that it could follow the intelligence signal from a carrier of .75 mV and .25 mV side band voltages decently with a time constant of 0.1mS. When the resistor in parallel with the capacitor was change from 1k ohms to 100 ohms, the time constant became too fast, so the voltage dropped 0 at times and seemed to follow the carrier signal the rest. When the resistor was increased to 10k, the time constant became too slow to follow the intelligence signal. When the voltages of the carrier and side-band signals were halved, the detector did a poor job of detecting the intelligence signal.
The biased diode detector is more sensitive because it ensures that the diode is always on, but the phase is a bit shifted from the intelligence of the input signals. The detected signal was much cleaner for the original signal and it performed well at even a tenth of the voltage of the original signals in simulation. Lab results very similar, but the detected signal started to clip at the lower peak with an input of about 600 mV. When the 1k ohm resistor in parallel with the capacitor was replaced with the a 100k ohm, the detector started to perform like an AC-DC converter, but it performed like a detector with a 10k ohm resistor. When connected to the audio amp and speaker, the speaker played a loud sound that was crisp and clean.
The CFP is similar to a CC amplifier modified to act as an AM detector. The modified CC amp does not perform well with weak signals. The CFP fixes this problem with the addition of a resistor and a pnp transistor to the circuit. The pnp transistor provides additional current for charging Ce1 in the CFP circuit schematic. The CFP had a very good detection signal in simulation, but in lab, it had a hump on the rising edge of the detected signal as if it were following the carrier wave. When connected to the audio amp and speaker, the speaker played a loud sound that was clean, but not as crisp as the sound from the biased diode detector.
