AM Detector

Amplitude Modulation (AM) is one method of transmitting a signal. In order to decipher the modulated signal, a detector is developed to output the intelligence signal.

There are multiple circuits that can be used to unmodulated the transmitted signal, discussed below are the simple diode detector, biased diode detector and the complementary feedback pair (CFP).

For all the circuit discussed, the same input is used, unless stated otherwise. The AM input is frequency 1230 kHz, with 1 kHz modulation. The amplitude varies as required.

Part 1: Simple Diode Detector

As the name implies, the simple diode detector is the simplest detector, However, the simplicity is its drawback; as the diode only allows only the positive half cycle of the input and while the capacitor rectifies the output, resulting in an output that is closer to the actual signal.

Additionally, it is important to keep in mind the RC time constant, the time it takes the capacitor to discharge over the resistor, as it greatly affects the behavior of the output signal.

To observe how the time constant changes the output, several different values for the resistor were used. It was observed that increasing the resistance increases the time constant, thus the capacitor is unable to completely discharge before the next positive half cycle, leading to a near flat output signal.

Part 2: Biased Diode Detector

In the simple detector, the detector only works as long as the input signal is strong enough to turn on the diode. However, if the signal is not strong enough, then nothing will go through, the biased diode detector does not suffer from this problem. This effect can be observed by comparing figure 2 with figure 4. The input signal in figure 2 has an amplitude of 1.5 V, compared to that of figure 4, which is 1 V.

Observing the output, figures 4-9, it can be seen how decreasing the output makes it harder for the circuit to detect the signal. While it is hard to pin down one amplitude that is best for this circuit, the range between 250 mV and 500 mV seems to have the output closest to a sinusoidal wave.

Part 3: CFP Detector

The Complementary Feedback Pair is the most complex detector discussed here, however, it has the best detection range. The two transistors turn on when the voltage is high, tracing the input well. If the input voltage is not high enough, then capacitors discharge, much like how a biased diode detector behaves.

Observing the output, figures 11-15, it can be seen how decreasing the output makes it harder for the circuit to detect the signal. While it is hard to pin down one amplitude that is best for this circuit, the range between 100 mV and 500 mV seems to have the output closest to a sinusoidal wave.