Lab 04: AM Detector

Amplitude modulated signals require two frequencies to operate: the first forms the body of the wave, and is referred to as the 'carrier' frequency, while the second is the actual information we are looking to broadcast. This signal is referred to as the 'intelligence' signal, and is typically at a much lower frequency than a carrier signal. The intelligence signal is created by modulating the amplitude of the carrier signal over time, resulting in a waveform that looks similar to this:

In this lab, I constructed an AM detector circuit. The function of this circuit is to filter out the carrier signal, leaving only the intelligence signal which will be amplified and sent to a speaker, to produce a sound output. The goal of this lab was to build an AM detector signal that filters out the carrier signal as cleanly as possible, leaving only the intelligence for a clean sound output.

For my first try, I built a detector circuit centered around a diode with a capacitor in parallel in order to alter the time constant. Ideally, the time constant will be properly sized to follow the intelligence signal while filtering out the carrier. This circuit functions ok, but the nature of the diode means that it has a turn on voltage, which introduces distortion to the circuit as you can see in the figure below. (Yellow is for input signal, blue is for output signal)

In order to get the diode detector circuit to function better I biased the diode, meaning that I applied a small voltage to the diode in order to keep the diode near its' turn on voltage and therefore make it more responsive. As you can see in the figure below, the biased diode circuit performed wonderfully, eliminating distortion and filtering out the carrier frequency.

Reducing the input voltage from 140 mV to a mere 42 mV did not phase the biased diode detector, it still followed the input and filtered out the carrier as seen in the figure below .

Finally I tested a complementary feedback detector, another circuit designed to function as an AM detector. This circuit produced an output with a higher voltage than the biased diode detector for the case of a low voltage 42 mV input, but it did not perform as well filtering out the carrier frequency, producing a thick output signal that was dirtier than that of the biased diode detector. The figure below demonstrates what I am describing.

After all I have learned from this lab, I chose to use the biased diode detector as the AM detector for my AM radio, due to its impressive filtering capability and its' ability to operate under low voltage input conditions.