ELEC 3030 Lab 4: AM Detector

In Lab 4, I studied the AM Detector circuit, whose job is to extract the audio signal from the AM signal. After testing a few circuits through simulations in LTspice, I built these circuits in lab and eventually connected one to the rest of my radio (the audio amplifier from the previous lab activity). The following exercises document the information of each of the circuits I breadboarded in lab.

First, I built a simple detector circuit (Figure 1) using a 1 kilo-Ohm resistor, a 0.1 micro-Farod capacitor, and a diode (D1N4148). Setting the input to 20 mVpp at a carrier frequency of 200 kHz, internal modulation for intelligence at 1 kHz, and 50% modulation, I recorded the output (blue) shown in Figure 2. As I increased the input voltage, I did not record an output voltage until the input reached 770 mVpp. Figure 3 shows the input and output at an input of 1 Vpp.

Figure 1: Simple AM Detector Circuit

Figure 2: Graph of Input vs Output of Simple AM Detector Circuit

Figure 3: Input vs Output at Vin = 1 Vpp

Using these settings, I changed the load resistance to 10 kilo-Ohms and measured the output, which slightly resembled a ripple voltage waveform because of the large RC time constant which impeded the capacitor from discharging fully before the next AC cycle. Then again changing the value of R1, this time to 100 kilo-Ohms, this effect was compounded.

Next, I breadboarded the biased diode detector circuit whose schematic is shown in Figure 4. I tested different input voltages for this circuit to determine which values worked best. First I tried 200 mVpp and was not satisfied with the output waveform (Figure 5). Next, I tried 500 mVpp which produced a very nice, clean sine wave (Figure 6). Finally, I tested the circuit with an input of 800 mVpp.

Figure 4: Biased Diode Detector Circuit

Figure 5: Input vs Output of Biased Detector Circuit at an Input of 200 mVpp

Figure 6: Input vs Output of Biased Detector Circuit at an Input of 500 mVpp

Figure 6: Input vs Output of Biased Detector Circuit at an Input of 800 mVpp

Next, moving on to the CFP (complementary feedback pair) detector (Figure 7), I constructed this circuit and performed some tests. Using an input value that I had previously calculated, I tested the circuit with an input (yellow) of 30 mVpp, which resulted in a very strange, triangle-wave output (blue) signal (Figure 8). Then, I changed the input (yellow) voltage to 50 mVpp, and this seemed to do the trick as my output (blue) was a nice sine wave (Figure 9).

Figure 7: CFP Detector Circuit

Figure 8: Input vs Output of the CFP Circuit with an Input of 30 mVpp

Figure 9: Input vs Output of the CFP Circuit with an Input of 50 mVpp

Finally, in the last part of the lab activity, I chose the biased diode detector circuit to add to my audio amplifier. After adjusting the settings of my function generator to accommodate the audio amplifier circuit and the biased diode detector circuit, I propagated an AM signal through the circuit and, to my delight, received a wonderfully clear, crisp tone from the speaker.

Although I did not know how exactly the AM detector was going to know, after hearing the signal through the speakers, I finally comprehended the function of the detector. To my great surprise and delight, I had great control over the frequency of the tone coming from the speakers. It also amazed me that such a clean sound could come from this small, inexpensive speaker. As I add more and more parts to my radio, my excitement for the finished project grows.