Lab 4 - AM Detector

The purpose of this lab is to experiment with different AM detectors and select the one that will be used in my design of an AM radio.

Simple Diode Detector

The first detector that I experimented with was a simple diode detector. The circuit that I built is shown in Figure 1.

Figure 1: Simple Diode Detector Circuit

An 200 kHz carrier frequency and a 1kHz intelligence frequency AM signal was generated using the function generator. A small signal of around 150 mV pk/pk was first used as input to the detector. Figure 2 is a picture of the input and output of the circuit.

Figure 2: Input and Output of Simple Detector with Small Signal

R1 was changed to 10k and the input peak to peak voltage was increased to 11.6 V. Figure 3 shown the input and output of this.

Figure 3: Simple Diode Detector with R1 changed to 10k ohms and a large input magnitude

With R1 changed to 10k Ohms, some of the carrier frequency is passed through the detector. Even if the diode were turning on this would be a fairly noisy signal.

Now I changed R1 to 100k ohms. Figure 4 is this circuit with a small input and Figure 5 is this circuit with a large input.

Figure 4: Simple Diode Detector with R1 = 100k ohms and a small input

Figure 5: Simple Diode Detector with R1 = 100k ohms and a large input

The simple diode detector did not work. This is because the diode needs 0.7 V to turn on. I am not sure why the detector did not work when the input was increased to 11 V pk/pk. One way to get around this issue is to bias the diode so that it is right around 0.7 V. This way a small positive voltage would turn on the diode.

Biased Diode Detector

Figure 6 is the circuit that was created and Figure 7 is the input and output of the circuit.

Figure 6: Biased Diode Detector Circuit

Figure 7: Input and Output of Biased Diode Detector

This detector doesn't work very well with signals around 10 - 20 mV pk/pk. If this circuit is chosen, this will have to be taken into consideration when constructing the RF amplifier

CFP Detector

The last detector circuit that was experimented with was the CFP Detector Circuit. This circuit is shown in Figure 8.

Figure 8: CFP Detector Circuit

With this circuit, I had some trouble with the function generator. The input had periodic high frequency noise that distorted to AM signal to a point that it could not be detected. However, considering the faulty input, the output seemed correct for the circuit. Figure 9 shows the input and output of this circuit.

Figure 9: CFP Input and Output

Detector Selection

The simple diode detector is completely passive which means it uses no power. This is a good advantage, however, the main disadvantage to this design is it's inability to work. I was unable to get the CFP working properly, however this could be an issue with the function generator. The biased diode detector worked very well and the circuit is simplistic. For these reasons, I chose the biased diode detector as my AM detector.

Adding the AM Detector to My Radio

For the test of the detector and audio amplifier, a 1230 kHz carrier frequency and 1kHz intelligence AM signal was used as input to the radio. I also added a 1000 uF capacitor across the power supply to cut down on some of the noise. The speaker was connected in place of the 10 Ohm resistor.

The sound quality sounded very close to that of a sine wave. With the audio amplifier that I chose, I was able to get the sound to be pretty loud before it distorted. When I lowered the input amplitude of the AM signal, strange distortion occurred that resulted in the audio changing pitch to different harmonics. The sound went down a fifth and then an octave lower than the intelligence frequency.

Conclusion

As long as I can amplify the RF signal to a proper amplitude, the radio as it stands right now should work well. The biased diode detector has a calculated quiescent power dissipation of 81.82 uW. This is almost negligible considering the power dissipation of the audio amplifier.