RF Systems Lab

Lab 4: AM Detector

Lab 4 Goals:

Become familiar with, analyze, and construct the following amplifiers:

Understand a simple diode detector circuit and analyze it with LTspice
Understand a biased diode detector circuit and analyze it with LTspice
Analyze a complementary feedback pair (CFP) detector with LTspice
Breadboard and test simple diode detector, biased diode detector, and CFP detector circuits
Add your chosen detector circuit to your radio’s audio amplifier and test

Lab 4 Equipment:

AFG1062 Arbitrary Function Generator
Tektronix MDO 3032 Oscilloscope
Omega HHM90 Digital Multimeter
USB Device
1/2 Watt 8 Ohm Speaker

4.1 AM Detector

The AM detector’s job is to extract the audio signal, or envelope, from the AM signal. This audio signal is then amplified and passed to the speaker. We will

investigate a very simple diode detector followed by a biased diode detector for better performance. Both of these will be studied in LTspice. Then, we

will simulate a complementary feedback pair (CFP) detector circuit which has clear advantages over the diode detector circuits. The pre-lab report for sections 4.1-4.4 is below.

PreLab_4.pdf

4.5 Constructing AM Detectors

First, the simple diode detector is constructed using a 1kΩ resistor, a 0.1µF capacitor, and a D1N4148 diode. The carrier frequency is set to 200kHz with an intelligence frequency at 1kHz and 50% modulation. Figure 1 shows the output with a 20mVpp input and FIgure 2 shows a 1Vpp input. If the value of R1 is changed to 10kΩ from 1kΩ, the output displays a ripple voltage rather than a sinewave due to the large RC constant.

Figure 1: Simple AM Detector with 20mVpp Input

Figure 2: Simple AM Detector with 1Vpp Input

Next, the biased diode detector circuit is constructed and input voltages from 200mVpp to 1Vpp are tested to determine at what input voltage the circuit functions best. Ultimately, an input value of 800mVpp seems to produce the best output signal. Figures 3, 4, and 5 below show the input and output waveforms at these inputs.

Figure 3: Biased AM Detector with 200mVpp Input

Figure 4: Biased AM Detector with 500mVpp Input

Figure 5: Biased AM Detector with 800mVpp Input

Then, the complimentary feedback pair detector is constructed and analyzed. Based on the pre-lab simulations, 30mVpp is chosen as a beginning input voltage. However, the output resembled a sawtooth wave rather than a sinewave. The input is then adjusted to 50mVpp and a sinewave is produced as the output of the circuit, as shown in Figure 6.

Figure 6: CFP AM Detector with 50mVpp Input

4.7 Add the AM Detector to the Amplifier

Finally, the biased diode AM detector is added to the audio amplifier from the last chapter. A large electrolytic capacitor is also added to reduce noise in the circuit. When the load resistor is replaced with the speaker, a clear and relatively loud tone is produced. Based on the previous performance of my radio, I was surprised at how loud the speaker was able to produce the tone without distortion or static.

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