Lab 4: AM Detectors
Lab 4 was designed to illustrate how an intelligence signal can be extracted from an AM signal and compare some different AM detector circuits, including a basic diode detector, biased diode detector, and complimentary feedback pair detector.
Part 1 of section 4.5 was to construct a basic diode detector, consisting of just a 1N4148 diode, resistor and capacitor to provide the time constant, and DC blocking capacitors on the input and output. The completed circuit is shown below in Figure 1.
Figure 1: Basic diode detector
Parts 2 through 4 involve feeding the detector with a 200kHz, 50% modulation signal at increasing amplitude to see how the detector circuit behaves. At a low signal amplitude, 100mVpp, nothing is detected on the output of the circuit. This is shown in Figure 2 below.
Figure 2: Input (ch 1) and output (ch 2) signals from the diode detector with a 100mVpp input amplitude
As the input signal level is increased, the output begins to resemble the intelligence signal. This is shown in Figures 3 and 4. The best signal was found with an input amplitude of approximately 620mVpp.
Figure 3: Input (ch 1) and output (ch 2) signals from the diode detector with a 375mVpp input amplitude
Figure 4: Input (ch 1) and output (ch 2) signals from the diode detector with a 618mVpp input amplitude
Part 5 exchanges the 1kΩ resistor in the detector circuit for a 10kΩ resistor. This caused the waveform to change from that in Figure 4 to the one shown below in Figure 5. Because the time constant is higher, the circuit does not discharge fully between pulses.
Figure 5: Input (ch 1) and output (ch 2) signals from the diode detector with a 10kΩ detector resistor
Part 6 increases the resistance again to 100kΩ. This caused the waveform to change to the one shown below in Figure 6. Because the time constant is even higher, the circuit discharges even less between pulses, smoothing out the signal considerably.
Figure 6: Input (ch 1) and output (ch 2) signals from the diode detector with a 100kΩ detector resistor
Part 7 modifies the diode detector into a biased diode detector by using a voltage divider to provide a DC offset to the input signal. This allows the detector to work at much lower input levels than the basic diode detector. The input and output waveforms for this detector are shown below in Figure 7. This detector offers a clear and well-reproduced output signal even with a 300mVpp input signal.
Figure 7: Input (ch 1) and output (ch 2) signals from the biased diode detector
Section 4.6 analyzes the detector circuits further using the oscilloscope's spectrum analyzer functionality. The biased diode detector was driven with a 1230kHz carrier signal with 200mVpp amplitude and 50% modulation. The input and output of the detector was measured and the markers were used to find the frequency and amplitude of the carrier and sidebands. Figure 8 below shows the input of the detector and Figure 9 shows the output.
Figure 8: Carrier and lower sideband of the input signal
Figure 9: Carrier and lower sideband of the output signal
The output spectrum has a lower amplitude overall due to a significant component of the signal being filtered out. The sidebands also represent a greater proportion of the overall signal in the output. This is due to much of the carrier frequency being filtered out.
In sections 4.7 and 4.8 the detector circuit is added to the amplifier stage built in previous labs. The circuit that results from this is shown below in Figure 10. After assembling the full circuit an AM signal was fed to the detector and the output was sent through the amplifiers to the speaker. The audio output waveform is shown in Figure 11, and the sound quality I observed was quite good.
Figure 10: Amplifier stages with biased diode detector as the input
Figure 11: AM signal input and audio output of the circuit in Figure 10