9.7.2021 -- The purpose of this lab is to build an AM detector circuit, which will read the intelligence signal of an AM signal. We built two different AM detector circuits for this lab: the biased diode detector and the complementary feedback pair circuit.
4.1 Simple Diode Detector
First, we built a simple diode detector in LTSpice to see the effect of the capactior on the circuit. The diode restricts voltage to the resistor when the input voltage is negative, so only half of the input voltage wave is recorded. See Figure 4.1 for the circuit graphed input and output voltage. Notice that the output wave only starts getting measured when the input wave is at least 0.7 V.
Figure 4.1 Simple diode circuit - half wave rectifier
Now a capacitor is added in parallel with the load resistor. The capacitor begins charging when the input voltage is positive and then discharges while the input voltage is decreasing and negative. The RC time constant determines how fast the capacitor discharges. In Figures 4.2 and 4.3, the load resistance was varied, and from that we can conclude that the RC time constant increases when the resistance increases (and the same applies to the capacitor value). The higher the time constant, the flatter the output voltage wave will appear.
Figure 4.2 Load resistor = 1 kΩ, RC(RC time constant) = (1kΩ)(0.1µF) = 0.1 ms
Figure 4.2 Load resistor = 10 kΩ, RC(RC time constant) = (1kΩ)(0.1µF) = 10 ms
4.1a LTSpice Simulation
See rectifier circuit in Figure 4.1 and Figure 4.2 for transient simulation results. I was able to produce both waveforms in the LTSpice simulations from the two rectifier circuits.
See Figure 4.3 for AM detector results.
See Figure 4.4 and 4.5 for transient simulation results with different R1 resistances. When R1 = 100 Ω, the intelligence signal that is read from the carrier signal is shaped like the output signal from a half wave rectifier. This is shown in Figure 4.4. It doesn’t quite follow the intelligence signal completely. The trace for this signal gets thicker when the Vout signal starts increasing. When R1 = 1kΩ (shown in Figure 4.5), the read intelligence signal looks more like the output signal in Figure 4.2. Since Figure 4.2 and Figure 4.5 have similar output signals, the capacitor C2 (C1 in lab manual) must be charging with positive voltage and then discharging with the negative voltage.
The output signal becomes a much flatter version of the wave in Figure 4.5 when all the signal voltages are divided by 2. It still does not completely follow the intelligence signal on the carrier wave. See Figure 4.6 below for the simulation with the halved input voltages.
Figure 4.3 Simple Rectifier - AM Detector
Figure 4.4 Carrier signal and Intelligence signal when R1 = 100 Ω
Figure 4.5 Carrier signal and Intelligence signal when R1 = 10 kΩ
Figure 4.6 Halved input voltages
4.2 Biased Diode Detector
The simple diode detector from Section 4.1 works well, but its abilities are somewhat limited in detecting the intelligence. For the simple circuit to work, there must be enough voltage for the diode to turn on. A very weak signal will cause the diode to not turn on, and a signal that is too strong will distort the output signal. The biased diode detector will help solve these problems.
4.2a LTSpice Simulation
See Figure 4.7 for AM detector results.
It appears that the amplitude of the Vout signal decreases as the input voltages decreases. The output wave still follows the intelligence wave decently but as well as it could be. See Figure 4.8 for the transient simulation results.
The theory in the previous question still holds true. We can conclude that as the input voltage decreases, the less the Vout wave will produce the desired intelligence wave from the carrier wave. See Figure 4.9 for the transient simulation results.
Figure 4.7 LTSpice simulation of biased detector circuit
Figure 4.8 LTSpice simulation of biased detector circuit with halved input voltages
Figure 4.9 LTSpice simulation of biased detector circuit with original input voltages divided by 10
4.3 Complementary Feedback Pair (CFP) Detector
The CFP detector is able to detect the intelligence signal from weaker signals, but it's prone to noise interference. The CFP configuration consists of npn and pnp transistors arranged so that at least one of the transistors is always on and the emitter capacitor is always charging. See Figure 4.10 for the CFP circuit.
Figure 4.10 Complementary Feedback Pair (CFP) circuit
4.3a LTSpice Simulation
See Figure 4.11 for simulation results.
See Figure 4.12 for simulation results.
See Figure 4.13 for simulation results. Rc1 is needed for there to be a current across the collector terminal of the NPN transistor and the base terminal of the PNP transistor. This way, the capacitor Ce1 will continue to charge and discharge and display a good intelligence signal on oscilloscope graph. Rc1 provides additional current to charge Ce1 quickly.
Figure 4.11 common collector-based detector – input and output waves
Figure 4.12 common collector-based detector with reduced input signals
Figure 4.13 Output for CFP Detector for a weak input signal
4.4 Guided Design: Maximum Detector Range (bonus)
In the prelab, the amplitude range of the carrier frequency wave was about 8 mVpeak to 160 mVpeak. The diode AM detector circuits had a hard time picking up low voltage signals. It also did not have the best reading of the intelligence wave. However, the complementary feedback pair (CFP) detector was very good at picking up carrier wave signals at 8 mVpeak. In the simulation, the output wave followed the intelligence wave of the carrier signal very well. Although the CFP detector is very good at detecting low voltage signals, it is prone to picking up noise interference.
For now, the diode detector is probably the best choice for now to detect AM signals. I’ll have to look for a way to reduce the noise interference if I want to use the CFP detector.
I’m thinking about using the two-stage op amp and class AB amplifier for my radio because there is a good gain for this circuit. That way if a low voltage signal is detected, then the input signal will be amplified well. Below are my simulation results from Lab 3: Audio Amplifier for the op amp and class AB amplifier.
Figure 4.14 Transient simulation of two stage amplifier – op amp and class AB amplifier circuit
4.5 Constructing AM Detectors
Construct the circuit in Figure 4.5.
Input AM signal: fc = 200 kHz, fi = 1 kHz, 50% modulation, carrier amplitude = ~100 mVpp.
Observing the input and output on the oscilloscope
Although the signal output signal in Figure 4.15 is very fuzzy, the simple diode circuit is detecting the intelligence signal of the input AM wave. It looks like the simulated results from Figure 4.3, so this part of the experiment was a success! It appears that the output only measures the positive side of the intelligence signal.
When R1 = 10kΩ from the circuit in Figure 4.3, the capacitor appears to charge and discharge like the signal in Figure 4.5. There is a distinct bump for where the capacitor charges and a downward slope for when the the capacitor discharges.
When R1 = 100kΩ from the simple diode detector circuit, the output wave becomes much flatter than the previous signal.
The biased diode output did not look like what I expected. It looked more like the the results of the simple diode detector rather than the simulation results in Figure 4.7.
I was not able to get my CFP design to work properly. In fact, my CFP circuit kept detecting an AM signal.
Figure 4.14 Measured result from simple diode detector circuit - R1 = 1kΩ
Figure 4.15 Measured result from simple diode detector circuit - R1 = 100 kΩ
Figure 4.16 Measured result from biased diode detector circuit
Figure 4.17 Measured output (in blue) of the CFP circuit
4.6 Bonus: Spectrum Analyzer (skipped)
4.7 Add AM Detector to Your Radio
I decided to use the AM radio in the schematic below because my original circuit idea didn't have as good of a gain as the one from the lab manual. This AM radio consists of the biased diode circuit (to read the intelligence signal from the input AM signal) and athe CE-CC audio amplifier (to amplifiy the intelligence signal from the AM detector).
Figure 4.18 AM radio circuit
4.8 Adding the AM Detector to the Audio Amplifier
Notice that the capacitor Cbig is present to cut down on the noise interference. The input is an AM signal at 1230 kHz for the carrier frequency, 1 kHz for the intelligence frequency, and 50 % modulation. There was a clean sound coming from the speaker with little noise interference.
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