“The Electronics Of Radio” NorCal 40B Transceiver Build Lab Notes: Problem 4 (Also An Interesting Way To Simulate Amplitude Modulation In LTspice)
This is a continuation of my solutions to the problems contained within “The Electronics of Radio” text by David B. Rutledge. These posts are probably only useful if you also are following along with the book and building the transceiver.
[Update: The links to all problem solutions as I go through them will be posted here.]
[Refer to this post for a how-to on building a breadboard amplitude modulation – demodulation circuit.]
The following is a way to simulate amplitude modulation in LTspice. Specifically I followed this video to learn this.
First place two voltage sources. Ground the negative aspects. Label the positive nets. In this case it is the 0.5V 1000KHz carrier signal and the 5V 1KHz message signal.
Add an “arbitrary behavioral voltage source” which is listed simply as “bv” in the components menu. This will be the mixer and you can multiply the carrier signal by the message signal as shown below (when you right click on the component). Under value, input the function: V=V(Carrier)*V(Message).
When you run the simulation, you can see the carrier (blue) and message (green) signal waveforms.
And below the amplitude modulated resultant waveform (red) superimposed on the carrier (blue) and message (green) signals.
Attach the positive terminal of B2 to the demodulator circuit composed of the 1N4148 switching/rectifier diode and the RC low pass filter. Optionally, label the output of the demodulator stage “Demodulated_Signal”. Rerun the simulation. I have changed the output colors: now the modulated signal is gray and the demodulated signal is pink.
And a closer up view of the modulated (gray) and demodulated signals. Note that the demodulated signal has the same frequency as the modulated signal. The book calls the demodulation circuit the “detector circuit” and I think incorrectly states that the its output should be a “1-kHz sine wave like the modulating waveform”. Keep in mind, I have not bothered to add a DC offset to the modulating wave, and therefore this is not a complete picture of amplitude modulation and demodulation, but it is an interesting start to understanding all of this.
Now to the actual parts of Problem 4.
Note, when considering part B, the
C. Below is the effect of reducing the carrier frequency from 1MHz to 100KHz on the demodulated signal waveform (blue). I have also added a 5V DC offset on the message signal (green). Unfortunately, I have no easy way of changing the modulation depth, so all of my circuits are at 100% modulation depth. The problem calls for 70% modulation. I really think I need to upgrade my signal generator.
D. 100% modulation results in the peaks of the modulated signal product reaching 0V and the peaks doubling the carrier wave voltage.
That is all I have to say for Problem 4. It would probably be worth redoing this section once I acquire a signal generator with the ability to amplitude modulate a signal.
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