# Basic AM modulator circuit?

I am having trouble finding a good resource for understanding the design of a very basic AM modulator circuit.

Specifically, I am using a simulation tool which provides an AM wave, which consists signal frequency and a carrier frequency. It produces a nice AM looking signal.

I am trying to create that signal from more basic components, but I cannot find any working schematics.

I am using a handy simulator found here: http://www.falstad.com/circuit/circuitjs.html

In my case, I am using a 40Hz signal on a 300Hz carrier. The AM element is configured the same way.

My 300Hz element is AC, -2.5V <-> 2.5V. My 40Hz element has a base voltage, so it goes between 0 -> 5V.

I'm pretty sure I'm missing a lot, but can someone help me fill in the blanks? One common way of doing AM modulation is to multiplying a baseband signal with a square wave that is at the carrier frequency, and then run the chopped baseband signal through a band-pass-filter tuned to the carrier frequency.

The reason this is done is that multiplying by a square wave is pretty easy to do with as little as one high frequency switching transistor.

If the signal plus DC bias is connected through a resistor to the source of a FET, and a square wave at the carrier frequency is connected to the gate, then you will get multiplication by a square wave. When the FET is off the output signal is equal to the input (multiplied by 1), when the FET is on the output is 0 (multiplied by 0).

A square wave at the carrier frequency can be decomposed into an infinite series of sinewaves that are multiples of the carrier frequency. So if we multiply the input by a square wave at the carrier then the output is equal to the input multiplied by a sine-wave at the carrier (which is exactly what we want for AM), plus the input multiplied by sine-waves at the harmonics of the carrier (which is something we don't want).

If the output of the chopper is run through a band pass filter which passes only the signals near the carrier, then the signals at the harmonics are rejected and we are left with only what we want (the AM modulated signal).

See...
http://homepages.udayton.edu/~hardierc/ECE401/ECE401L/ECE_401L_Lab2.pdf

http://www.analog.com/library/analogDialogue/archives/43-09/EDCh%204%20rf%20if.pdf

Make two changes.

(1) AC couple the output, i.e. insert a capacitor between the drain and your 200K load. As you want to suppress the modulating signal and pass the carrier, pick a frequency between 40 and 300Hz - say, 120Hz.

RC = 1/(2*Pi*f) where R = 200 kilohm and F = 120Hz. This will give you a value for C.

Now the modulated waveform (across the 200K resistor) should be more or less symmetrical about 0V.

(2) Noting that the negative half cycles are almost completely cut off, add a small positive DC term to your modulating (40Hz) waveform. Some experimentation will be required, but start with about 0.5* the modulating amplitude, and adjust for best output symmetry. By inspection I think that'll be at about 0.7* the modulating amplitude but that will depend on the FET's characteristics (which, in production, will vary from FET to FET).

Your simulation shows it is working -- although there is a lot of distortion in the AM output.

To get better than that you actually need a multiplier function of some type -- it multiplies two signals together. In general, those circuits are not trivial.

• Isn't that what the FET does? – jdizzle May 22 '16 at 1:14
• Not unless you operate it in the linear region. Or one might say, sort of, but quite crudely. And of course even with a linear modulator you still have to avoid overmodulation. – Chris Stratton May 22 '16 at 1:29