Heterodyning a PWM Sine Wave

[Raven Luni]

Greetings,

Following the discussion in the microcontroller section, I have an idea for producing higher frequency sine waves without sacrificing resolution:

As you know, modulation results in the sum and difference of the original frequencies, so if a sine wave is modulated with itself (after sending it through a pair of independent buffers), the result will be twice the frequency plus a DC offset (0 hertz) which can be filtered with just a capacitor.

Is there any reason why this cannot be done with multiple stages to achieve a high frequency?

 

[Harald Kapp]

In theory this is right.
Do you have a good multiplier/modulator at hand that has minimal distortions? Distortions will increase with the number of modulator stages.
I think that may be the reason why this approach is not commonly used.

 

[Raven Luni]

Theres that. I suppose a combination of a decent op amp and filtering of each stage would have to do. I can see how the circuit complexity would start to overtake that of a good analog generator, but if the original goal is to have precise control of the frequency, and the maximum stages would be no more than 3 or 4, then maybe its still worth it?

 

[Harald Kapp]

It all depends on what frequencies you are talking about. Today its easy to build a DDS (direct digital synthesis) generator for frequencies up to 400MHz using just one single chip. 400Mhz is a frequency where most OpAmps have gone flat long before.