Re: Dual sine wave generator with variable frequency and 90 degree phase difference

On Wed, 20 Aug 2008, Steve wrote:

I'm looking for a waveform generator that outputs two sine waves of
the same frequency with 90 degree phase difference (sine and cosine).
I need a variable frequency between 0.05 Hz and 10 Hz. Is there a
design that uses a single potentiometer or perhaps is voltage
controlled ? Low distortion is not a requirement.

Steve


The classic solution is a tri-state filter, with feedback to make it
oscillate.

If you really need one pot to control it, then you use some gain element
as the variable in the filter, and use one pot to control each.

It was all covered well in Don Lancaster's "Active Filter Cookbook" and
for that matter in all kinds of material about electronic music.

Of course, if distortion isn't an issue, especially given the low
frequencies, you might want to generate two signals 90 apart, easily done digitally with a couple of flip-flops, and then use some method to convert it to stepped sine waves. Walking ring counters and summing
resistors on the output were the way decades ago, well covered in
Don Lancaster's "CMOS Cookbook" among other places. It seemed to
be the method used in a lot of ICs. And done right, you can generate
the two 90 degree apart waveforms with one walking ring counter and
suitable summing resistors.

Michael

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