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

On Sep 3, 6:42 am, Steve <st...@xxxxxxxxxx> 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).

Now that I think of it, there IS an analog solution: the old
CA3080 transconductance amp application notes had the
key item, I think it was ICAN-6668. National Semiconductor
LM13700 data ***, fig. 17, is similar.

<http://www.national.com/ds/LM/LM13700.pdf#page=1>

Basicallly, it's a phase-shift oscillator using four amplifiers
with four matched capacitive loads on the outputs.
One tunes the frequency by modulating the output impedance
of the transconductance amplifiers (all four amps tuned with
a matched variable current). Your adjustment pot feeds
two resistors (the dual amps are matched, or it would be
four resistors) to the four program pins.

Dual OTA amps are still available (LM13700), though not the
'3080' (so sad, RIP). The oscillator with three sections rings at 60
degree phase shift; you want to use four sections, 45 degrees
shift on each, to get sine and cosine outputs. Remember, an
odd number of sections have to be inverting amplifiers!

(sorry about reposting, I muffed the reference)
.

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