[sdiy] 3 phase sine VCO

[Magnus Danielson]

From: Scott Gravenhorst <music.maker at gte.net>
Subject: [sdiy] 3 phase sine VCO
Date: Tue, 28 Dec 2004 07:46:21 -0700
Message-ID: <200412281546.iBSFkK301647 at linux6.lan>

> A few days ago, someone posted (from EDN, I think) about a 3 phase sine
> wave VCO.  Did any of you super-brain people explain how this works - and
> would it be linear?  (I've had some SDIY list problems, can't receive posts
> on this address, but I can send posts...)  

OK, I haven't seen this one, but here is an idea:

Set up a classical sine/cosine VCO, i.e. biquad/state-variable solution.

Now, set up two summation networks (opamps + resistors) to mix the sine and
cosine in suitable mixture to turn the reference (cosine) +/- 120 degrees.

For +120 degrees phase you want -0.5*cos and  0.866*sin.
For -120 degrees phase you want -0.5*cos and -0.866*sin.

Doing this is trivial and linear. Trimming might be needed if being picky
about phase and amplitude. By making the 0.866 term trimable (it's a numerical
approximation) and by making the gain of the summer op-amp trimable (feedback
resistor may be suitable) can phase be trimmed individual from the amplitude.

The trouble may be the phase-stability between the cosine and sine.

Anyway, this is just applied trigonometry and the linear algebra of complex
vectors.

Cheers,
Magnus