[sdiy] 3 phase sine VCO

[Scott Gravenhorst]

Magnus Danielson <cfmd at bredband.net> wrote:
>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:

It was Tim Parkhurst who sent this URL:

http://www.edn.com/article/CA149120.html?spacedesc=designideas

It's a VCO made of 3 inverters, each inverter output supplies one of three phases.

>
>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
>
>--0-593696390-1104251567=:66497--
>

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