[sdiy] quadrature VCO - a different approach

[Magnus Danielson]

From: "JH." <jhaible at debitel.net>
Subject: [sdiy] quadrature VCO - a different approach
Date: Sun, 2 May 2004 10:08:25 +0200
Message-ID: <000901c4301c$c3a75f00$2a70b9d9 at jhsilent>

Hi Jürgen,

> Still working on my FS-2 project (it goes very slow - most of my spare time
> is consumed by that big project of building a house now),

Now, tell us... any neat modules in that project? ;O)

> I wanted to make a better QVCO than the first time. Something that tracks as
> good as a normal VCO, and which has lower THD than the first version.
> 
> I made a real big circuit, 3 times as many components as the first version,
> but I wasn't really satisfied. It's really hard to have exact 90 degree
> phase difference from 1/60 Hz  to 15 kHz, and also keep the triangle
> levels constant (for precise sine conversion with a diode network),
> and also keep dc offset low (to avoid carrier bleedthru in the FS),
> over the whole wide range.
> 
> So I thought about a different approach: Using an ordinary VCO
> and another dome filter. (Like some commercial FS with input for
> an external oscillator). The catch is than now I need a dome filter
> that goes down to 1/60Hz, in order to make slow modulation
> with a period of 1 minute. 
> 
> A dome filter with 2 x 6 stages can create a phase difference
> of 90 deg (+/- 0.3 deg) from 15Hz to 15kHz.
> 
> I made some calculations and found that I only need 2 x 12 stages
> to do the same thing for a range from 15mHz to 30KHz.
> This was less than I had expected, and so I've started to build
> it. It will use no caps larger than 4.7 uF (no electrolytics needed)
> and no resistors larger than 10 MegOhm (no special care for isolation
> needed). The largest time constant is 30 seconds, so the filter should 
> be able to work nicely a few minutes after power-on. With
> careful component selection, I expect a phase error < 0.5 deg
> in the audio range, and < 2 deg in the LFO range (where you
> don't hear the carrier anyway).

Have you looked at the Polyphase way of doing things? There is both ideas for
the phase-difference-filter as well as for direct oscillators.

Maybe you are going for a traditional dome-filter instead.

> I've only started to build this - I'll let you know about the progress.
> Schematics will be published when I know that it really works.

Eager to see it!

Cheers,
Magnus