thru zero VCOs (was: Re: [sdiy] Temperature Compensated VCO attempt - help?

[Magnus Danielson]

From: jhaible at debitel.net
Subject: thru zero VCOs (was: Re: [sdiy] Temperature Compensated VCO attempt - help?
Date: 3 Feb 2003 12:48:56 +0100,Mon,  3 Feb 2003 12:48:55 +0100

Hi Jürgen,

> Speaking of the VCO in my FS-1, this is *not* optimized for fast
> transitions thru DC. It's optimized for precise, but slow transitions;
> using a slow, low-offset opamp.
> (Magnus: even when the actual current switching n the VCO core 
> comes for free, and works fast, you also need a precise and ultra-fast
> circuit that gests the switching signal and a full wave rectified
> signal from your audio rate modulation signal, which isn't trivial.)

Actually, no... I never separate the sign and the absolute value like you do.
I let them all the way into the 4-quadrant multiplier that the Gilbert-cell
supplies. This comes from the observation that a Gilbert-cell actually has
three input signals, the standing current, and the two diffrential inputs.
The standing current of the Gilbert-cell comes from a normal expo-setup where
as the two different input signals each apply it's polarity switching magic on
the current. The beauty of this design decission is that signals requiring less
precission enters the Gilbert-cell on the base inputs (which has the annoying
distorsion) and the really sensitive signal, the expo-current, enters the
common emitter current path for which the other transistors just sets current
ratios for.

What I didn't do however was to describe how a phase quadrature oscillator
is designed. However, with the sparkling intellect you guys have, it should
come as apparent once you've understood my initial idea.

> And while I don't like a BFO concept for a frequency shifter oscillator
> (locking and/or drift issues), it would be an excellent choice for
> audio rate thru-zero FM, wouldn't it?

It would. When I looked at the 1630 schematics and description I came to
realize that it would have been a piece of cake to make it through zero, but
instead the work has been centered on single-side modulation.

Cheers,
Magnus