[sdiy] Temperature Compensated VCO attempt - help?

[Magnus Danielson]

From: Don Tillman <don at till.com>
Subject: Re: [sdiy] Temperature Compensated VCO attempt - help?
Date: Mon, 3 Feb 2003 10:35:07 -0800

Hi Don!

>    > Date: Mon, 03 Feb 2003 10:02:43 +0100 (CET)
>    > From: Magnus Danielson <cfmd at swipnet.se>
>    > 
>    > My bid is:
>    > 
>    > Put a Gilbert-cell on top of an expo-converter. Put the linear
>    > (sign-swapping) modulation on one of the Gilbert-cell input pair,
>    > put the Schmitt-triggered output waveform on the other input pair
>    > of the Gilbert-cell. The current output of the Gilbert-cell is
>    > connected to an integrator and this is followed by a
>    > Schmitt-trigger. The current in the Gilbert-cell is modulated
>    > from the expo-pair. This should be both a compact and pretty
>    > well-behaving oscillator and since there is non abrupt switching
>    > of current but rather linear change in current balance I think
>    > there is a good chance of a nice behaviour.
> 
> Well that's certainly elegant.

Thanks! ;O)

> But I think something's missing (or perhaps I'm missing something).

OK. Let us see...

> Say we have it set to a positive frequency, the integrator has just
> hit the positive reference voltage, the Schmitt trigger changes the
> direction to slew down, and then at that point, if we set the control
> voltage to negative, the integrator slews back up to the reference
> voltage, but the Schmitt trigger is still in the positive state so it
> won't notice to change direction again.

Ah, right. You've spotted exactly where I made a mental note "fix this".
You win a pint of beer next time we're in the same bar! ;O)
(Fat chance... but I did enjoy Antonios Nuthouse back in '96)

What you want is to ensure the sign of the load current and the sign of the
schmitt trigger is correct, or you find yourself another way of dealing with
it. The good point is that we've moved it off to a more "logical" part of the
design. Those things can be solved. I will try a few ideas out with pen and
paper.

> Graceful thru-zero a very difficult problem.

Indeed. The main point with my proposal was to build a new core, and see what
happends when we elaborate on that problem.

Cheers,
Magnus