HFT

[Magnus Danielson]

From: "Ian Fritz" <ijfritz at earthlink.net>
Subject: Re: HFT
Date: Thu, 30 Dec 1999 11:11:37 -0700

> Hi Folks --
> 
> Just to make sure this is clear, the diode plus resistor scheme has been
> around for many years. I picked it up from Hal Chamberlin's book, but it has
> appeared several other places alse. I can't find the original reference, so
> I'm not sure who did it originally. Franco? Hemseth? Maybe someone has a
> reference? Its purpose is to compensate for the emitter bulk resistance of
> the converter transistor, and the detailed analysis shows that it does so
> exactly, provided that the temperature dependence of the diode and the
> transistor base-emitter junction are the same. I recommend not leaving the
> diode out.

But this is why we tend to use matched transistors with tigth temperature
connectivity, to remove bulk resistance errors out of the exponential
properties. But then, there are surely many ways to acheive the same effect.

> At one time I experimented with different tracking schemes for the SAW VCO
> and got the best results combining both the Rbulk correction and the reset
> time correction (small resistor in series with the timing cap).  You can see
> that the design on my site has both!

Ah. We are still talking about two diffrent issues then.

I thougth someone had gone of and invented some way of adjusting the reset
timing error by sligth reengineering of the expo stage. While I am sure that
approximations can be acheived I'd still have to be convinced (I'd love to see
such a thing!) that you can do it and acheive the same wide frequency
correction while keeping on scale as the resistor trick does. Fixing the
expo stage does have the advantage of not breaking the waveform shape and its
DC offset and this is improtant stuff for cases where you use your sawtooths
trough waveshapers to acheive good canceling etc.

Cheers,
Magnus