ARP VCO (was:More on Fav Designs)

[Haible Juergen]

Hi Joachim (and DIY list) !

>Actually, there are components which aren't considered to be part of
>a VCO design at all, but which can affect stability, such as calibration
>trimmers. A rheostat (variable resistor) -connected trimmer will drift
>with temperature, while a potentiometer (variable potential divider)
>will not, provided that the end-points are connected to low-impedance
>sources (such as pwr supply lines or op-amp outputs).
>
>[ARP used the latter approach for both tuning offset *and* scaling
>trims, and came up with a simple stable VCO design in 1974 which they
>never needed to change.]

You're right !! I didn't remember having seen something like this in my
Odyssey docs, so I looked again this weekend: They changed the usual
rheostat scaling to potentiometer scaling on the later Odyssey revisions
indeed!
I am still a bit in doubt that it does make such a differenc, nevertheless.
Mostly the scaling is done by braking a resistor into 90% fixed value and
10% rheostat trimpoti, so the tempco of the whole thing would still be 
dominated by fixed resistor. But the potentiometer approach is surely the 
better way.

Now, as I had the Odyssey docs in front of me, something else caught
my attraction: The feedback path from the reset pulse over a low pass filter
back to the expo converter. Now what is this ?!
Looks like some strange HF-tracking circuit. Just  wondering why they have
done it *that* way ... The lp corner is at several hundred hertz, so it would
even change the waveform at low frequencies, wouldn't it?
Does this circuit have any advantages compared with other HFT schemes?

>In some designs MOSFETs cause serious overshoots and glitches,
>because of their capacitive nature (insulated gate). JFETs have
>higher on-resistance, but are better for precision waveform
>generation, IMO.

Very interesing point! I have never thought about that.

JH.
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