[sdiy] Active VCO temp compensation

[George Hearn]

Yes Jims design is very clever!  I won't be performing the compensation
locally in each oscillator though, the temp sense voltages from each
oscillator would be multiplexed into the DAC reference voltage to provide
the scaling.  Tempco resistors simply aren't accurate enough and not being
on the transistor die they are not instant enough either, and heating up the
expo pair is a bit crude and creates a problem of extended warm up time
after power on.  I think sensing the -Vbe point of the exponential converter
pair was a good idea put forward.  If this were inverted and amplified this
would have exactly the right Vt dependence however this voltage may not be
clean enough and may vary slightly as the input to the exponential converter
(base voltage) is modulated and the opamp tries to compensate, I would have
to see.  Better I think would be to use a spare transistor on the same die
of the expo pair, though I'm not quite sure which way to go with this.
Perhaps just a constant current through a diode connected transistor would
give me the desired vbe drop proportional to Vt? Vbe=vt*log (Id/Is).
However, doesn't 'Is' have a temperature dependence too?  

-----Original Message-----
From: karl dalen [mailto:dalenkarl at yahoo.se] 
Sent: 21 December 2008 23:56
To: synth-diy at dropmix.xs4all.nl; George Hearn
Subject: SV: [sdiy] Active VCO temp compensation

Hi George!

The CEM 3374 concept, yes, Xpander, Polaris and OBMx etc.

Mat04 are waste of money in analog polys, all
scaling should be done by the MCU, the simplest
would be to sense the -Vbe point of the expo
then create a scaler routine that takes that
voltage wich are directly temp dependant of the
expo into a tuning offset, lets say at the rate
pitch/temp change. However that would ruin the
Lin FM.
Or just sense one of the trannys in the LM3046
and maintain the Lin FM possibility!
Or place a Tempco on each array and sense that!
Or use Rene S multiplier solution, i.e CEm3340,
but that eats two arrays, one for each VCO!
Or just heat upp the expos!

What about the questions i had for you earlier on George? ;-)

Reg
KD


--- Den sön 2008-12-21 skrev George Hearn <georgehearn at btinternet.com>:

> Från: George Hearn <georgehearn at btinternet.com>
> Ämne: [sdiy] Active VCO temp compensation
> Till: synth-diy at dropmix.xs4all.nl
> Datum: söndag 21 december 2008 19.37
> I'd like to modify my polysynth design to use active
> temperature
> compensation by making the VCO's output a temperature
> reference voltage that
> has a TC of 3300ppm/c.  This would then be used to derive
> the DAC reference
> voltage in order to compensate for the VCO exponential
> converter temperature
> drift.  I would like to do this by using a MAT04 quad
> matched pair or
> LM3046, using two transistors to make the exponential
> converter and somehow
> use the others to generate a voltage proportional to Vt. 
> Does anyone have
> any ideas?  The CEM3374 had this arrangement and it was
> successfully
> employed in the Oberheim XPander and Matrix 12.  Regards,
> George
> 
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> Synth-diy at dropmix.xs4all.nl
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