Tempco measurements on ASM-1

[Joachim Verghese]

Hi,

> Without compensation, the error is quite small att the high
> frequency range. At 7V CV (4186 Hz) it is 19 cents sharp. But with
> lower CVs the error increases linearly. At 0V CV (32,7 Hz) the error is
> a hefty 300 cent.

>From what I recall, this is because of how the ASM-1 expo
converter is dimensioned. The scaling drift will be virtually
zero when the collector currents of the two transistors are
equal, i.e. 15/150k = 100 uA. This yields an oscillation
frequency of approx. 9 kHz, which is pretty high, IMO. The
further you move from this frequency, the greater the error
without compensation, as your measurements proved.

If you were to use this VCO without compensation, you might
want to experiment with higher values for R15. For example,
1.5Mohms would shift the stable "spot" to around 900 Hz, which
is closer to the mid-point of the frequency range of a VCO
in "normal" use.

(ASM-1 VCO schematics at:
http://www.it.kth.se/~e93_mda/synths/friends/stopp/asm1vco.new.pdf
)

Just a thought.

Thanks for posting the results, BTW.

-joachim