[sdiy] Difficult 2164 behaviour between different sections.

[Tom Bugs]

Hi,

I've been farting around with a big design for 18 months already at 
least (!!) and turned up some behaviour in the 2164s that I've not 
encountered before, so wondered if anyone else on the list has such 
experiences or other comments - or posting as a potential warning..

The design uses two 2164s, so 8 VCAs used as follows:
1 for Tri-Core VCO plus one for temperature compensation
2 for SV-Filter plus one for temp comp
1 for VCA plus one for linearization
1 for VC-decay
Most of the circuit approaches are pretty standard and ones which I have 
used many times before.

Having made what I thought was the final prototype and overall enjoying 
the results, I then noticed that the adjusting the VCA and VC-Decay 
settings were slightly altering the VCO frequency by about 10cents each 
- argh!
In that version the TriCore + Temp Comp were on the same 2164 as the 
VC-Decay and the VCA (not the linearization section of it).

I did another prototype sharing the 2164 sections differently - so the 
VCO sections were on the chip with the SVF core (not the temp comp for it).
It solved the original problem - but then shifted it elsewhere! Now 
adjusting the VCF cutoff would affect the VCO freq (by 40cents)

Voltage refs are stable, power lines are solid and bypassed, overall 
layout approaches are, I think, good - so I can only now put it down to 
interaction between 2164 sections.
I have identified that the output of the VCO Temp-Compensation section 
is varying as VCF cutoff is adjusted (again, without the voltage sources 
or the temp-comp voltage varying).

One thing I have not massively investigated is the biasing of the 2164 - 
there is the graph on the datasheet suggesting that minimum CV 
bleedthrough should be achieved with something like a 560k resistor. 
Tried a bit, but without success.

I've already drawn out a new prototype with different temp-compensation 
approaches, but I wondered whether anyone has any smart ideas on 
something I may have missed.

Best! Tom

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