[sdiy] expo accuracy? or integrator accuracy?, or both?

[Czech Martin]

An expo vco has at least two components:
expo current converter and integrator.
The expo converter is always pretty much the same
structure, the integrator can come as analog computer
integrator (feedback) or simple capacitor with buffer.

There are some published results from such vcos
(EN as well as WWW). The problem is that they
treated both components together (aka vco), most of
them stated that they couldn't really understand
why the vco was going sharp or flat or strange in that
particular way, it remained open.

A few mails ago there was a thread about what transistor
pair should be used. 
There is much writing about expo converters, tempcos etc.
I will not bore you to death with that.
But what about the integrator?
How can one say that this or the other transistor pair
is sufficient if the error source can not be distinguished?

The integrator can have the following problems:
-capacitor dielectric absorption
 (this is ugly, because it is different for different
  circuits and frequencies, so component manufacturers
  can not spec it. It is what I understand least.
  Polystyrene, polypropylene 
  tend to have low absorption, I have seen that by experiment,
  but different brands
  are different, I thought that mica was good, but
  Bob Pease published that it isn't)
  Anyway, it can make the integration slower, by 0.1% - 1%.
-limited gain of amplifier
 (this is easier to understand, the inverting integrator
 circuit will get slower if gain goes down. Gain can go
 down with heat, lower supply, 
-offset voltage (fortunately no issue)
-amplifier input bias current (easy to understand,
 steals current, will get worse with temperature,
 leads to shift of some Hz down, parts with less then
 30pA are expensive and sometimes slow)
-dominant pole of amplifier (most compensated amplifiers
 will have a pole at ~ 10Hz, with -20dB per decade, substantial!)
-output resistance of amplifier (inverting integrator)
-other sources of leakage (reset, sync)

I do not know yet, but I feel that the total integrator
error can be substantial and it could be that it can 
be of the same magnitude than expo source error.
I do not need to mention that said effects introduce
also temperature dependency into the system, this is certainly
important for leakage and absorption.

Since some of the effects are not really observable or 
can not be manipulated in experiment, I thought it would
help to set up a simulation model. I know that (non)linear 
differential equation solvers of the SPICE type have problems,
but very soon we have third order (amplifier with single pole,
integration cap with soakage cap), this is very cumbersome
to solve with analytic methods.


What's up with Martin Czech, is he ill to ask such questions?
No, he's just curious. And he wants to measure the performance
of the japanese dual transistors (finally) with a current
integrator and he wants to know how much error the measurement
instrument will possible have. If it's about 1% it will be useless.

I apologize that I torture your mind with such a long mail.

m.c.