[sdiy] Dial-a-tempco

[Czech Martin]

It's not entirely clear what you mean, do you have a schematic
of said circuit?

And: why is your frequency measurement error +- 2Hz?
I thought that with a period measurement much more acuracy
is a very simple thing... Or is there some jitter problem?
Let me see...
The simplest counter ICs (Intersil) run with 10MHz, giving theoretical
100ns resolution in period mode.
A 8000Hz gives 125us, 100ns is 800ppm of that.
The counters can be set so that 10 or 100 periods are measured.
This will then give even better figures...


Of course, the quartz could be a bit out of tune, with a bad quartz
perhaps several 100ppm. But that is only interesting for absolute
frequencies, and this is not interesting for tracking questions...

m.c.

-----Original Message-----
From: Ian Fritz [mailto:ijfritz at earthlink.net]
Sent: Mittwoch, 28. Mai 2003 04:13
To: synth-diy at dropmix.xs4all.nl
Subject: [sdiy] Dial-a-tempco


Hi all --

I had a chance this week to build and test the tempco-resistor corrector 
idea I posted a few weeks ago.

The basic idea is simple.  A perfect tempco resistor has a PTAT response, 
i.e., R = AT, where A is any constant and T is Kelvin temperature.  This 
gives a tempco of d(Ln R)/dT = 1/T = 3350 ppm/K at 25 deg C.

What if the 25 C coefficient is something other than 3350 ppm/K?  Well, 
it's a fundamental property of metals that their resistance is very nearly 
linear with temperature around ambient temperatures, so the tempco 
deviation has to be understood as a finite intercept at zero temperature, 
i.e., R = AT + B, where B is a constant.  So to turn an imperfect tempco 
into a perfect one just requires a simple circuit to cancel the B term.

I made a converter using this idea with a trimmer to zero out B, using two 
OPA2227 opamps and a CA3083 transistor array.  I separated the converter 
from the oscillator core so I could heat it alone and characterize just its 
own drift.  Since I had already measured the bare tempco's response to be 
about 3270 ppm, I knew in advance approximately how much correction to 
apply.  Starting from that value I was easily able to dial in the proper 
compensation.  Testing the octave from 4 kHz to 8 kHz from 25 C to 45 C I 
saw no observable frequency-ratio change.  So the tracking stays constant 
to the error of the frequency counter (call it 2 Hz) over a 20 C 
range.  This makes the scale-factor drift under 25 ppm/K, or about 1% of 
the uncompensated drift (2330 ppm/K in frequency).

For a sanity check, I figured out what value of B corresponded to the 
(measured) compensation voltage and got 50 Ohms.  This agrees pretty well 
with the value of B = 35 Ohms deduced from the direct tempco measurement.

There was also some linear frequency drift, which could be caused by offset 
voltages, resistor/power supply drift etc.  This, along with the drift of 
the core requires separate compensation.

As a final observation, I would note that the converter is reasonably 
quiet, in contrast to the active compensation schemes I have tried, which 
have extra noise because low-level circuitry must be used for the multipliers.

   Ian