[sdiy] Dial-a-tempco

[Magnus Danielson]

From: "Czech Martin" <Martin.Czech at micronas.com>
Subject: RE: [sdiy] Dial-a-tempco
Date: Wed, 28 May 2003 09:45:01 +0200

> 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...

I recommend reciprocal frequency counters, they will count both the number of
reference clocks during the gate time as well as the number of cycles. The
period time and frequency can both be calculated by division of those two
numbers. Precission counters also use analog interpolators to get additional
time-resolution.

Jitter is a problem, but when averaging over a longer time the effect can be
reduced. Frequency errors due to incorrect reference can be compensated for if
the error is known, but a good oscillator isn't too hard to find.

Cheers,
Magnus - count me in