[time-nuts] ACAM GP22 Chip

Attila Kinali attila at kinali.ch
Wed Nov 25 18:36:10 EST 2015


On Wed, 25 Nov 2015 14:19:37 -0000
"Thomas Allgeier" <th.allgeier at gmail.com> wrote:

> You guessed my origin right from my name.
> It is not that I don't trust ACAM (we are one of their customers) but from 
> the source I am measuring I get a lot of what you guys probably call jitter. 
> So I want to be sure the jitter comes from the oscillator I am measuring and 
> not from the GP22.

Ah, so it's about verification of the results you already got.
Then I would rent some TIC for a day or two from one of the many shops
that rent out measurement instruments. That's probably cheaper (especially
if you include your work time) and you don't have to guestimate the noise
that your reference circuit makes, but get the noise floor values from
the instrument manual.

> Absolute accuracy is not the problem here, unless the 32768 Hz clock on the 
> eval board actually drifts. In other words if we are 10 ns out on the period 
> measurement nobody cares if it remains the same 10 ns all the time. As in 
> many applications in the weighing industry what really matters is the change 
> in weight, not how heavy the mass is in absolute terms.

I don't really see how a 32kHz crystal is used in a scale, but there are
many way to do things :-)

> It sounds like the coil must be wound what we used to call "bifilar" in my 
> school days, i.e. self-cancelling the inductivity. That should be doable, 
> and on the other stuff I will read up a bit first.

Again, please do not think of a coax in terms of wires. It's a transmission
line. 99% of the wave energy is between the inner and the outer conductor.
There is (almost) no magnetic field that you will be able to cancel by
building a bifilar coax spool. How your signal looks like depends on the
intrinsic properties of the cable. There is very little dependence of the
electrical parameters on the way how you arange the cable, whether you use
a straight line or roll it up.

On Wed, 25 Nov 2015 14:23:40 -0000
"Thomas Allgeier" <th.allgeier at gmail.com> wrote:

> I will report back any outcome that looks plausible / presentable.

Please also report back when you have negative results. It's almost
always more instructive to see what went wrong and figure out why.

				Attila Kinali
Reading can seriously damage your ignorance.
		-- unknown

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