[time-nuts] ACAM GP22 Chip

Attila Kinali attila at kinali.ch
Tue Nov 24 11:56:34 EST 2015


On Tue, 24 Nov 2015 14:04:32 -0000
"Thomas Allgeier" <th.allgeier at gmail.com> wrote:

> We want to use this chip to measure the period of a square wave, of
> around 13 kHz i.e. in the 70 µs range. As the application is potentially
> high-accuracy we need to know the period to within 1 ns or better.

That's some modest requirement and should be doable with the GPS22
quite easily (or any other TDC for that matter). BTW: when specing
something like this, please make sure to mention whether 1ns is
1sigma, 3sigma or worst case/peak-to-peak. These 3 are quite different

May I ask why you want to verify the specs of the GP22?
The specs say that it does something between 39ps and 70ps (1sigma),
which is probably way better than what you need. And as Acam is
a german company, I expect the datasheet to be accurate.

BTW: 1ns over 70us is approximately 14ppm. The GP22 uses the attached
crystal for absolute calibration. Please be aware that 14ppm will
require at least a TCXO to reach that level over the whole temperature
range, and depending on what TCXO you use, you might need to calibrate
the TCXO post-production and again after a couple of years of use.
Even if you don't need calibrate, I would add a TCXO frequency measurement
to the production test.

BTW2: the "we have a x ppm TCXO" value is usually misleading,
as that's the best-case, pre-soldering, pre-aging, pre-anything value.
The end-value can be 3 times as large... easily.
(unless you happen to choose one of the more honest manufacturers,
for example, like Abracon)

> In order to evaluate the chip I was planning to replicate John A’s
> experiment with the coaxial delay line from the HP5370b 

For those wondering: "John A" is John Ackermann and the experiment
in question is documented at http://www.febo.com/pages/hp5370b/

> – but as my interest
> is in “measuring range 2” of the GP22 I need a delay of 500 ns or more
> (actually 1 µs sounds a better start). This is the equivalent of a 200 m
> length of cable. I fear trouble with this: Am I not getting unwanted
> inductivities if I use a coil of that size?

The coax is a transmission line. Yes you have inductance and capacitance,
but it does not make that much sense to talk about that anymore,
the impedance is the right thing to talk about.
Your output will not be as sharp as your input due to dispersion,
but that can be easily recovered using some buffer gate.

Please make sure that your coil is reasonably temperature stabilized
and, if it's cheaper cable, also humidity stabilized, as both parameters
change your delay. (putting it into an isolated box should be good
enough for this kind of measurment).

			Attila Kinali

It is upon moral qualities that a society is ultimately founded. All 
the prosperity and technological sophistication in the world is of no 
use without that foundation.
                 -- Miss Matheson, The Diamond Age, Neil Stephenson

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