[time-nuts] ACAM GP22 Chip

Thomas Allgeier th.allgeier at gmail.com
Wed Nov 25 09:19:37 EST 2015

Moin Attila,

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.
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.

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.

----- Original Message ----- 
From: "Attila Kinali" <attila at kinali.ch>
To: "Discussion of precise time and frequency measurement" 
<time-nuts at febo.com>
Sent: Tuesday, November 24, 2015 4:56 PM
Subject: Re: [time-nuts] ACAM GP22 Chip

> Moin,
> 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
> requirements.
> 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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