[time-nuts] ACAM GP22 Chip

paul swed paulswedb at gmail.com
Tue Nov 24 10:17:24 EST 2015

Welcome to the group. I am sure others will comment.
Many of us have a very wide range of experience and expertise so you should
feel comfortable with any question.
To the coax delay question. You are not pushing the limits.
But its important to understand the impacts of such long lines.
They need to be driven and terminated and the rise time will suffer from
the line capacitance. Essentially a fast rise time will become a slow
risetime on teh other end. There are lumped lc network delay lines. I have
experimented with them. They have the same effect. But you can cascade them
and use an inverter or buffer between each one.Each inverter also adds
delay. This helps the rise time issue. But the buffers add jitter and each
also adds delay thats temperature sensitive.
For cascaded delays of very short duration I have actually used 74LS244s
74HC244 line drivers cascaded and they work really well but only good for
each drivers delay.
Others will have better answers.

On Tue, Nov 24, 2015 at 9:04 AM, Thomas Allgeier <th.allgeier at gmail.com>

> Hello,
> I have an ACAM GP22 TDC chip and evaluation board which I am looking at
> for “work” purposes – I work for a company active in the weighing and force
> measurement world.
> I should say from the start that I am new to time and frequency
> measurements and not even an electronics engineer – but then I have been
> exposed to high-precision electronics for the last 25 years hence have
> picked up some dangerous degree of half-knowledge.
> 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.
> In order to evaluate the chip I was planning to replicate John A’s
> experiment with the coaxial delay line from the 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?
> So, to come to the point: Am I pushing the concept of a coax delay too far
> with 1 µs and are there other (simple/reliable) ways to achieve this kind
> of delay? I have tried it with a shorter piece of cable (around 2 ns which
> is measured in “range 1”), there I seem to get consistency virtually to
> within 100 ps. But I need to know if the device sticks to this level of
> performance when the periods are much longer, and thus measured in “range
> 2”.
> Thanks and best regards,
> Thomas.
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