Dear Paul,
Thanks for the reply and thanks to all other contributors. Seems I have
subscribed to the right list!
As it happens this is a sideline project. So I have the luxury to ask for
advice and even consult books before advancing with care.
I will probably try and go down the coax route, starting with a shorter
length first, and reading up a bit.
If any useable results can be obtained I will post them for future
reference.
Best regards,
Thomas.
----- Original Message -----
From: "paul swed" <paulsw...@gmail.com>
To: "Discussion of precise time and frequency measurement"
<time-nuts@febo.com>
Sent: Tuesday, November 24, 2015 3:17 PM
Subject: Re: [time-nuts] ACAM GP22 Chip
Thomas
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.
Regards
Paul
WB8TSL
On Tue, Nov 24, 2015 at 9:04 AM, Thomas Allgeier <th.allge...@gmail.com>
wrote:
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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