Hello Bob,
That kind of approach is what I had in mind and as others have commented if
done carefully (which for me means a bit at a time) should get me there.
The buffers may present a bit of a challenge to a mechanical engineer
(unless they are the kind that can be salvaged from railway waggons) but the
AoE book as suggested by Hal should set me straight.
Thanks again,
Thomas.
----- Original Message -----
From: "Bob Camp" <kb...@n1k.org>
To: "Discussion of precise time and frequency measurement"
<time-nuts@febo.com>
Sent: Tuesday, November 24, 2015 11:24 PM
Subject: Re: [time-nuts] ACAM GP22 Chip
Hi
If you head down to your local big box store, they will happily sell you a
thousand foot spool
of RG-6 coax for next to nothing. If their prices are still to high, the
auction sites will sell it for
even less. It has a 75 ohm impedance and a bandwidth of several GHz. The
rather convent
formula of RT = 0.35 / BW then comes in. A 3.5 GHz cable will limit you to
a 100 ps rise time.
In all likelihood, you will be unable to generate a signal with this fast
a rise time.
You also will have some loss effects in the cable that are frequency
dependent. The calculation above
assumes you have done a few tricks to take care of this. If not, to get a
10 ns rise time, you need to maintain
a 35 MHz bandwidth. That works fine if you have a buffer every 500 feet.
No tricks, just a CMOS buffer
chip.
As noted by others, it *is* coax. You need to drive it and terminate it
with 75 ohms. At 35 MHz, a cheap
75 ohm resistor will do the trick just fine. At 3.5 GHz you may need to
get a bit more careful.
So is the 500’ limit an issue? I’d suggest that it’s not. Consider
chopping up the spool in a binary series of
400, 200,100,50,25,12.5, 6.5, 3.25 feet. You now have a set of buffered
lines that can be arranged to give you
a nice set of 256 time steps. Yes, the delay of the buffers will get in
the way a bit. The actual line lengths will
be a bit shorter as the lengths drop.
So how much delay do you get from a 400’ line? Velocity factor comes in
here. Best guess is that
your foam RG-6 has a 0.78 velocity factor. The "speed of light” in the
coax is 78% of the speed of light
in vacuum. Your 400 foot coax has about a 520 ns delay. Your stack comes
out just a bit over 1 us.
Bob
On 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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