[time-nuts] ACAM GP22 Chip

[Bob Camp]

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.allgeier at 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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