[time-nuts] ACAM GP22 Chip

Thomas Allgeier th.allgeier at gmail.com
Wed Nov 25 09:27:41 EST 2015

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,
----- Original Message ----- 
From: "Bob Camp" <kb8tq at n1k.org>
To: "Discussion of precise time and frequency measurement" 
<time-nuts at 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.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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