[time-nuts] ACAM GP22 Chip

[Bob Camp]

Hi

To me the easy way to do the buffers:

!) Grab some buffers or inverters in a reasonable package. The 74ACT14 
in a PDIP-14 package seems to sell for about 50 cents. 

2) Power the gate off of 5V, it’s TTL input so it will trigger at 1.4 V

3) Hook the inputs of 5 inverters to the output of the 6th. The input to 
the gizmo will be the input to this 6th inverter.

4) Terminate the input to the 6th inverter to ground with 75 ohms.

5) Run a (75 x 5 = 375 ohm) resistor from the output of each gate to a common
point. That point will drive your coax. 

Other than connectors, that’s it. You will have roughly 2.5V logic on the “far end” 
of the coax. that will trigger the TTL inputs nicely. Each buffer has a parts cost of 
maybe $1.  Each buffer will add about 14 ns to the delay of the coax. If you buffer
both the input and out of the line, that would give you 28 ns. 

I realize that may not make much sense at this point. It is a recipe you can point
somebody at and they can say “yes, I can do that”. It’s also a way to give you
a rough cost for the parts. 

Bob

> On Nov 25, 2015, at 9:27 AM, Thomas Allgeier <th.allgeier at gmail.com> wrote:
> 
> 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" <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.
>>> _______________________________________________
>>> time-nuts mailing list -- time-nuts at febo.com
>>> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
>>> and follow the instructions there.
>> 
>> _______________________________________________
>> time-nuts mailing list -- time-nuts at febo.com
>> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
>> and follow the instructions there. 
> 
> _______________________________________________
> time-nuts mailing list -- time-nuts at febo.com
> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.