[time-nuts] ACAM GP22 Chip

Thomas Allgeier th.allgeier at gmail.com
Wed Nov 25 09:12:19 EST 2015

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 

Best regards,
----- Original Message ----- 
From: "paul swed" <paulswedb at gmail.com>
To: "Discussion of precise time and frequency measurement" 
<time-nuts at 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
> On Tue, 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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