Re: [time-nuts] ACAM GP22 Chip

[Attila Kinali]

N'abend

On Wed, 25 Nov 2015 14:19:37 -
"Thomas Allgeier"  wrote:

> You guessed my origin right from my name.
> 
> It is not that I don't trust ACAM (we are one of their customers) but from 
> the source I am measuring I get a lot of what you guys probably call jitter. 
> So I want to be sure the jitter comes from the oscillator I am measuring and 
> not from the GP22.

Ah, so it's about verification of the results you already got.
Then I would rent some TIC for a day or two from one of the many shops
that rent out measurement instruments. That's probably cheaper (especially
if you include your work time) and you don't have to guestimate the noise
that your reference circuit makes, but get the noise floor values from
the instrument manual.

> Absolute accuracy is not the problem here, unless the 32768 Hz clock on the 
> eval board actually drifts. In other words if we are 10 ns out on the period 
> measurement nobody cares if it remains the same 10 ns all the time. As in 
> many applications in the weighing industry what really matters is the change 
> in weight, not how heavy the mass is in absolute terms.

I don't really see how a 32kHz crystal is used in a scale, but there are
many way to do things :-)

> It sounds like the coil must be wound what we used to call "bifilar" in my 
> school days, i.e. self-cancelling the inductivity. That should be doable, 
> and on the other stuff I will read up a bit first.

Again, please do not think of a coax in terms of wires. It's a transmission
line. 99% of the wave energy is between the inner and the outer conductor.
There is (almost) no magnetic field that you will be able to cancel by
building a bifilar coax spool. How your signal looks like depends on the
intrinsic properties of the cable. There is very little dependence of the
electrical parameters on the way how you arange the cable, whether you use
a straight line or roll it up.

On Wed, 25 Nov 2015 14:23:40 -
"Thomas Allgeier"  wrote:

> I will report back any outcome that looks plausible / presentable.

Please also report back when you have negative results. It's almost
always more instructive to see what went wrong and figure out why.


Attila Kinali
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Re: [time-nuts] ACAM GP22 Chip

[Attila Kinali]

On Wed, 25 Nov 2015 14:19:37 -
"Thomas Allgeier"  wrote:

> It is not that I don't trust ACAM (we are one of their customers) but from 
> the source I am measuring I get a lot of what you guys probably call jitter. 
> So I want to be sure the jitter comes from the oscillator I am measuring and 
> not from the GP22.
> Absolute accuracy is not the problem here, unless the 32768 Hz clock on the 
> eval board actually drifts. 

Oh.. there is one thing i forgot to mention: 32kHz oscillators are usually
optimized for low power. Modern 32kHz oscillators can get below 1uA current
consumption. This optimization for power has of course consequences, one
of them is that these oscillators are fairly noisy. People using 32kHz
oscillators usually don't care as they use these to count seconds anyways,
where the instability is dominated by the temperature coefficient of the
crystall and noise mostly averages out. I wouldn't expect the noise to
be in the ns range, but i wouldn't surprised if it was a few 10ps.
I have never done any measurements though, and I don't think i've
ever seen any jitter measurements for 32kHz oscillators, so take
this value as rough guestimate.


Attila Kinali

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Re: [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  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" 
> To: "Discussion of precise time and frequency measurement" 
> 
> 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  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 

Re: [time-nuts] ACAM GP22 Chip

[ed breya]

I don't want to muddy the water too much, but I'd second going 
all-digital, along the lines of what Bob L. recommended earlier. It 
seems like you just need to make some specific delay times, so various 
logic counting or shifting circuits should be just fine - and 
well-defined and understood.


If you go with coaxial delays, the loss and dispersion may cause much 
analog grief to determine how much compensation and amplification is 
needed, and the thresholds for comparison for squaring it up. This is 
especially aggravated with long lines.


In the hundreds of meters range, it may be better to go with optical 
fiber instead. The bandwidth can be huge, and losses will be tiny, even 
into km lengths, and all kinds of E/O and O/E conversion parts are 
available for the ends. There can be some timing jitter due to the noise 
of the conversions, and AGC issues, but probably less than the noise 
associated with adapting long coax cables to this task.


Another option may be acoustic glass delay lines. I don't know if any 
are made nowadays, but they were common in TVs and VCRs, with time delay 
somewhere in the horizontal line duration range, and very compact. It 
could be that there are many more kinds for various applications (or 
maybe all obsolete - replaced by digital).


Ed
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Re: [time-nuts] ACAM GP22 Chip

[Attila Kinali]

On Wed, 25 Nov 2015 12:35:44 -0800
Hal Murray  wrote:

> If you poke around on the web you can probably find a graph of frequency vs 
> temperature, maybe even for that particular vendor/model of crystal if you 
> can read the label.  Beware: the graph for a 32 KHz crystal is probably much 
> different than a high frequency (10 MHz) crystal.

Yes, 32kHz crystals are usually XT cut tuning forks and exhibit a
negative quadratic detuning from the inflection point (usually 25°C)
Because it's quadratic and does not have the nice cubic form of AT cuts,
this can get quite big. E.g. if we use the Abracon ABS07[1] an example
for a typical speciem that you might use in such an application 
(i.e. not one of the cheap china crystals with an initial offset of >200ppm)
you see that it has a typical temp coefficient of -0.036ppm/°C^2 relative
to the infection point. Let's say we have a device that is used indoors
in some place that is more or less well ventilated, then we can expect
a maximum temperature of 45°C. This results in an frequency offset of
14.4ppm. Which is already above the limit of 1ns/7us=14.2ppm. 
And that's best case. It does not account yet for the inflection point
being off (+/-5°C variation, which give another 8.1ppm) or the coefficient
being higher (can go up to -0.040 -> +1.6ppm).

So already with a relatively good 32kHz crystal and very moderate
temperature requirements, the crystal change its frequency by 25ppm
and thus by 1.75ns @ 70us.

There is reason why I said one needs temperature compensation for
this kind of stability requirement ;-)

Attila Kinali



[1] http://www.abracon.com/Resonators/ABS07.pdf

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Re: [time-nuts] ACAM GP22 Chip

[Tom Van Baak]

>> In order to evaluate the chip I was planning to replicate John A’s
>> experiment with the coaxial delay line from the HP5370b 
> For those wondering: "John A" is John Ackermann and the experiment
> in question is documented at http://www.febo.com/pages/hp5370b/

Maybe I misunderstand, but I would not suggest testing a time interval counter 
by using a fixed ns delay -- that's almost never how the real world works and 
those tests tend to produce bogus ADEV plots that have -1 slope forever (a clue 
that something's wrong with the test).

A selection of fixed delays is slightly better. But best, and much easier, is 
to use uncorrelated A, B, and LO (ext ref) signals. A fixed delay may land on a 
sweet spot or honey bucket. Linear sweeping the range covers all spots, and 
gives you best case / worst case / rms statistics as a bonus. In other words, 
what you want is a set of random (but known, or knowable) delays; not a set of 
hardcoded delays.


> of the GP22 I need a delay of 500 ns or more (actually 1 µs sounds a better 
> start).

Are you sure you want a hardcoded delay of N ns or N us? Or is a variable or 
even varying delay sufficient?

What I use in cases like this is two stable oscillators that slowly drift apart 
(i.e., close, but not the same frequency). For example, if they differ in 
frequency by 1e-12 your signals drift 1 ps/s. Or if they differ by 1e-10 your 
signals drift by 1 ns / 10 s. You get an uncorrelated, very low-noise, linear 
phase sweep "for free".

This sort of slow varying phase relationship is ideal when making counter 
tests; much better than a fixed delay. You can use a laboratory counter to 
monitor their exact phase difference in parallel with your DUT. That is, you 
then compare TIC "truth" against what your DUT reports.


> 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.

I may have missed it in the thread -- but how quickly do you need your 
measurements? Is one measurement every 1 or 10 or 100 seconds ok? (in which 
case an ACAM chip is total overkill). Or is this some sort of sub-second 
real-time application that require both modest resolution (1 ns / 70 us = 15 
ppm, easy) and fast response (hard)?

/tvb
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Re: [time-nuts] ACAM GP22 Chip

[Angus]

>I wouldn't expect the noise to
>be in the ns range, but i wouldn't surprised if it was a few 10ps.
>I have never done any measurements though, and I don't think i've
>ever seen any jitter measurements for 32kHz oscillators, so take
>this value as rough guestimate.

On the GP21 board I used it was more like 10ns+. I don't know if
that's typical, but it wasn't a even particularly cheapo crystal as
the original one was faulty and I had to replace it. The official acam
board might be better, but the data sheet specifically talks about the
32K osc having a lot of jitter.

One of the problems of the data sheet is that it is really tailored
for those using it as a flow converter - some of the text does not
even make sense otherwise. That was one of the reasons I was planning
to test it more as a TDC, but just getting back to doing that now.

Angus.
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Re: [time-nuts] ACAM GP22 Chip

[Angus]

Hi Thomas,

Another option for generating a range of delays would be to use a
stable oscillator like an ovenised one. This is actually similar to
what the GP22 does for measuring on mode2. 

If you gate the output of the oscillator you can get a start and stops
at 1us or 70us or whatever. It's useful because it allows you to test
the whole measurement system rather than just the TDC noise which
should be way below 1ns peak-peak.

Angus.



On Tue, 24 Nov 2015 14:04:32 -, you 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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Re: [time-nuts] ACAM GP22 Chip

[David C. Partridge]

FWIW I think the two books T versus AofE complement one another quite well 

Regards,
David Partridge 

-Original Message-
From: time-nuts [mailto:time-nuts-boun...@febo.com] On Behalf Of Attila Kinali
Sent: 25 November 2015 10:25
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] ACAM GP22 Chip

On Tue, 24 Nov 2015 11:44:04 -0800
Hal Murray  wrote:

> th.allge...@gmail.com said:
> > 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.
> 
> Do you have a copy of Art of Electronics by Horowitz and Hill?

He is german. He most likely has Tietze, which I consider to be the 
better book than AoE* :-)

Attila Kinali

* I haven't had a look at the 3rd edition yet, so no comment on that.
--
It is upon moral qualities that a society is ultimately founded. All the 
prosperity and technological sophistication in the world is of no use without 
that foundation.
 -- Miss Matheson, The Diamond Age, Neil Stephenson 
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Re: [time-nuts] ACAM GP22 Chip

[Thomas Allgeier]

Hello Hal,

The book has been sourced and is on its way to me - a good pointer, thanks. 
With it and the comments I have had from you guys I expect I will get pretty 
much what I want, given a little time which thankfully I have.


I will report back any outcome that looks plausible / presentable.

Kind regards,
Thomas.
- Original Message - 
From: "Hal Murray" 
To: "Discussion of precise time and frequency measurement" 


Cc: 
Sent: Tuesday, November 24, 2015 7:44 PM
Subject: Re: [time-nuts] ACAM GP22 Chip




th.allge...@gmail.com said:
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.


Do you have a copy of Art of Electronics by Horowitz and Hill?



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?


The trick is that your coil has 2 wires.  It's a transmission line.

Here is the handwaving explanation: The current on the return path is 
going

in the other direction and cancels out the inductance.

Twisted pair will work almost as well as coax, maybe better than cheap 
coax.


AoE has an appendix on transmission lines.  You can find lots of info on 
the

web.  You need to terminate it.  You will want a scope to check the
termination.



--
These are my opinions.  I hate spam.



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Re: [time-nuts] ACAM GP22 Chip

[Thomas Allgeier]

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" 
To: "Discussion of precise time and frequency measurement" 


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  
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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Re: [time-nuts] ACAM GP22 Chip

[Hal Murray]

th.allge...@gmail.com said:
> Absolute accuracy is not the problem here, unless the 32768 Hz clock on the
> eval board actually drifts. ...

It will drift with temperature.  Not much, but this is time-nuts.  Coax or 
R-C delay lines will probably drift more.

If you poke around on the web you can probably find a graph of frequency vs 
temperature, maybe even for that particular vendor/model of crystal if you 
can read the label.  Beware: the graph for a 32 KHz crystal is probably much 
different than a high frequency (10 MHz) crystal.

The question is how much does it drift and/or is that significant relative to 
other sources of drift.  Probably not.  When you get things working, try 
hitting the crystal with a heat gun or cold spray and see if you can see any 
change.

You may be able to see it on a scope if you trigger on one edge, use the 
delayed sweep to look at the next cycle (or several cycles out) and then zoom 
way in.

--

The really really simple approach is to get a signal generator that will do 
it all.  I don't have a model number to suggest, but some of them have two 
knobs, one for the frequency of the basic signal and another for the delay 
from the basic signal to a secondary signal.

--

[delay line suggestion]

> Thanks for the suggestions - these sound a bit beyond my current level of
> skills and kit.

If you want long delays, it's often easier to do it with digital logic rather 
than analog.  (I'm embarrassed that I didn't think of it.)

Assume you have a 10 MHz clock and you want to delay 10 microseconds, that's 
100 cycles.  With faster clocks you have to delay more cycles.  ...

The really simple case is to delay by one cycle.  That's just another 
flip-flop.  Assuming that your signal is in one FF, take that signal as input 
to another FF.  The output of that second FF will be the first FF delayed by 
one clock.  If you have a signal generator you can use it for the clock.  
Adjusting the frequency will change the delay.

To delay a few cycles, you can use a shift register.  That's just shorthand 
for a collection of FFs all wired up in a chain.  They come 8 bits to a DIP 
package.  Look at the data sheet for a HC299.

For longer delays, you want a counter.  The simple approach is to pick your 
count to be a power of two.  If it counts every cycle, the carry out will 
tick every 2-power-of-N cycles where N is the number of bits in the chip.  
You can get counter chips with a load option.  That's an easy way to adjust 
the delay.  Assume you have an 8 bit counter.  If you load it with 0, it will 
take 255 cycles to get a carry out.  If you load it with 155, it will only 
take 100 cycles.

For really long delays, move to software.  An Arduino is probably the right 
approach for something like this.  The basic idea is to turn on one output 
pin, wait a while, then turn on another pin.  The wait-a-while is just a loop 
that does nothing for the right number of iterations.  If you need fine 
control, you probably need to use assembly language and count each cycle.  If 
you can live with rough answers (20.3 microseconds rather than 20), then you 
can probably do it with C and the standard Arduino software package.



-- 
These are my opinions.  I hate spam.



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[time-nuts] Nortel GPSDO

[Jim Sanford]

All:

Several years ago, I purchased (from the ePlace) one of these units that 
many of you commented favorably on.


While it worked and communicated properly with Lady Heather, I never got 
it to completely lock in -- some form of periodic noise on the 
oscillator voltage.  It has been gathering dust.  I'm sure it can be 
made to work (maybe with a better power supply) but I just don't have 
time to mess with it.


I ran across a couple of Meinberg units (and their 
antenna/downconverter) and they work fine, so I have no need for the 
Nortel unit, and would rather it not collect dust.


If anybody would want to make a go of this thing or use it for parts, 
please contact me off list.


Jim
wb4...@amsat.org


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Re: [time-nuts] ACAM GP22 Chip

[Thomas Allgeier]

Moin Attila,

You guessed my origin right from my name.

It is not that I don't trust ACAM (we are one of their customers) but from 
the source I am measuring I get a lot of what you guys probably call jitter. 
So I want to be sure the jitter comes from the oscillator I am measuring and 
not from the GP22.
Absolute accuracy is not the problem here, unless the 32768 Hz clock on the 
eval board actually drifts. In other words if we are 10 ns out on the period 
measurement nobody cares if it remains the same 10 ns all the time. As in 
many applications in the weighing industry what really matters is the change 
in weight, not how heavy the mass is in absolute terms.


It sounds like the coil must be wound what we used to call "bifilar" in my 
school days, i.e. self-cancelling the inductivity. That should be doable, 
and on the other stuff I will read up a bit first.


Thanks,
Thomas.
- Original Message - 
From: "Attila Kinali" 
To: "Discussion of precise time and frequency measurement" 


Sent: Tuesday, November 24, 2015 4:56 PM
Subject: Re: [time-nuts] ACAM GP22 Chip



Moin,

On Tue, 24 Nov 2015 14:04:32 -
"Thomas Allgeier"  wrote:


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.


That's some modest requirement and should be doable with the GPS22
quite easily (or any other TDC for that matter). BTW: when specing
something like this, please make sure to mention whether 1ns is
1sigma, 3sigma or worst case/peak-to-peak. These 3 are quite different
requirements.


May I ask why you want to verify the specs of the GP22?
The specs say that it does something between 39ps and 70ps (1sigma),
which is probably way better than what you need. And as Acam is
a german company, I expect the datasheet to be accurate.

BTW: 1ns over 70us is approximately 14ppm. The GP22 uses the attached
crystal for absolute calibration. Please be aware that 14ppm will
require at least a TCXO to reach that level over the whole temperature
range, and depending on what TCXO you use, you might need to calibrate
the TCXO post-production and again after a couple of years of use.
Even if you don't need calibrate, I would add a TCXO frequency measurement
to the production test.

BTW2: the "we have a x ppm TCXO" value is usually misleading,
as that's the best-case, pre-soldering, pre-aging, pre-anything value.
The end-value can be 3 times as large... easily.
(unless you happen to choose one of the more honest manufacturers,
for example, like Abracon)


In order to evaluate the chip I was planning to replicate John A’s
experiment with the coaxial delay line from the HP5370b


For those wondering: "John A" is John Ackermann and the experiment
in question is documented at http://www.febo.com/pages/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?


The coax is a transmission line. Yes you have inductance and capacitance,
but it does not make that much sense to talk about that anymore,
the impedance is the right thing to talk about.
Your output will not be as sharp as your input due to dispersion,
but that can be easily recovered using some buffer gate.

Please make sure that your coil is reasonably temperature stabilized
and, if it's cheaper cable, also humidity stabilized, as both parameters
change your delay. (putting it into an isolated box should be good
enough for this kind of measurment).


Attila Kinali

--
It is upon moral qualities that a society is ultimately founded. All
the prosperity and technological sophistication in the world is of no
use without that foundation.
-- Miss Matheson, The Diamond Age, Neil Stephenson
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Re: [time-nuts] ACAM GP22 Chip

[Thomas Allgeier]

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 
reference.


Best regards,
Thomas.
- Original Message - 
From: "paul swed" 
To: "Discussion of precise time and frequency measurement" 


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
WB8TSL

On Tue, Nov 24, 2015 at 9:04 AM, Thomas Allgeier 
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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Re: [time-nuts] ACAM GP22 Chip

[Thomas Allgeier]

Hello Bob,

Thanks for the suggestions - these sound a bit beyond my current level of 
skills and kit. But I do have people to call on in the office who may be 
able to rig something like that up for me should the coax route fail.


Best regards,
Thomas.
- Original Message - 
From: "Robert LaJeunesse" 

To: 
Sent: Tuesday, November 24, 2015 6:25 PM
Subject: Re: [time-nuts] ACAM GP22 Chip


If the goal is to create two signals consistently spaced near 70us apart 
why not use a good, fast 8-bit serial-in, parallel out shift register, 
clocked cleanly at 100kHz? Using the outputs from stages 1 and 8 would 
result in a 70us delay between signals. The data in would be fed 100KHz 
divided by 10 (or 16, or anything greater than 8) at whatever duty cycle 
is available. This allows the GP22 to see the combined instabilities of 
the clock and the shift register, which could be down in the nanosecond 
range, possibly less since the shift register delays would inherently 
cancel all but their differences.


Bob LaJeunesse


Sent: Tuesday, November 24, 2015 at 9:04 AM
From: "Thomas Allgeier" 
To: time-nuts@febo.com
Subject: [time-nuts] ACAM GP22 Chip

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.


...

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.

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Re: [time-nuts] ACAM GP22 Chip

[Attila Kinali]

On Tue, 24 Nov 2015 11:44:04 -0800
Hal Murray  wrote:

> th.allge...@gmail.com said:
> > 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. 
> 
> Do you have a copy of Art of Electronics by Horowitz and Hill?

He is german. He most likely has Tietze, which I consider
to be the better book than AoE* :-)

Attila Kinali

* I haven't had a look at the 3rd edition yet, so no comment on that.
-- 
It is upon moral qualities that a society is ultimately founded. All 
the prosperity and technological sophistication in the world is of no 
use without that foundation.
 -- Miss Matheson, The Diamond Age, Neil Stephenson
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