[time-nuts] Comparing PPS from 2 GPS units

Bruce Griffiths bruce.griffiths at xtra.co.nz
Mon Dec 17 21:58:36 UTC 2012

Its actually a relatively low performance implementation of a classic 
The ramp capacitor reset switch (a saturated transistor) has a 
relatively slow turnoff and poorly defined reset voltage.

For shorter time delays lower capacitance current source and reset 
devices are required.

Decoupling of the current sensing device in the control loop from the 
current source device emitter is desirable.

ECL delay chips using this technique complete with a DAC to set the 
comparator threshold used to be available.

A classic reset switch uses a current driven pair of matched shottky 
diodes to ensure fast switching coupled with a relatively low offset 

Techniques to produce stable longer delays:

1) Use a stable (phase locked) gated oscillator to produce a well 
defined long delay using pulse counting together with a triggered ramp + 
DAC + comparator for fine adjustment.
The HP5359A delay generator is an example of this technique.

2) Use a synchroniser plus counter for the coarse delay followed by a 
triggered ramp + DAC and comparator.
The synchroniser delay is measured and the result used to adjust the 
fine delay to compensate for the variable synchroniser delay.
Analog techniques where the fine delay ramp is stopped and held by the 
synchroniser outpt and then resumed by the digital delay output 
transition have also been used.


M. Simon wrote:
> Nice pulse delay generator:
> http://www.edn.com/file/14660-Figure_1.pdf
> The article that goes with it:
> http://www.edn.com/design/analog/4323671/Dual-flip-flop-forms-simple-delayed-pulse-generator
> The delay is analog - charging a capacitor until it crosses a threshold. So I don't know if it is good enough. But I did find it interesting. The order of the delay is 1 to 250 uSec. Faster parts would get you into shorter delays.
> If you had two of these operating on a common trigger...
> Simon
> Engineering is the art of making what you want from what you can get at a profit.
>> ________________________________
>> From: David<davidwhess at gmail.com>
>> To: Discussion of precise time and frequency measurement<time-nuts at febo.com>
>> Sent: Monday, December 17, 2012 6:56 PM
>> Subject: Re: [time-nuts] Comparing PPS from 2 GPS units
>> On Mon, 17 Dec 2012 10:19:43 -0800, Hal Murray
>> <hmurray at megapathdsl.net>  wrote:
>>>> A fifth solution is to use a pulse delay generator like a DG535. I use this
>>>> to create high-resolution early/late 1PPS sync pulses. They show up on eBay,
>>>> but aren't cheap. For bargains, watch for older model programmable pulse
>>>> delay generators by BNC (Berkeley Nucleonics Corporation).
>>> Thanks.  Those are more $$$ than I'm interested in right now, but might be a
>>> useful tool sometime in the future.
>>> Another approach is to use a scope: trigger on one PPS and adjust the delay
>>> (which might be negative) and sweep speed so you can see the other PPS
>>> signal.  Maybe I'll play with this to see what sort of results I can get.
>>>> Lastly, there are cute little delay boxes (www.ebay.com/itm/150962422699)
>>>> that might work. Not sure how stable they are at the ns level. But it would
>>>> be fun to measure. If someone opens one of these please tell us if it's a
>>>> coil of wire, some kind of LRC filter delay, or if they use those Dallas
>>>> delay chips. Which is another solution for you -- google or eBay search for:
>>>> silicon delay line.
>>> You can make a reasonable delay line by using the lumped circuit
>>> approximation for the L and C for the appropriate impedance transmission
>>> line.  I assume that's what's in the delay boxes.  I should try that
>>> sometime.  Thanks for the reminder.
>>> The delay chips I've looked at before used gate delays.  I think they were
>>> Motorola rather than Dallas.  I just poked at a few Maxim data sheets.  I
>>> didn't find out how they implemented the delays.
>>> I think some of the clock recovery chips tune delays by tweaking the
>>> threshold voltage.
>> I have been testing just using adjustable RC delays into a logic gate
>> to generate pretrigger pulses for sampling oscilloscopes.  Accuracy
>> depends on a complete reset of the capacitor and tracking between the
>> RC charge voltage and gate threshold voltage.  Worst cast jitter for
>> TTL has been in the 100s of picoseconds range because of supply
>> voltage sensitivity.  Different families of TTL and CMOS logic all
>> performed about the same.
>> Here is the jitter measurement that came from the RC logic gate delay
>> test:
>> http://www.banishedsouls.org/c2df3757f1/PG506/PDJ%20lolcat.jpg
>> Much better is to use a differential comparator or differential input
>> ECL which solves the threshold variation errors and a fast (it really
>> isn't all that fast) ramp generator with a precision reset.  The
>> differential input allows the ramp rate and threshold voltage to be
>> linked allowing ratiometric operation to reject power supply or
>> reference voltage variation and noise.
>> My next pretrigger generator is going the differential comparator or
>> differential ECL route with a fast ramp and precision reset.  I expect
>> jitter to be significantly better than 10s of picoseconds for delays
>> up to about 100 nanoseconds.  If I get down to 10 picoseconds of
>> jitter, I will be happy since I have no real way to measure much below
>> that.
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