[time-nuts] hello and first questions

Bruce Griffiths bruce.griffiths at xtra.co.nz
Wed Dec 24 15:11:54 EST 2008


The answer to your question depends on your accuracy and resolution

The easiest method is to obtain a time interval counter like the HP5370A
(or HP5370B), the SR620, the Agilent 53131A. Agilent 53132A, etc.

Alternatively you can build your own:

1) If your resolution requirements are more modest (eg 100ns) then you
can easily build your own time interval counter. With some ingenuity its
possible to use a suitable micro to do this.
The best method being to sample a free running counter (extended in
software as required) with the synchronised leading edge of the PPS
pulses and subtract the corresponding time stamps. Done correctly, this
ensures that no PPS pulses are lost due to processing time.

2) If you need higher resolution you can use a gate array and
microprocessor together with a faster clock (100MHz or more depending on
the gate array).
Its also possible to use a suitable DSP (eg an Analog Devices Blackfin
DSP) to timestamp the leading edge of a PPS pulse with a resolution of a
few nanosec (<3) with little external logic required.

3) Subnanosecond resolution requires an interpolator as GHz clocks are
usually impractical and/or expensive.
There are many interpolation techniques including:

a) Sampling a a pair of quadrature phased sine waves. The clock usually
has the same frequency as and is phase locked to the sinewave.

b) Charging a capacitor for the time interval from the leading edge of a
PPS pulse to the next active clock edge. Use an ADC to measure the
voltage across the capacitor, combine this with the digital time stamp
derived by sampling a continuously clocked counter and reset the
capacitor for the next PPS pulse. A variant of this is to charge the
capacitor with a current I and discharge it with a lower current such as
I/1000 and measure the time taken to discharge the capacitor to zero
using a conventional counter. This technique can increase the resolution
by a factor of 1000 or more.

c) Triggered phase locked vernier oscillators as used in the HP5370 (A
or B).

d) Use a tapped delay line consisting of a long string of gates within
an IC (custom or FPGA) as in the Acam time interval measurement chips.

All the clocks need to be phase locked to a low noise frequency standard
to maintain accuracy for longer time intervals.

For short time delays between the 2 PPS pulses an interpolator circuit
by itself may suffice.

Thus the relevant questions are:

1) What is the maximum range you need/want to be able to measure?

2) What resolution do you need/want?

3) What accuracy do you need/want?


Trevor Higgins wrote:
> Hello!
> Just a couple of backslashes probing the world of insanely accurate time.
> My first question, which will doubtlessly lead to many more, is where 
> can I find some information on the circuitry to derive the difference 
> between two PPS signals and read it in some sort of digital format.
> I'd also be interested in finding some information on deriving PPS 
> signals from a raw 10Mhz signal, since it seems that rubidium frequency 
> standards are a fair bit cheaper than 1PPS standards.
> I hope to build my first GPS disciplined atomic standard within a few 
> months, learning all sorts of things along the way. Sorry if I'm already 
> asking some questions that have obvious answers, but i wanted to start 
> on the best possible information out there, and anyone on this list 
> probably has the best advice that I could ask for.
> I've got a lot of raw potential, and have been feeding my interest in 
> timekeeping with a ton of random information for a long time, and it is 
> time to do something with it.
> Glad to be part of a group that might understand how fascinating this 
> all is to me.
> Obsessed with all things precise and accurate,
> \\

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