[time-nuts] Frequency dividers
Dr Bruce Griffiths
bruce.griffiths at xtra.co.nz
Tue Sep 26 01:30:31 EDT 2006
In August you asked
> First off, what I am about to do is ask a REALLY STUPID question, but
> more and more of the GPS stuff I do is drifting towards the precision
> timing end of things, so I thought I should ask.
> I have been seeing a lot of traffic concerning making 10MHz frequency
> dividers using PIC's. While they provide an elegant solution to
> providing an accurate 1PPS from a precision source, I have to ask if
> there is a reason for going this route? I am just using three HCT40103
> down counters hooked to a DS4000 to get what I think is a very stable
> 1PPS. Am I missing something? I realize 40103's are as old as dirt (I
> guess I am showing my 4000 series CMOS days), but the HCT series have
> plenty of bandwidth.
> Please be gentle....
As long as you have synchronously cascaded the HCT40103's the PPS
stability will be reasonable.
However if you need timing stability in the nanosecond region the
temperature dependence of the clock to output propagation delay will be
a problem if the temperature varies over a wide range.
The rms timing jitter due to the dividers should be reasonably low (in
the few hundred picosecond region), the relatively poor DS4000
oscillator phase noise will limit the PPS jitter to around 100
You can reduce the PPS jitter by resynchronising it to the clock using a
74AC74 (inherent jitter ~ 25-50 picoseconds).
If the oscillator phase noise were sufficiently low one could use an ECL
or similar high speed low jitter flipflop to resynchronise the PPS
signal and reduce the PPS jitter to a few hundred picoseconds.
Using a PIC or any other computer chip to divide down a clock signal to
produce a PPS signal has the same problem in that the delay from the
input clock to the PPS output will vary significantly if the temperature
changes over a wide range. Simultaneous switching noise will also
introduce jitter in the PPS output pulse if the PIC isn't always doing
the the same thing at the PPS transitions.
A dedicated external flipflop can be used to reduce the timing jitter to
a few tens of picoseconds or less.
However if a few nanoseconds matters when comparing with a GPS derived
PPS signal then the GPS antenna, antenna cable (particularly if its
length is greater than 100m ), and receiver will all need to be
temperature controlled to stabilise the GPS derived PPS signal timing to
better than a few nanoseconds.
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