[time-nuts] Comparison of Logic Standards for Clock Distribution
Dr Bruce Griffiths
bruce.griffiths at xtra.co.nz
Wed Oct 25 09:41:24 EDT 2006
Stephan Sandenbergh wrote:
> Hi Bruce,
> Thank you for the elaborate answer covering different logic types. Funny
> enough, I have just read the excellent book you recommended cover to cover -
> probably the origin of many of my questions.
> As I said in reaction to Said's response - I am not surprised that analog
> (sine wave) transmission is superior. But, it takes a lot more effort to do
> it well.
> You mentioned that locking to a crystal at the receiving end as an option.
> Does this mean that signal transmission is primarily plagued by short term
> I have never really touched the topic of optical fibre, but I realise that
> it is superior to conventional methods. The superiority of optic fibre is
> probably not as pronounced at short distances, is it?
> I realise that a better reference clock will only improve a system's
> performance up to the point where the jitter and phase noise of the other
> components in the system begins to dominate. However, I would like to have a
> good grip on the basics. Are there any good books you can recommend on the
> topic of clock distribution?
> Kind regards,
> Stephan Sandenbergh
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Whilst there are many books available of clock distribution within
digital systems (eg http://cva.stanford.edu/books/dig_sys_engr/) and
within VLSI chips, there is little specific info published in book form
on precision clock/frequency standard distribution on the larger scale
such as within or between buildings.
The difficulties associated with high speed clock distribution in
silicon VLSI chips currently limits the maximum clock speed of such
chips to around 4GHz or so.
Simply using higher resolution lithography to produce smaller
transistors doesn't help much. Optical techniques for clock distribution
on chip are being investigated.
One can glean some idea of how this may best be done by looking at how
NIST, USNO, NIST, PTB, etc pipe the standard frequencies from their
various atomic standards around. Current designs for modern radio
telescope arrays such as the Atacama millimeter array and upgrades of
various existing instruments give a good indication of current best
practice for such instruments that require state of the art timing
For VLBI where the radio telescopes may be on different continents it is
not possible to actually transmit reference frequencies between stations
with adequate stability.
Usually each station has its own hydrogen maser. The frequencies of
these masers may be steered in the long term (weeks) by GPS all in view
observations and other techniques. Signal integration times of up to
10,000 seconds are used when observing cosmic radio sources. As long as
the offsets and drift of the various clocks are relatively stable and
not too large the data reduction software can compensate for them.
Ensuring that the various antenna clocks in an interferometer array are
synchronised to within a few picoseconds (preferably femtoseconds) is
difficult when the antenna separation is large and the cabling/fibre is
subject to large temperature variations.
That said, low frequency radio frequency interferometry has been done
using GPS all in view techniques to synchronise antenna clocks. It
doesn't matter too much if the various antenna clock frequencies wander
around as long as they all do so in lockstep.
When using passive filters in receivers and distribution amplifiers, the
temperature coefficient of the filter phase shift can limit the phase
stability of the received clock signal.
Low temperature coefficient inductors and capacitors are necessary and
high Q tuned circuits have relatively high phase shift temperature
coefficients. For the ultimate in phase stability it may be necessary to
regulate the temperature of such components. Even coax cables have delay
tempcos ranging from 50-100 ppm/K. The tempco of some optical fibres can
be significantly lower. When using coax one should avoid PTFE
dielectrics if the cable temperature approaches 15C where PTFE undergoes
a phase transition which can be observed by plotting the cable delay as
a function of temperature.
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