From: Daniel, Erik S. (Daniel.Erik@mayo.edu)
Date: Tue Apr 24 2001 - 21:09:55 PDT
> Humble request to anyone who has sub-picosecond jitter
> requirements: please describe how you achieved that
> spec in a real system?
I'll preface my comments by saying that the applications and implementations
I'll describe here may be very different from those many of the SI-list
readers are familiar with -- research and development scale, not production
scale; in some cases, racks of equipment, not a compact PCB; more exotic
IC processes than Silicon. Also, I won't claim to be the expert in the
details of any of these systems -- in each case, we've collaborated with
other groups who have had more direct experience with the clock systems.
The ADC systems we have worked with requiring low phase noise clocks
typically start with a low phase noise oscillator such as the HP8662 (pretty
good) the Poseidon SLCO (apparently the best you can buy --
http://www.psi.com.au/resonator.htm) or some homebrew system. These things
are typically big rackmount devices that generate very pure sine waves.
This low noise oscillator is used to drive the guts of the ADC which is
fabricated in a very high speed transistor process (e.g. GaAs HBT, SiGe HBT,
InP HBT) where the ft's are high enough that the clock can be distributed
with minimum additional phase noise, and the latches can have very fast
regeneration times, reducing aperture uncertainty. These buffers and
latches burn a LOT of power to meet the specs.
One more exotic project we are working on uses a mode locked laser to
generate *optical* pulses which serve as a very low jitter clock.
Some of the most knowledgable folks in this area seem to be the NIST
timekeeping division (http://www.boulder.nist.gov/timefreq/), though they
are typically interested in the phase noise spectrum between "zero" and 1 Hz
offset from the carrier, whereas I'd guess most of us reading this list are
interested in a range from, say, 10s of kHz up to our clock frequency.
Erik Daniel, Ph.D. Voice: (507) 538-5461
Mayo Foundation Fax: (507) 284-9171
200 First Street SW E-mail: email@example.com
Rochester, MN 55905 Web: www.mayo.edu/sppdg/
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