Re: simple frequecny multiplier

On Thu, 07 Apr 2005 05:36:27 GMT, "colin"
<no.spam.for.me@xxxxxxxxxxxx> wrote:

>"John Larkin" <jjSNIPlarkin@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote in
>message news:qve95150hjeho69si4nqn7raqsvtn2qqg9@xxxxxxxxxx
>> Why are picosecond delays interesting here?
>> What's the application?
>
>its for an experiment trying detect any relative change in speed of light.
>
>> 1 ps RMS jitter over 100 ms time will be very difficult. A good
>> crystal oscillator can do a few tens of ps over 100 ms; a very good
>> ocxo can hit 1 ps over that time.
>
>im using the latest VCTCXO from cmac cfpt-9000 series wich are suposed to be
>very good, certainly from stability anyway although they dont quote phase
>noise/jitter explicitly, i think its suposed to be one of the lowest,
>certainly stability is very good. maybe better over short term than an ocxo
>as it operates at lower temperature and has no thermal feedback loop. i
>beleive you can also now get these as a oven controled version too. its
>useful to know the figures you mentioned tho.
>

TCXOs have a temperature transient problem: the temp comp sensor never
has the same thermal time constant as the crystal itself, so whereas
the compensation averages very good, a millikelvin delta-t over 100 ms
can cause a lot of phase shift. A good OCXO will have a huge thermal
isolation system and also operates the crystal at its "turning
temperature" where the inherent TC is zero. All that makes a huge
difference in close-in phase noise. If you use a TCXO, put it in a
heavy aluminum can to slow down temperature transients; that alone can
cut thermally-induced phase noise 10:1.

A good SC-cut OCXO is a few hundred dollars new and is the best you
can do without going atomic. You can get a used rubidium for about the
same, but I'm not sure the short-term stability is necessarily better
than the SC.

Considered an optical interferometer?

John


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