[time-nuts] OT: leaching was, Alternative time interval interpolation technique

Bernd Neubig bneubig at t-online.de
Tue Aug 17 04:12:37 UTC 2010


On Aug 16, 2010, at 18:35 Said Jackson [saidjack at aol.com] wrote:

>It's very hard to find a vendor making good (sc-cut) low PN crystals that
are not in an ocxo. They are mostly designed to work at inflection points
around 90C.
>No good very low PN vcxos around unfortunately..

It is the nature (or definition) of the SC-cut that this cut yields an
inflection point at around 95°C. This makes it not usable for VCXO or clocks
without temperature control, becasue the frequency vs. temperature various
rather strongly in a normal -20C to +70C environment.
There are compromizes like the IT and the FC cut, whose inflection points
are at 75C or 50C respectively. But still the frequency excursion in
"normal" operating temperature ranges is rather large.

PN of VCXO is larger than that of OCXO because
- mostly fundamental mode or low overtone crystls are used, which have lower
Q that tose crystals used in OCXO
- the phase noise is increased due to parametric effects of teh varactor
diode
Nevertheless there are VCXO on the market which provide much lower phase
noise than teh mass-produced standard models.
See for example the AXIS10LN on the AXTAL website www.axtal.com. There may
be other vendors too ;-)

Regards

Bernd Neubig DK1AG





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