[time-nuts] frequency reference for portable operation

John Miles jmiles at pop.net
Mon Mar 11 16:20:09 EDT 2013

> >
> > With most modern lightweight Rb's the OCXO is integrated into the same
> heater block as the physics package. That makes it a bit tough to heat one
> without heating the other. .
> On LPROs the OCXO sits on the opposite side of the board.

Pretty sure that's not an OCXO.  If it is, it's a tiny one that will warm up
very quickly.  I doubt it's even a TCXO, though.

Of the Rb standards I've seen, only the PRS-10 and HP 5065A use OCXOs.  I'd
assume that the old-school Tracor and R&S models also used OCXOs, but for
the cheap telecom modules it would have been seen as a waste of money.

Also, the HP 5065A has separate ovens for the lamp and resonance cell, but I
believe the smaller units use a single heater for both.  They won't lock
until the heater is up to temperature.   

> So, I have bought a lot of 10 MHz OCXOs from eBay over the last few
> years. The best phase noise baseline reference I have found so far is my
> Z3805. I have lots of OCXOs in the 2x2x1.5 inch size. Many had good
> specs pointed at by the listings or word of mouth. When I used them with
> my board and SA most were pretty crappy compared to the Z3805. A couple
> of the ones I bought were Morion MV89As. - supposedly good, but what I
> saw didn't look very great. One of the best ones I have is a small
> 2x2x.75 inch Wenzel I bought a few years back. It has a custom part
> number of 500-11935. But don't buy by name. I recently picked up a 1x1x3
> Wenzel 10 MHz with sma output connector and its phase noise looks pretty
> horrible.

The MV-89A DOCXOs are good below 10 Hz but they have a high white PN floor
beyond 1 kHz, at about -150 dBc/Hz, with AM noise at a similar level.  They
aren't actually 10 MHz OCXOs, but doubled 5 MHz units, so there's a lot of
extra circuitry to add noise between the oscillator stage and the output
jack.  On the upside, this makes them relatively insensitive to load pulling
and injection locking.

Bob is right in that any scheme that involves multiplying 10 MHz directly to
X band is going to result in a relatively noisy output carrier, of course.
In the 8566B they multiply 10 MHz to 100 MHz, then to somewhere around 400
MHz, and only then does the sampler loop multiply that to reach 2-6 GHz. 

-- john, KE5FX
Miles Design LLC

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