[time-nuts] frequency reference for portable operation

Bob Camp lists at rtty.us
Mon Mar 11 19:50:16 EDT 2013


A few numbers on the 10 (or 5) MHz to 100 MHz stuff:

The most likely frequency range is 100 Hz to 100 KHz. That covers CW operation/filters as well as reasonable adjacent channel issues on SSB. 

At 10 KHz and beyond you likely will be at floor on your 100 MHz oscillator. With a good design that will be in the -165 to -175 dbc/ Hz range. The offset between 10 MHz and 100 MHz would be 20 db in this range. That would require a -185 to -195 dbc / Hz 10 MHz OCXO to be "as good as". The normal DOCXO's run in the -160 range. The 100 MHz likely wins by > 20 db.

At 100 Hz, you are doing pretty well to hit -125 dbc / Hz. Some luck will get you to -135 dbc / Hz.  At 100 Hz, you are likely to get some noise from your multiplier. A 23 db offset is probably a good guess here. That puts your 10 MHz OCXO in the -148 to -158 dbc / Hz range. The TBolt OCXO will hit these sort of numbers. Most DOCXO's run out around -145. At best you are same / same with the multiplier. Most likely you are 10 db better with the 100 MHz. 

Between 100Hz and 10 KHz it's going to be a "that depends" sort of thing. The 100 MHz will always win. It's a question of ho much it'll win by. 

The simple approach is to set up a PLL with a 3db (not Wn) loop bandwidth in the 10 to 20 Hz range. That should keep the 10 MHz well below the 100 MHz by the time you get to a 100 Hz offset.  


Next step is to run up from 100 to something and then from there to 10+ GHz. If you get to the next step by multiplying, then the -175 dbc phase noise might be useful. If you are running into some sort of phase detector or most SRD's then you may not get the full benefit of noise that far down. Without knowing what the next step is going to be, you really can't fully spec out the 100 MHz oscillator. 


On Mar 11, 2013, at 4:20 PM, John Miles <jmiles at pop.net> wrote:

>>> 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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