[time-nuts] more is revealed about OCXO harmonics & level

Bob Camp kb8tq at n1k.org
Thu Jan 28 00:26:16 UTC 2016


> On Jan 27, 2016, at 6:33 PM, walter shawlee 2 <walter2 at sphere.bc.ca> wrote:
> I decided to rip the add-on board apart, and go right to the oscilloquartz 8633 output.
> no surprise, >0dBm level, but alas, 20Mhz harmonics are only -35 dB down, and there's one at 30, etc..  hmm, what does this really mean?  well, when in doubt, go back and look at the data:
> http://www.sungwhatech.com/product/pdf/aging/8663.pdf
> rats, *the spec is only -25dBc for harmonics*.  phase noise looks good, as does stability, but
> now I understand why I am seeing these spurs, it's in spec!  it does look like my level is out of spec though, I cannot get +4dBm even right from the module, perhaps that's why these units became surplus.  the date code on my OCXO 8663-XS is 2008.  I cannot identify what the X part of the dash number means, maybe lower output with my luck?

The X probably means “OEM spec”. That could cover anything.

> to make the board a bit safer to use, I made up a resistive splitter with 2 51R carbon resistors, and slipped ferrite beads on the leads to help control high frequency harmonic content.  I can now run the counter and SA at the same time, and get maybe -2dBm outputs, closer to 0dBm if I use only one.
> if I add my outboard 10MHz bandpass filter, harmonics drop into the grass at -60dBm, but level goes to -5dBm.

The output may or may not have a pad between it’s output filter and the pin you are working with. If there is no pad, anything you hang on the output will interact with the internal filter. The result is best described as “unpredictable”. The solution if you have a lot of output is to put on a pad. Since you have low output, an amp and a filter are your best bet. 

> clearly not as nice as the FE5680A Rb unit, but I will leave it in there just for comparison.

It likely is *much* better in terms of phase noise, spurs (close in), and ADEV at < 100 seconds. 

> I think I will filter one output, and leave one raw, since I have 2 jacks already at the front for outputs. but this was very instructive, I really did not expect to see so much harmonic content from a precision OCXO sinewave crystal oscillator.  a quick cross check on some of my other hp/FE ovenized standards in the lab does not show this kind of unwanted content.
> what kind of results does everyone else get?

On the Chinese stuff:

A lot of what I get has maybe -7 dbm out or gets really hot…The ones that don’t fail at that
point don’t meet any sort of reasonable spec when run over temperature. 

On the North American stuff:

They seem to be fine on output, temperature and usually do pretty well over temperature. 
The only knock on them has been ADEV. 


> all the best,
> walter
> -- 
> Walter Shawlee 2, President
> Sphere Research Corporation
> 3394 Sunnyside Rd.,  West Kelowna,  BC
> V1Z 2V4  CANADA  Phone: (250) 769-1834
> walter2 at sphere.bc.ca
> WS2: We're all in one boat, no matter how it looks to you.
> Love is all you need. (John Lennon)
> But, that doesn't mean other things don't come in handy. (WS2)
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