[time-nuts] what is acceptable harmonic content & level for a 10Mhz standard?

Bob Camp kb8tq at n1k.org
Tue Jan 26 23:06:52 UTC 2016


If your OCXO is putting out less than 0 dbm, it’s broke. Given the way these parts are treated
in the salvage loop, damage is not unusual. There are a *lot* of examples of surplus OCXO’s
with low output. Normally it’s a damaged output cap or something like it.  I’ve seen roughly half the units
in some lots come in that way ….


> On Jan 26, 2016, at 2:52 PM, walter shawlee 2 <walter2 at sphere.bc.ca> wrote:
> I have been working on a compact portable 10Mhz bench standard
> using both an FE FE5680A Rb oscillator and an Oscilloquartz ovenized
> crystal oscillator.  I was curious to see how their short and long term stability
> would work out, and I needed something to drive my 6502 amp to pipe the signal to some bench instruments which could benefit from better master references.  all of these were inexpensive ebay acquisitions, so the total cost to make the package is maybe $250 plus a few weekends work and a left-over but nice looking tilt-bail case & Condor 65W switching power supply.
> I drove everything from some independently, heavily post-filtered +15VDC power.
> to try and keep any unexpected interaction to a minimum, the outputs exit though floating BNCs, so they don't have an unwanted ground loop at the case.  only the FE5680A is physically grounded to the case for heatsinking, so the connection is a single point.
> while looking at the outputs of the two sources, I saw some surprising 20Mhz harmonic content on my spectrum analyzer.  I expected a bit from the 5680A, but it was actually much bigger on the OCXO, which had a chinese add-on dual buffer board attached.  I had thought there might be some, so I also scored some little 10Mhz BP filters at the same time and did some before/after tests.
> here's what I got:
> FE5680A 10Mhz (+5dBm), 20Mhz (-55dBm)  (with filter)
> OCXO       10Mhz (-2dBm), 20Mhz (-35dBm) as received
> OCXO       10Mhz (-5dBm), 20Mhz (-60dBm) with filter
> I hate to distribute any spurious RF around the lab, so I thought less harmonic content was the better plan, although I realize that as a counter time base source, it is probably irrelevant, but might play a role in my 8656B RF generator as added unwanted spurs.  I was a bit disturbed by the low level and harmonics from the OCXO assembly, so I thought some external comments about norms were needed at this point before I go further.
> what kind of harmonic content does everyone else see in their 10Mhz standards?  the -5dBm level seems too low to me, I am used to 0 to +5-7dBm from sources, but I have not tried to run it through the distribution amp yet, so possibly that's enough.  I can try and dig into the totally undocumented surface mounted chinese add-on board to improve things, and increase the level, but hard to say what will happen there.
> all comments and data welcome.
> 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
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