[time-nuts] what is acceptable harmonic content & level for a 10Mhz standard?
paulswedb at gmail.com
Tue Jan 26 16:40:30 EST 2016
Walter that OCXO sure looks odd I would expect unfiltered to be a lot
better then that. I would guess that several possibilities exist.
The actual crystal is a 5 MHz and it feeds a doubler.
The chain has something thats gone non-linear. -5dbm seems low to me.
Oscilloquartz have a very fine reputation on time-nuts. I do not own one
nor most likely never will.
So you are right to ask and suspect.
On Tue, Jan 26, 2016 at 2:52 PM, walter shawlee 2 <walter2 at sphere.bc.ca>
> 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
> 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
> I drove everything from some independently, heavily post-filtered +15VDC
> 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 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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