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

walter shawlee 2 walter2 at sphere.bc.ca
Tue Jan 26 19:52:41 UTC 2016

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 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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