[time-nuts] FE-5680A's suitability for use as a 10 MHz reference for microwave transverters

Rob Kimberley robkimberley at btinternet.com
Tue Feb 14 13:56:37 UTC 2012

Nice job!!


-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of C. Turner
Sent: 13 February 2012 21:03
To: time-nuts at febo.com
Subject: Re: [time-nuts] FE-5680A's suitability for use as a 10 MHz
reference for microwave transverters

Last week I noted that the FE-5680A's "barefoot" output was found to NOT be
a suitable 10 MHz reference for microwave transverters.  
Specifically, I tested it on two different 10 GHz transverters and found
there to be objectionable levels of "grunge" on signals caused by low-level
phase modulation internal to the '5680A and at 10 GHz the result of this
phase modulation was a racket of audible and subaudible noises on CW
carriers that made it difficult to find zero beat!  In comparison, the 10
MHz outputs of the Z3801, Isotemp VCXO and LPRO-101 yielded results at 10
GHz that were quite clean.  Related observations were also made by N8UR in
his web page comparing various units.

In order to clean up the output of the FE-5680A I did the obvious thing,
disciplining a homebrew VCXO to its output - details are found here:


While the comparison frequency is fairly high (1.25 MHz) the loop gain and
bandwidth are quite low so it's pretty much the Butler oscillator VCXO that
determines the phase noise of the 10 MHz output and I can't detect any
audible artifacts from the '5680A at all.  At the moment I don't have the
means of generating a "pristine" test signal at 10368 MHz, but from what I
can determine, the resulting CW notes from the transverter (being locked to
the "regenerated" output of the the '5680A) compared to the other 10 MHz
sources sound the same.  At some point I hope to do a more-detailed

Had I a low-noise "canned" 10 MHz VCXO around, I'd have probably used that
rather than go through the hassle of building the oscillator, but none of
the 10 MHz VCXOs that I *did* have on hand produced as good a CW note as the
Butler built around a cheap microprocessor-type crystal.  I also had on hand
some 10 MHz ovenized VCXOs which would have worked fine, but not only were
these too large to fit in the box, they would have added even more current
consumption to an already power-hungry frequency source - an important
consideration when operating from a battery!

There are, no doubt, a number of ways one could do this same thing, but it's
clear that this simple of a circuit will do an admirable job of extracting
the frequency stability of the FE-5680A without the synthesis-related
artifacts.  Of course, the regenerated 10 MHz output will have a degree of
variable phase offset with respect to the '5680A's "barefoot" 10 MHz output
over varying conditions (such as temperature) but when used only as a
frequency reference these rather slow changes are unimportant.



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