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

[paul swed]

Clint a good read and a fine approach.
By accident I looked at the original filter schematic first and believe it
has an error for the first output amplifier. The 470 ohm resistors in the
wrong location.
Others on the list have mentioned the same thing about the 5680s output
being dirty. You have that fixed up nicely.
Regards
Paul
WB8TSL

On Mon, Feb 13, 2012 at 4:03 PM, C. Turner <turner at ussc.com> wrote:

> 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:
>
> http://www.ka7oei.com/10_MHz_**Rubidium_FE-5680A.html<http://www.ka7oei.com/10_MHz_Rubidium_FE-5680A.html>
>
> 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
> analysis.
>
> 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.
>
> 73,
>
> Clint
> KA7OEI
>
>
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