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

Bob Camp lists at rtty.us
Thu Feb 9 00:40:15 UTC 2012


Hi

Here is a little more on how much of a problem you have.

If you would like the spurs to be down 70 dbc at 10 GHz. They go up by 20 log N. in this case N is 1000. That gets you 60 db. Spurs at 10 MHz would have to be down at -130 dbc to make it at 10 GHz.  

If you want noise over 10 KHz to be 60 db down, it goes by 10 log BW. That gets you to 1 Hz noise at -100. The same 60 db to 10 MHz then applies. You would need -160 dbc phase noise at 10 MHz to hit that.

Both of those would be hard to hit with any Rb. Fortunately you can use a multi step multiplication chain. Once you make that decision, noise and spurs on the Rb are not a big issue.

Bob




On Feb 7, 2012, at 1:52 PM, John Ackermann N8UR <jra at febo.com> wrote:

> I am just finishing my promised stability and phase noise measurements on a batch of inexpensive Rb standards; I hope to publish the results tomorrow evening.
> 
> In the meantime, I've looked at two of the FE-5680s and their phase noise is significantly worse than either the Efratom FRS or the Datum LPRO -- in particular, there is a forest of spurs all the way from 1 Hz on out, most of them at around -80dBc or worse.  By the time you multiply that to 10 GHz, that's only about 20 dB below the carrier! Apart from the spurs, the noise floor is significantly higher than the other two types.
> 
> A clean-up oscillator would be an interesting add-on.
> 
> John
> ----
> 
> 
> On 2/7/2012 12:48 PM, C. Turner wrote:
>> Hello,
>> 
>> As has been mentioned here before, the output of the "non-tunable"
>> FE-5680A's has been noted to have low-level spurs in it - no doubt due
>> to the way the various frequency loops are derived within, some using
>> DDS techniques. It is for this reason that when I packaged my FE-5680A
>> in its own, stand-alone enclosure I included a fairly narrow band (+/-6
>> kHz @ -6dBc) crystal-based bandpass filter in the output.
>> 
>> After more recent testing of two FE-5680A's using two different 10 GHz
>> microwave transverters, I've determined that this filtering just isn't
>> enough. At first, it was assumed/hoped that the racket that I was
>> hearing was coming from somewhere else - perhaps the switching
>> up-converter or some other interaction - or just something "odd" about
>> my homebrew 10 GHz transverter, but this is, unfortunately, not the case.
>> 
>> In testing with a DownEast Microwave 10 GHz transverter fitted with an
>> N5AC synthesizer, the CW notes sounded nice and clean when locked to the
>> Z3801 and there was only a trace of modulation that I'd not really
>> noticed before when I used the Efratom LPRO-101, but when the '5680A was
>> connected, the incidental PM was bad enough that it was difficult to
>> determine where, exactly, zero beat was! Since the synthesizer uses a
>> fairly high reference frequency internally there was little impediment
>> to the low-level phase modulation on the reference.
>> 
>> I compared this with my own homebrew 10 GHz transverter. This unit uses
>> an 18.4 MHz Butler VCXO that is multiplied to 110.4 MHz which is then
>> fed to a "brick" oscillator with the 110.4 MHz being compared to the 10
>> MHz reference using a harmonic mixer, locking to the 400 kHz residual.
>> Since this unit has a comparatively low loop bandwidth in the VCXO the
>> grunge was considerably reduced, but still objectionable, giving some
>> hope that a simple VCXO scheme might make the '5680A usable.
>> 
>> I still have yet to do a more-detailed analysis of the phase modulation
>> that is appearing on the 10 GHz signals, but I can clearly hear a low
>> frequency modulation source (perhaps the lock-in amplifier) plus a
>> myriad of other audio frequency components and their harmonics. Again,
>> with the LPRO-101 was very "clean" by comparison and I could *just* hear
>> some similar, very low-level noises in the background that I'd not
>> really noticed before.
>> 
>> As it is, the '5680A-based reference is unusable with the N5AC
>> synthesizer and its wide loop bandwidth and "almost" usable with my
>> homebrew transverter and its comparatively narrow loop bandwidth. I'm
>> now bent on making the '5680A usable as a microwave reference, but my
>> current plans are to build a simple 10 MHz Butler VCXO and then lock it
>> to the '5680A using a very "slow" loop filter: In that way, I'm hoping
>> that the phase noise will be largely that of the 10 MHz VCXO and its
>> cheap CPU-type crystal rather than the '5680A!
>> 
>> Clint
>> KA7OEI
>> 
>> 
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