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

[EWKehren at aol.com]

Use a MV89 or something like it and lock it to the 5680A with a 10 to 100  
Hz analog filter. You will like the results at 10 GHz.
Bert Kehrenn
 
 
In a message dated 2/7/2012 12:48:57 P.M. Eastern Standard Time,  
turner at ussc.com writes:

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


_______________________________________________
time-nuts  mailing list -- time-nuts at febo.com
To unsubscribe, go to  
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the  instructions there.