[time-nuts] 9390 GPS RX

Ed Palmer ed_palmer at sasktel.net
Tue May 28 21:15:10 EDT 2013


I have some good news.  I decided to copy your test by powering up an 
FRK that's been sitting in a box for over a year.  Once it locked, I 
measured the frequency and was surprised to find that it was about .06 
Hz (6e-9) low.  I added about 890 ohms between the two external 
frequency adjustment leads and got an error of about 7e-11.  So your 
unit might be in much better shape than I thought. Sorry if I misled you 
on that.

I'm also looking at the Allan Deviation.  It's only been running for a 
few hours, but one thing that I noticed is that the frequency is 
drifting as the unit warms up and works the kinks out.  This drift 
degrades the AlDev numbers by looking like aging or a high noise floor.  
Try using Hadamard Deviation instead of Allan Deviation. Hadamard is 
insensitive to aging.  So far, my AlDev values aren't much different 
than yours but as I run the graph out beyond 800 sec., the numbers do 
get a little better.  Also, remember that most Rb standards aren't 
optimized for low AlDev values since most applications don't need that.


On 5/28/2013 4:10 PM, Ed Palmer wrote:
> The frequency offset of 3e-9 is still very high.  Since the 9390 will 
> discipline the FRK, the C-field adjustment of the FRK doesn't really 
> matter.  Before you open up the FRK again, see if the C-field can 
> change the frequency.  If it can, and if the range is similar to the 
> specified range, then the C-field is working and there's no point in 
> opening the FRK.
> As for your AlDev numbers, in no particular order.....
> Have you measured the noise floor of your measurement system?
> Check the power supply for hum and noise.
> Is the crystal control voltage stable or is it noisy?
> A weak Rb lamp might give you a noisy signal, even though it's 
> locked.  Is the lamp voltage noisy?
> Check for a bad cable or some other bad connection.  If possible, use 
> only double-shielded cables.
> Running down problems like this can drive you crazy.  The levels are 
> so low that it could be almost anything from the computer you're 
> running Timelab on to your neighbor's wireless phone!
> A good OCXO often takes a month or more to stabilize after you pick it 
> up in an auction.  Rb standards aren't usually that leisurely about 
> settling down but I'd just let it run and see what happens. This might 
> be a good time to put it back in the 9390 and see what happens.  Does 
> the 9390 have commands that will show you how well the FRK is performing?
> You also might have a unit that's working properly but is a little 
> noisy.  The specs don't cover something like this.  You're well 
> outside the time for 'short term stability' but haven't yet got to the 
> 'aging' spec.  If you're using the 5370 by itself, you can't measure 
> the short term stability at all.
> Ed
> On 5/28/2013 11:37 AM, Mark C. Stephens wrote:
>> Ed and Bob,
>> The FRK has settled down to 0.03Hz out after 2 days undisturbed (back 
>> in its black box).
>> Allan deviation @ 800 seconds is 4.88E-12 which I am not terribly 
>> happy about.
>> -mark

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