[time-nuts] Looking for Wavecrest Visi

SAIDJACK at aol.com SAIDJACK at aol.com
Mon Apr 9 01:12:52 UTC 2007


 
In a message dated 4/8/2007 16:49:00 Pacific Daylight Time,  
bruce.griffiths at xtra.co.nz writes:

Since  Wavecrest, 53132A etc have no specifications for the effect of the 
input  circuit noise with a finite slew rate input, the only way to make 
a more  precise comparison is to actually make some measurements. The 
integral and  differential nonlinearity of the Wavecrest do not seem to 
be specified,  nor are the channel delay mismatches. Are thes internally  
calibrated?



Hi Bruce,
 
yes, the unit calibrates out the inputs using two reference signals  that are 
swapped during the measurement. All that is needed are two SMA cables,  and 
two SMA grounding plugs. Best of all, the internal calibration only consists  
of one screw for the Vectron 100MHz OCXO, and the power supply voltage  
adjustments. All other calibrations are done in software automatically.
 
I did see some jitter differences when feeding square waves versus sine  
waves into the unit. This was more pronounced on the newer SIA3000 units. I was  
doing the tests with our Jackson-Labs Fury reference GPSDO which has both Sine  
and CMOS outputs, the CMOS outputs having slightly less jitter.
 
> Wavecrest is likely to have a  trigger jitter ~ 10ps rms (when  the input 
> comparator noise is taken into account with the finite input  sinewave 
> signal slew rate)
 
Not so, it's better: when measuring the internal 100MHz reference (there is  
a Sine-Wave output with -4dBm 100MHz in the back) then the RMS jitter  is 
about 2.7ps, this doesen't change much from 5 to 1000 sample averages. This  is 
about the number I get from other good 10MHz OCXO sources as well. It's in  line 
with what the Wavecrest reps said the timebase typically can do.
 
Once I get the Windows software running, I was planning to split a signal  
using a power splitter, delay one side of the signal with a longer cable, and  
feed both inputs into the A to B measurement. That should give a  
source-independent value for all internal noise sources.
 
For now, here is a hint of the precision that is achievable:
 
In cable-length measurement mode, the unit uses its' two reference outputs  
to generate two 200MHz sine waves. these are feed via two SMA cables to the two 
 inputs, and the unit calibrates itself to 0.0ps cable length.
 
Then, one can insert an additional cable into one of the two feeds to  
measure the electrical cable length of this added segment.
 
The LCD display updates the measurement about 20-30 times a second (guess)  
and the values do not jitter more than about +-300 femtoseconds over a period 
of  several seconds. I would guess they use internal averaging to get to the 
number  the LCD is displaying since the resolution is "only" 800 femtoseconds.
 
Now one can slowly unscrew one of the SMA connectors effectively enlarging  
one of the cable lengths by very small amounts.
 
By doing this, you can actually observe the measured value increase very  
slowly, one can even observe the sub 1ps values increase! Doing this, you can  
see about 3ps of added delay for every single turn of the SMA connector ground  
nut.
 
Not sure many other instruments can do that.
 
Will report raw capture data once I have the software running.
 
bye,
Said 
 
 
 
 
 



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