[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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