[time-nuts] Looking for Wavecrest Visi

[Magnus Danielson]

From: SAIDJACK at aol.com
Subject: Re: [time-nuts] Looking for Wavecrest Visi
Date: Sun, 8 Apr 2007 21:12:52 EDT
Message-ID: <cb8.f427e8a.334aed14 at aol.com>

Said,

> 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.

This is to be expected as the slew-rate of the "CMOS" signal is higher than the
sine of the same frequency.

> > 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.

What you measure when you measure the instruments timebase is a filtered
variant of its phase-response. You get a high-pass filter effect due to the
time-delay difference, but it also contains a bunch of nulls in the response
which is to be expected. Thus, the noise you measure is being reduced. Also, as
for systematic errors you measure the noise-out average of a certain point of
the interpolator, not the full range.

> 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.

No. It is better than feeding the internal timebase to a single input.

> 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.

Cool!

> 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.

For that measurement, yes.

> 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.

Handy number to have in memory! Thanks! :-)

> Not sure many other instruments can do that.

You should be able to get similar result on a network analyzer.

> Will report raw capture data once I have the software running.

Keep us posted.

Don't you have the programmers manual so your can set up the GPIB yourself?
You should be able to do that in modern OSes.

Cheers,
Magnus