[time-nuts] Looking for Wavecrest Visi

[Dr Bruce Griffiths]

SAIDJACK at aol.com wrote:
>  
> 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.
>   
Said

This is probably due to the fact that the CMOS gates have lower 
bandwidth and input noise than the Wavecrest input comparators.
For a given fixed signal frequency there is, particularly for lower 
frequencies, a more optimum signal conditioning circuit than a simple 
comparator that will minimise the output timing jitter of a logic level 
square wave. However such circuits require very good temperature control 
and are optimised for the known input 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.
>   
We are not comparing the same thing here, the amplitude and slew rate of 
the input signal are important.
With a 100MHz signal of amplitude >= +7dBm. one would expect the 
internal noise (~ 3ps rms) to dominate.
The observed noise with a 10MHz input signal will depend on the signal 
amplitude and the (unspecified) input comparator noise.
I assumed around 300uV rms as a reasonable guess for the (unspecified) 
input noise bandwidth (> 1GHz ??)
It could be somewhat lower. Using a lower amplitude, known low slew rate 
signal should make this dominant and would be useful in getting some 
idea of the actual effective value of this noise. You could try 
inserting an attenuator between the 10MHz OCXO output and the Wavecrest 
to obtain a lower slew rate input signal so that the this noise may be 
determined.
Once the effective input comparator noise is known it is then possible 
to make a rational choice between counters particularly for lower input 
frequency low slew rate signals.
>  
> 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.
>   
Only if the delay isn't too long for the particular source's noise 
characteristics.
Otherwise you've just built a delay line discriminator.
>  
> 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.
>  
>   
Unlikely to update the actual display at that rate (20-30 times per 
second - 20-30 times a minute is more likely.) as it would become 
unreadable, particularly the least significant digits. If the pp display 
jitter is about 600fs and the input pp jitter is about 25ps then about 
2000 measurements need to be averaged to achieve this.
> 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.
>  
>   
There's no particular reason that they cannot if they have adequate 
resolution and stability and a sufficient number of measurements are 
averaged.
This should be possible even with an HP5370A/B albeit with a slower 
response time.
> Will report raw capture data once I have the software running.
>  
> bye,
> Said 
>   
Bruce
>  
>  
>