[time-nuts] Looking for Wavecrest Visi

[Dr Bruce Griffiths]

SAIDJACK at aol.com wrote:
> Hi Bruce,
>  
> again, I did say in my original post that my math only works if their noise  
> is Gaussian, and that I did not know if it was.
>  
> Also, please note that they do claim 25ps Accuracy , and 800 femtoseconds  
> Hardware resolution, not the other way around.
>  
> They claim +-25ps accuracy with 3-sigma confidence. The user manuals  are 
> available on the Wavecrest website.
>  
> There is no mention in their documents that averaging is needed to make use  
> of their 800 Femtoseconds hardware resolution; quite the contrary in some 
> older  white-paper documents they show how they achieve this single-shot 
> resolution  (when looking inside one of these units it's quite amazing  technology).
>  
> Keep in mind that these instruments are on a totally different level than  
> the 53132A or SR-620: these units cost anywhere from $50,000 to $90,000  new.
>  
> BTW: the only point I was trying to make is that you sometimes can get  these 
> things for <$500 and they are a lot better than the above mentioned  
> instruments for time-interval measurements.
>  
> Bye,
> Said
>   
Said

The simplest way of comparing the counter specifications is to compare 
their rms measurment  noise with say a +7dBm 10MHz input signal (eg from 
an HP10811).

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)
HP53132A has a trigger jitter ~10 ps rms (difficult to work out from the 
specifications as the input circuitry noise isn't specified however 
internal jitter (3ps) is comparable to the Wavecrest)
SR620 will have a trigger timing jitter ~ 8ps
HP5370A/B has a trigger timing jitter ~ 35ps (dominated by the internal 
hardware jitter trigger jitter is about 3.3ps rms with this input signal.

WAVECREST
The 800fs hardware resolution is largely irrelevant when the input 
circuit produces a jitter of that is 12x larger than this. As long as 
the resolution is better than the rms timing jitter it will not have a 
significant effect on the measurement noise (~ 10ps rms).

53132A
Since the hardware resolution of the 53132A (~ 150ps) is much larger 
than its timing jitter (10ps) with this signal, the resolution will 
dominate the measurement noise (~45ps rms). Its a pity that the 
resolution is so poor otherwise the performance could have been 
comparable to the Wavecrest.

SR620
Since the hardware resolution of the SR620 (~ 25ps typical ) is 
comparable with its trigger timing jitter (8ps) with this signal, both 
will have a significant contribution to the measurement noise (~11ps 
rms). Its a pity that the resolution is so poor otherwise the 
performance could have been comparable to the Wavecrest.

5370A/B
The 20ps hardware resolution has little effect on the measurement noise 
(35ps typical).

Thus for this signal the Wavecrest is perhaps only about 4.5x better 
than the worst of the other counters, not >100x as you claimed earlier.
With higher frequency and or higher slew rate input signals the 
Wavecrest performance should be somewhat better.


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?

Counter internal delay channel mismatches and other systematic errors 
(as in the HP5370A/B and the SR620 etc) can be  measured and corrected. 
However such constant channel delay mismatches have little or no effect 
when measuring the Allan variance of a source, nor do integral and 
differential nonlinearity over portions of a counter's range that are 
not used in such measurements.

If one were to design/build ones own time interval counter, then a 
resolution of 10ps is possible using a CMOS TDC, whereas subpicosecond 
resolution should be possible using dual ADC's sampling a pair of 
~200MHz sine waves in quadrature phase. Comparator jitter of around 
100-200 fsec is possible with a input signal with a sufficiently large 
slew rate.

NB it has been assumed that the input signal noise is negligible (< 50uV 
rms).
The effect of timebase reference noise has also been neglected. This 
should be negligible if a 1 period (of the input signal) time interval  
measurement is made.

Bruce