[time-nuts] Looking for Wavecrest Visi

Magnus Danielson cfmd at bredband.net
Sun Apr 8 22:13:09 UTC 2007


From: SAIDJACK at aol.com
Subject: Re: [time-nuts] Looking for Wavecrest Visi
Date: Sun, 8 Apr 2007 15:42:15 EDT
Message-ID: <cd2.eb71859.334a9f97 at aol.com>

Bruce, Said,

> 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).

Indeed. They directly interpolate each trigger event to 800 fs (DTS-2075),
200 fs (SIA-3000) or 300 fs (SIA-4000). The trigger noise, variations due to
environmental etc. eats on the accuracy as expected. How these instruments work
is fairly open, it is a traditional charge-cap-and-A/D the time-error converted
into voltage. The SIA-3000 for instance uses a 200 MHz coarse clock and then
interpolates with a factor of 25000. Interpolation like that allows for a 1-2
clock interpolation and a standard 16 bit DAC ought to do it. I haven't checked
exactly which A/D they are using.

> 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.

Indeed. These instruments fall into the SR-620 category and not the 53132A
category in the terms of that article. The 53132 is an HP style el cheapo. It's
not a bad instrument, but it doesn't cut as deep down as you can be fooled to
beleive, and this is the point of that article. That point is however not at
all relevant for the Wavecrest devices.

> 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.

Certainly.

Cheers,
Magnus




More information about the Time-nuts_lists.febo.com mailing list