[time-nuts] Fury Realhamradio listing

[Dr Bruce Griffiths]

SAIDJACK at aol.com wrote:
> Hi Bruce,
>  
> never doubted that it was technically possible to get this type of  
> resolution/accuracy. I myself mentioned the 15 year old Wavecrest units achieve  800 
> femtoseconds resolution, single shot.
>  
> The point was
>  
> A) that type of resolution is not needed in a TI unit where the intrinsic  pk 
> to pk noise on the TI intervall is >100ns (more than three orders of  
> magnitude above 100ps).
>  
> B) that implementing it with that kind of resolution and getting a  
> meaningful accuracy (say 250ps 6-sigma accuracy) is not easy while at the same  time 
> keeping the cost to "Three-four transistors and a handfull of caps and  
> resistors." In mass production a handfull of caps and transistors/resistors cost  less 
> than $0.20.
>  
> Again if it was that easy and cheap, HP would have done it in their 5334A's  
> or even the 5335A for example which have 1 or 2ns resolution I  believe
>  
> SRS would have given us 100ps resolution on their PRS10 time-stamping input  
> - what better place to do it than in a highly-accurate frequency  reference.
>  
>   
Said

Actually the interpolator in the PRS10 has 200 picosec resolution.
However they have not provide a means of accurate autocalibration of the 
interpolator offset and gain.

>> The reason that the 53132A doesn't have resolution and accuracy better  
>> resolution than 150ps, is that a design choice was made to implement it  
>> all (counters plus interpolators) in a CMOS chip using the delay of a  
>> CMOS inverter to set the resolution. This reduces the cost  and 
>> complexity significantly and allows faster cycling of  the interpolator 
>>     
>
> Bingo. QED.
>  
> People choose not to do 100ps resolution in their  products because of cost 
> and complexity, even in >$3K products such as the  53132A - let alone in $750 
> products.
>  
>   
Not exactly the point, the entire counter complexity and PCB area was 
reduced by using a single chip for the counters etc., and it was easy to 
incorporate an interpolator with sufficient resolution for the target 
market. One doesn't usually add a synchroniser with more than the 
required resolution even if it only costs a few dollars, if a device 
with adequate resolution can be obtained at very little added cost when 
its incorporated within the counter chip itself. Having decided to 
implement the counter in a CMOS chip, the  jitter due to internal cross 
coupling and noise within the chip would have  made it difficult to 
achieve a usable resolution much better than the 150ps actually achieved 
even if a cheap high resolution external interpolator were used.
> C) I don't believe the Z3801A has 100ps single shot resolution and accuracy  
> (for resolution doesn't do anything without accuracy) until someone will  
> prove it to me. And even then it would be wasted resolution since the GPS 1PPS  
> source noise will totally swamp out any benefit a 100ps resolution would  give.
>  
> On top of that, all GPSDO's do heavy averaging of this time intervall, with  
> a PRS10 typically doing 7 hours or more of averaging. 100ps per-second  
> resolution in that kind of averaging window is meaningless, since the OCXO  cannot 
> perform that well - it would require 4E-015 stability in a 7 hour window.  Not 
> possible without a high-end Cs/Rb/H source. Certainly not possible with  the 
> 10811 that's inside a Z3801A.
>  
> Still hoping someone knows the TI hardware used in the Z3801A's...
>  
> bye,
> Said
>   
Bruce