[time-nuts] Question about precise frequency / phase measurement

[Bruce Griffiths]

Attila Kinali wrote:
> On Thu, 19 Apr 2012 23:39:32 +0200
> skywatcher<skywatcher at web.de>  wrote:
>
>
>    
>> BTW i'm using the
>> Parallax 'Propeller' controller
>> which has 8 cores running at 80 MHz each, and can measure time intervals
>> with 12.5 ns resolution.
>>      
> [...]
>
> Looks like an interesting thing. But also very specialized.
> If you have already experience with it, ok. But if you start
> from scratch i would recomend to use one of the Cortex-M3
> or Cortex-M4 processors. There is a lot more knowledge available
> for these and you get a lot of tools just for free.
>
>
>
>    
>> I think the DDMTD could be a good solution.  The question is, if 74HCxx
>> parts would be good enough
>> to get<  1 mHz resolution for a 10 MHz frequency with an update rate of
>> <  1 sec.
>>      
> That's a simple calculation. 1mHz of 10MHz is a precision of 10^-10.
> Sampling at a second you need a time resolution of 1s*10^-10 = 0.1ns.
> Modern CMOS (ie not HC/HCT) have a jitter in the region of 1-5ns.
> (ECL are in the region of a couple 100ps)
>    
Not in this application, where DDJ and other pattern dependent jitter is 
absent.
4ps per inverter of flipflop is more typical for HCMOS.
> Ie, if you build a DDMTD with just HC/HCT, your jitter will be dominated
> by the logic circuit and you have to average several samples to get
> below the 10^-10 you want. ECL would be definilty better.
>
>    
Try it and you'll be pleasantly surprised.
CERN do much better than you speculate (White Rabbit project)
> Also keep in mind, that in this region of precision, you have to model
> your digial circuit partially as analog. Especially taking into account
> that you have a a finite rise time, input and output jitter, power supply
> noise, signal noise etc pp. All these will limit the precision you will
> achieve for single measurements.
>
>    
Yes, but in practice is easy to achieve hitter about 2-3 orders of 
magnitude lower than your speculation in a DDMTD.
> 			Attila Kinali
>
>    
Bruce