[time-nuts] Sub Pico Second Phase logger

[Joe Gwinn]

Bruce,

At 10:16 PM +0000 12/23/08, time-nuts-request at febo.com wrote:
>
>Message: 1
>Date: Wed, 24 Dec 2008 08:47:21 +1300
>From: Bruce Griffiths <bruce.griffiths at xtra.co.nz>
>Subject: Re: [time-nuts] Sub Pico Second Phase logger
>To: Discussion of precise time and frequency measurement
>	<time-nuts at febo.com>
>
>[snip]
>  >>>>>
>  >>>>  Using a DDS avoids the requirement for a pair of low phase noise VCOs.
>  >>>>
>  >>>  If we can control the spurs, many DDS chips are very good.
>  >>>
>  >> [BG] A DDS, unlike a conventional digital frequency divider, 
>doesn't suffer
>  >> from aliasing of phase noise into the output passband.
>  >
>>  [JG] How true is this, in practice?  A DDS is at the mercy of phase noise
>>  in its reference clock, by much the same mechanism as for a simple
>>  divider chain.  And the variable-factor dividers (the M and N above)
>>  work in a manner similar to a DDS, but with far coarser increments
>>  and limits.  Both DDS and M/N PLL chips use a PLL to clean up the
>>  resulting ref signal.  Many DDS chips incorporate a M/N PLL to
>  > multiply the ref frequency.
>  >  
>For NIST's measurements of this effect see:
>http://tf.nist.gov/timefreq/general/pdf/1380.pdf

Thanks for the reference.  Very interesting paper, although speckled 
with typos and gaps.   For instance, equation 5 is not grammatical, 
and appears to have been mangled.  There is a later and longer IEEE 
paper by the same authors that I'll get in January.

One would have expected this paper to have appeared 15 or 20 years 
earlier.  I guess the effect wasn't elucidated until many people 
tried to lock microwave sources to 10 MHz.

It also shows how to avoid the aliasing effect: Put an antialiasing 
filter and comparitor between stages.  This is what Ascarrunz does in 
his US patent 6,278,330, specifically Figure 4.  (This patent is what 
ref 3 of paper 1380 became.)


>[snip]
>  >> The simplest way of achieving the required performance is preferable.
>>>    
>>
>>  Yes, but aren't we Time Nuts?
>>
>>
>Usually with finite budgets.

Few dollars, many hours.


Joe