[time-nuts] DMTD mixer question

Bruce Griffiths bruce.griffiths at xtra.co.nz
Thu Dec 4 01:04:38 UTC 2008


pablo alvarez wrote:
> thanks for the positive feedback,
>
> In xilinx fpgas, for example, the recovery time after a metastability
> issue is quite fast as reported in this paper
>
> http://www.xilinx.com/support/documentation/application_notes/xapp094.pdf
>
> The capture window of metastable state is 0.01fm (page 2). Probably
> this window is moving around a bit, but this 0.01fm sounds promising.
>
> In page 3 "When granted 2 ns of extra settling delay, the problems
> caused by metastability are almost eliminated, as their MTBF exceeds
> millions of years."
>
> So we can just solve the glitch problem by adding a shift register as
> Bruce suggested.  I think the major concern may be crosstalk, but
> using lvds or ecl logic and placing the IOs far away one from each
> other may help to reduce it a bit.
>
>
> Now a DMTD architecture can be almost completely based on a FPGA where
> some LVDS  IOs would contain the D flip-flop mixers, with their clock
> input connected to the reference frequency and the D input to the
> clocks under test. The FPGA would contain a 32 bit free running
> counter clocked by the reference clock. Every time I detect a
> transition on my LVDS IOs the free running counter is latched and
> passed to a FIFO. Then the work can be passed to a data analysis
> program (of course through an LVDS serial link)  to do all sort of
> funny calculations.
>
> I wonder how good this system could eventually be if we reduce
> crosstalk to a minimum.
>
>
> Pablo
>
>
>
> On Wed, Dec 3, 2008 at 9:01 PM, Bruce Griffiths
> <bruce.griffiths at xtra.co.nz> wrote:
>   
>> Lux, James P wrote:
>>     
>>>>> Cheers
>>>>>
>>>>> Pablo
>>>>>
>>>>>
>>>>>
>>>>>           
>>>> Pablo
>>>>
>>>> Flipflop mixers tend to produce glitches at the beat
>>>> frequency transitions.
>>>> A digital PFD in a PLL doesnt produce a beat frequency output
>>>> when locked so such glitches arent a problem,
>>>>
>>>>
>>>>         
>>> I don't know that this is the case with modern PLL PFDs.. If only because glitches at the transitions would cause other problems, so there's an incentive to get rid of them.
>>>
>>>
>>> Jim
>>>
>>>
>>>       
>> Surely metastability may occur when the mixer flipflop D input
>> transitions occur close to its active clock transitions?
>>
>> There is at least one patent (US5053651) that claims to eliminate such
>> glitches in a digital mixer, however its output beat frequency is half
>> the difference between the input frequencies.
>>
>> Bruce
>>
>>     
Pablo

You could also run several such digital mixers in parallel to exploit
statistical process variations and increase the number of transitions
associated with each coincidence of the clock and D input transitions.
Crosstalk in this case may be even more problematic, however statistical
process variations have been exploited to improve the resolution of
arbiters by driving several of them in parallel.

An FPGA based DMTD should reduce costs considerably and would perhaps
put such systems within the financial reach of more time nuts.

Bruce




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