[time-nuts] DMTD mixer question

Lux, James P james.p.lux at jpl.nasa.gov
Wed Dec 3 18:36:50 UTC 2008


> -----Original Message-----
> From: time-nuts-bounces at febo.com
> [mailto:time-nuts-bounces at febo.com] On Behalf Of pablo alvarez
> Sent: Wednesday, December 03, 2008 7:50 AM
> To: Discussion of precise time and frequency measurement
> Subject: [time-nuts] DMTD mixer question
>
> Hi,
>
> I have been looking at several Dual Mixer Time Difference
> designs. As far as I know the basic architecture is based on
> generating a tone with an small frequency offset respect to
> the nominal clocks frequency, analogue mixing of the clocks,
> low pass filtering, then a slow zero crossing detector and
> finally a time interval counter or time stamp unit.
>
> I have this naïf question: would not it be better and simpler
> to use directly an ECL D flip-flop as a mixer instead of an
> analogue one? I suppose analogue mixers are preferred because
> they provide with a better "differential tempco", but using
> ECL logic can provide also with a good tempco.
>

That's an interesting question. I know that some folks building microwave synthesizers for radio science applications (where very low phase noise and Allan deviation are of concern) are using the Phase Frequency Detector (basically a flip flop) in parts like the Peregrine PE9701 or PE3236, as opposed to a standard mixer.

Mind you, the data sheet
http://www.peregrine-semi.com/pdf/datasheets/pe9701ds.pdf

Says "ultra low phase noise" but doesn't actually give any numbers.

But the PE3236 does give at least one number, however it's for the full PLL, not just  the PFD contribution, which will be very small)
http://www.psemi.com/pdf/datasheets/pe3236ds.pdf

One reference (Cook, et al., 2004)  gives numbers of (at some unknown offset or measurement condition)
-165 dBc/Hz for TTL
-150 dBc/Hz for ECL
-130 dBc/Hz for CMOS

But those are easily 10 year old numbers, if not more, and may not be representative of modern technology (particularly the new CMOS processes).

I seem to recall seeing a number like -190dBc/Hz or -200dBc/Hz for these hot stuff PFDs, but I can't recall where, maybe Banerjee's PLL book?



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