Re: Crystal drift

From: Phil Hobbs (pcdhSpamMeSenseless_at_us.ibm.com)
Date: 09/24/04 

Date: Fri, 24 Sep 2004 15:32:00 -0400

John Larkin wrote:
> On Fri, 24 Sep 2004 11:28:06 -0400, Phil Hobbs
> <pcdhSpamMeSenseless@us.ibm.com> wrote:
>
>
>
>>>Also, depending on the frequency, a simple D-flop makes an excellent digital
>>>mixer. Put one input on the clock, the other on the D. The Q output will switch
>>>at the difference frequency.
>>
>>Not with a PFD--metastability will blow you right out of the water. Every
>>lost cycle equals lost lock. The D flipflop trick can work with a narrow loop
>>and a multiplying phase detector though--I used it when I built a pilot tone
>>generator for what I think was the first commercial direct broadcast satcom
>>system, in about 1982. (I had just got my bachelor's degree in astronomy and
>>physics at the time, and they hired me to look after all the ultrastable PLL
>>stuff--talk about being thrown in the deep end. For the frequency reference
>>board (different from the PTG) I had to invent a fractional-N synthesizer
>>based on resynchronized rate multipliers. It worked great, eventually.)
>>
>
>
>
> This one
>
> http://www.highlandtechnology.com/DSS/V880DS.html
>
> uses an EclipsLite ff as the phase detector, locking the local 155 MHz
> rock to the incoming OC-3 data stream. BW is about 10 KHz for acquire,
> 2 KHz when tracking, with the uP switching loop filters when it feels
> it ought to. About 200 of these are scattered all over the NIF laser
> to fire everything at the right time. You can barely make out the
> crystal oscillator sitting up on tiny springs.
>
> Biggest hassle was the (expletive deleted) 850 nm VCSELs for the
> optical-out option. What nasty parts!

Interesting. Is this just a single bang-bang detector, or something like an
early/late gate? If it's a bang-bang loop, how do you handle metastability?
  The loop is going to be trying desperately hard to create perfect
conditions for it. Of course, it's not as bad a problem using the FF as the
phase detector as when driving a PFD from a FF mixer.

Cheers,

Phil

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