[SI-LIST] Re: f vs. Tf vs. jitter

Steven,

"Analog frequency synthesis techniques" includes a PLL, even with digital
counters and dividers in it.  (As opposed to direct digital frequency
synthesis.)

As such, analog frequency synthesizers can be easily programmed for
different division ratios and operating frequencies, especially as far as
the output is concerned.  And they do not necessarily have the problem with
slew rate when generating low-frequency events.

There are also analog based synthesis techniques that don't use digital
counters and such, only analog circuits (though some are nonlinear,
frequency multipliers and such).  But these are less common.

If you are talking about a need to generate sine waves, then yes there is a
link between frequency and slew rate OF THAT OUTPUT SIGNAL.  But that is
because of the desired output waveform, not the method of frequency
synthesis.

But if you have a need to trigger something downstream, then a sine wave
output is potentially a poor choice, especially if there is a need for wide
frequency ranges and/or very low-frequency events.

In your situation I'd be tempted to stay with digital counters to get from
80MHz to 10Hz, as opposed to a PLL unless I needed an odd ratio or
something.  If phase tweaking is necessary, the presence of 80MHz ought to
give them the ability to minimize the amount of analog delay adjustment that
is necessary.

Yes, it's true that electronic (analog) delay lines generate lots of jitter
and that it increases with their delay time.

NIST has a number of references (such as the Time and Frequency User's
Manual, which I think they now have on-line) that talk about things like
locked lasers, in a little bit of detail that might give you some hints
about how it's done.

Seven orders of magnitude isn't necessarily an issue; I mean it's not like
trying to make a PLL itself operate over 7 orders of magnitude, or a linear
RF amp over a similar amplitude range.  If you think of it as a weak loop
where the second oscillator (laser) is mostly free-running and has low
jitter, and just needs occasional updates to tweak its phase, then it's not
so bad.  There are trade-offs and limitations (won't work if the second osc.
wanders too much), but if these are managed, 7 orders of magnitude is not
necessarily a problem.

Keep in mind ... if you will be talking to a room full of PhDs, they will
probably take over the conversation anyway and steer it where they want it
to go, and you can't help but be made to feel like a fool.  Just don't sweat
it.

Regards,
Andy


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