On 7/13/20 9:58 AM, Taka Kamiya via time-nuts wrote:
I'm sorry to interject a newbie question.... I changed the title to
distinguish from rest of the conversation.
I have heard this both ways about external references - whether it's used to
phase lock internal source and used directly after some conditioning. Both
come from people on this list I trust. Limiting discussion to HP counters from
70s to 90s, which is the truth? Were there exceptions? If so, why? (I'm not
interested in injection locking....)
If some are phase locking, what does it phase lock? Most counters have options
on internal reference (ie. HP53132A has standard, mid performance, high
performance, and ultra performance) Does it phase lock the standard that's
always there? Or try to phase lock optional reference? I really don't see the
need for phase locking, as only critical element is rise time - so rather,
signal conditioning makes sense.
At least for me, the general public, circuit diagram is not made available for
later models. I have no way to tell for sure what is being done inside.
---------------------
There are also "frequency locked" devices that are not "phase locked" -
they essentially discipline an internal oscillator by adjusting its
frequency, but not with any sort of phase locked loop.
The 33600 series function generators from Agilent/Keysight are in this
bucket. You can feed in a 10 MHz source and they'll (after a fairly
short time, a few seconds) produce a 10 MHz output that is "right on
frequency", but it won't have a predictable phase relationship with the
external reference.
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