Hi,
In HP5370A/B you mux sources. In SR620 there is a lockup PLL, as far as
I remember.
Cheers,
Magnus
On 05/09/2017 01:11 PM, Bob kb8tq wrote:
Hi
If you grab the schematic of pretty much any of these counters, you find a
fairly common approach.
Somewhere inside is a VHF oscillator. The internal TCXO, OCXO, or Rb acts as a
phase lock source
for that VHF oscillator. Typical PLL bandwidths are pretty low (10’s of Hz).
When you put in an external
reference, it acts as the reference to the same PLL.
This does a couple of things. You take care of spurs on the external reference.
That is the same
thing you do locking up a VHF oscillator to your GPSD for microwave work. You
get the long term
accuracy (1 second and out) of the external reference. Your phase noise (and
thus broadband mask
jitter) is improved.
Does every counter on the planet work this way? - of course not. Somewhere
somebody did it a
different way. As long as they *did* do it this way, the internal reference
does not matter once you
switch to an external standard. Even if they did it in some other way, if they
did it right, the same
statement would apply.
Bob
On May 8, 2017, at 9:02 PM, Tom Knox <act...@hotmail.com> wrote:
Hi All;
How important is the standard, medium, or high stability reference in counters
like the 53230A, or FCA3120 when locked to an ultra low phase noise external
reference? Particularly when making measurements and adjustments on other ultra
high performance references?
I know Agilent sold 53132A counters with an option (H01 I believe) that
bypassed the internal reference completely when and external reference is
applied, but I think standard configuration on most counter is to discipline
the internal reference.
Thanks for your thoughts.
Thomas Knox
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