Charles posted:
but the locked frequency will be different from both oscillators'
free-running frequency and
the EFC will not correctly indicate the test oscillator deviation
because it isn't the only control input in the system.
Good point and No argument (except for the deviation part)
Because the EFC is the only control input THAT IS VARYING.
Also what I have said in the past, maybe unclearly:
NOW, Frequency offset due to loading effects, that is another issue and a
possible problem, so I'm surprised it has not yet been brought up.
The effects you are talking about are there and can be significant and are
easy to measure (with a second TPLL tester) but the BIG IMPORTANT point is
they are a fixed type of offset error and constant and do not effect the
Delta EFC with delta freq change.
The effect you state is a limit for the absolute accuracy of the DUT's
frequency measurment that the tester can make but it does not limit the
delta freq accuracy.(which is all that is really required for ADEV.)
BTW the way I reduce that effect you are talking about so that it also
becomes significant, compared to the accuracy of the Ref osc, is to add
attenuator pads in both osc paths. This does raise the noise floor some, but
the noise floor is so low that it can be raised and still not be significant
compared to the Ref osc, so it allows a good compromise to be made.
simple Example:
Take an osc that has some IL, buffer up its output real good at 90 deg and
now couple just a little of that buffered signal into the osc output. The
1e-11 to e-12 freq shift that will cause does not cause the osc to become
unstable or have some other significant different EFC shape, It just causes
the freq to change a little (offset) about the same as any other fixed and
constant load would have.
In any case there are lots of little subtle things that are going on that I
can not address in a single email, so that is why all I clam is that the
simple TPLL is better than the reference Osc, so it is good enough.
And YES it can be made MUCH better, if one uses more than a single active
part, but so what? for most things it is good enough as is.
******************
Charles posted:
Warren wrote:
The thing that you (and maybe Adler?) are missing is that effect
goes away when the two frequencies ARE exactly the same.
I'm not talking close, I'm talking the exact same freq with phase
held in quadrature within single digit femtoseconds.
BIG difference, Once that is understood, then that sort of answers
your other comments.
Actually, this is not true. If either or both oscillators are
affected by injection locking (and they pretty much all are, to some
degree -- in this connection, note that you want to make measurements
down to E-12 or better [I thought you mentioned E-14 somewhere early
on], so even the least bit of IL will affect the results), what you
have is two control inputs to the controlled oscillator (the EFC and
the reference oscillator) and one control input to the "reference"
oscillator (the oscillator under test, which is itself controlled by
both EFC and the reference oscillator). They will reach equilibrium
(unless the recursive feedback is unstable), but the locked frequency
will be different from both oscillators' free-running frequency and
the EFC will not correctly indicate the test oscillator deviation
because it isn't the only control input in the system.
Best regards,
Charles
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