Bob posted
The gotcha is that the injection gain is phase angle dependent.
So what? and it is also dependant on a whole bunch of other stuff, amplitude
being a major one.
And the gotcha you to your gotcha is that the phase difference in the TPLL
is fixed at very close at 90 deg.
(zero on the PD with in uv, & a few femtoseconds variation)
Just to put this subject to rest, I tried to measure the ratio of EFC gain
to injection lock gain
at near zero PD output at the nominal signal levels I'm using.
The problem is this is analog.
Analog unlike digital has some limits of about 1 PPM (1e-6) and 50 nv
(5e-8)
The IL ratio seems to be below 1e-6 so I have been unable to measure the
ratio.
The effect seems to be below the noise level of the reference Osc so that
also makes it a bit hard to get a good number.
Anyone have any suggestion how to go about measuring an effect this low?
Until I can measure the IL effect more accurately,
I'm just going to have go with the more general statement that
the TPLL method is limited by the Reference Osc.
If the reference osc or DUT Injection Lock sensitivity is more than say
around 1e5 times the 10811,
It is best to add an external buffer to be save to keep the gain errors
below 10%.
ws
******************
[time-nuts] Advantages & Disadvantages of the TPLL Method
Bob Camp lists at rtty.us
Wed Jun 16 11:34:27 UTC 2010
Hi
The gotcha is that the injection gain is phase angle dependent.
Bob
******************
On Jun 16, 2010, at 1:57 AM, Magnus Danielson wrote:
On 06/16/2010 05:45 AM, Charles P. Steinmetz wrote:
Warren wrote:
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.
No, it's not. The strength with which each oscillator pulls on the other
also varies as the equilibrium frequency (the result of all three
recursive control inputs) moves around relative to the two instantaneous
free-running frequencies. How much EFC is required depends, in part, on
the strength of the pulling. There are three varying inputs.
Magnus suggested that the effect of injection locking may be enough
smaller than the EFC input that it has little practical significance.
That may be so, but when dealing with measurement accuracy in the
hundreds or tens ot ppt, this needs to be verified by the results of
carefully constructed experiments and hopefully also supported by
mathematical analysis.
What you get is a scale error. Consider that you have an amplifier gain of
1000 and the injection locking provide a gain of 1, that will result in
actual gain of 1001 and the gain error on the EFC will become 1000/1001.
Considering that Allan deviation estimation has problem of its own, this
scale error is not significant. What you do need to check is that the
relationship between intended gain and injection gain is sufficiently
different. Since oscillator frequency from EFC may not be completely
correct, we already want calibration of that scale factor (K_O) and the
gain error due to injection locking would be included into that correction
factor.
So, sufficiently small amount of injection locking gain will change the
apparent EFC coefficient K_O [Rad/sV] on which the scale of TPLL frequency
measurements depends. The fractional frequency observed is
y(t) = 2*pi*f_0 / K_O,eff EFC(t)
Cheers,
Magnus
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