Hi,
Well, both amplitudes can be measured. The method I refer to is one of
several out of NIST, so it's not one of my own invention. See their AM
and PM Calibration material.
Using multiple methods you can evaluate how well the method functions.
The side-tone method generates known PM with the uncertainty in relative
amplitude. It can be easier to validate than a phase modulator approach,
as it needs calibration.
Cheers,
Magnus
On 2022-07-08 03:57, Bob kb8tq wrote:
Hi
One consideration:
If you do signal injection for calibration, you have the amplitude
uncertainties on
both the “carrier” and injected signals. The slope at zero on the beat note is
likely
to be *much* more accurate ( even if gain measurement at audio gets thrown in …)
Bob
On Jul 7, 2022, at 5:19 PM, Magnus Danielson via time-nuts
<time-nuts@lists.febo.com> wrote:
Hi,
A well established method is to use a separate offset RF generator that you can
steer frequency to form suitable offset and amplitude to form known level. You
can now inject this ontop of a signal to measure. Consider that you steer your
offset frequency to be +1 kHz of the carrier you measure, and you set the
amplitude to be -57 dB from the carrier. This now becomes equivalent to having
a -60 dBc phase modulation at 1 kHz.
The RF generator does not have to be ultra-clean in phase noise just reasonably
steerable in frequency and amplitude.
Cheers,
Magnus
On 2022-07-07 12:47, Erik Kaashoek via time-nuts wrote:
Bob, others.
It has been explained that for the best phase noise level calibration on should
use a signal with one radian phase modulation and measure the output voltage.
The problem with this approach is the unknown gain of the path into the PC. And
due to the gain one can not modulate with one radian as this saturates the
whole path.
An alternative method for phase noise level calibration could be to create an
oscillator so bad its phase noise can be measured using a spectrum analyzer. To
make such a bad oscillator a 10MHz signal was phase modulated with noise. The
phase noise became visible on the spectrum analyzer just above 20 degrees of
modulation. The phase noise level saturated between 55 and 60 degrees which is
consistent with one radian (57 degrees). The spectrum analyzer could measure
the phase noise at a flat -80dbc/Hz ( yes Bob, I better use the right
dimensions)
The simple phase noise analyzer also measured the phase noise at -80dBc
providing evidence the level calibration was done correctly.
I also tried to increase the DUT drive into the mixer further above saturation
so see if this made any change in the measured level but once above 0dBm I did
not observe any change up to +10dBm drive. Any higher levels felt too dangerous.
There is still a lot of work to be done to further increase accuracy.
Erik.
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