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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