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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