[time-nuts] Re: Fixing PN degradation via ADEV measurement

[Erik Kaashoek]

Bob, Magnus,
Thanks, clear. A counter is for ADEV, not for phase noise.
I made a test setup to learn how to use the mixer/PLL approach.
First using 10MHz from both outputs of a DSS (Rigol DG990) to observe 
the DC shift with shifting the phase between the two signal.
Then by modulating one output with FM or PM.
There is a low pass filter after the mixer to get rid of the 10 MHz and 
its harmonics but the LPF is measured flat till about 10kHz.
The output signal from the mixer was kept within 10% of the full voltage 
swing to stay in the (hopefully) linear range.
Using PM creates a low frequency output from the mixer that is 
proportional to the phase shift (region 0-1 degree) and constant in 
amplitude with change of frequency. Also when using external modulation 
from an audio signal generator created the expected behavior with drive 
level and no frequency impact
Using FM with 0.1 Hz frequency deviation the mixer output amplitude 
decreases very fast with increasing frequency (range 0.1 to 10 Hz)
Also when using 1 Hz or more frequency deviation. The higher frequency 
deviation leads to higher output levels as expected.
Can someone help me understand how this FM signal (0.1 to 1000 Hz 
modulation and 0.1 to 1 Hz frequency deviation) translates to the 
calibration example mentioned in the document on phase noise measurement 
as linked by Bob. (0.1 Hz deviation at 1 kHz rate leading to a sideband 
(at 1kHz?) level of -86 dBc)
At a 1kHz rate I see (yet) no output from the mixer where at 1Hz there 
is a lot of output. Why is this output frequency dependency?
Is this a problem with the signal generator?  Or the mixer?
Then I tried to use the modulated signal from the SG PLL locked to a 
10MHz VCO. Results where the same. FM output signal is frequency 
dependent, PM not.
Erik.