[time-nuts] First PN measurement results at 1 Hz to 20 kHz from carrier

[Erik Kaashoek]

Thanks to all the great help from people on this list I was able to make 
some progress in doing close-in phase noise measurements.
The setup consists of a VC-OCXO going into the LO port of  an ADE-1 
mixer, The DUT into the RF port, the IF port is low pass filtered and 
used to steer the VC-OCXO and is send to a high quality 24 bit USB audio 
capture unit connected to a PC running ARTA.
The ADE-1 mixer was selected because that all ports are completely 
isolated from each other, there is no common ground, which helps to 
reduce ground loop problems a bit.
The log plots from ARTA confirm a 130dB dynamic range and the resolution 
bandwidth is supposed to be about 1Hz. Each plot was averaged over 10 
measurements
All input signals where normalized by attenuators to have their carrier 
at 0dB in the plot.
Using a generator at variable frequency offset it was confirmed the 
audio input is flat down to 1Hz.
Using a generator with phase modulation down to 0.001 degree the 
sensitivity of the measurement chain was checked. (20dB level reduction 
with every factor 10 reduction in phase modulation depth)
It is expected to have at least +/-5dB level inaccuracy.
The DUTS measured where:
- a fairly clean XO (PN_XO.JPG)
- a rather bad GPSDO output (PN_GPSDO.JPG)
- The not so famous cheap Chinese TCXO (PN_TCXO.KPG)
- The output of a Rigol SG (PN_Rigol.JPG)
The XO is the cleanest
The TCXO shows odd spurs between 10 and 40 Hz and the PN does not drop 
down as it should (spec states: -135dBc/Hz at 1kHz offset)
The GPSDO is terrible, this demonstrates you can have a 1e-10 ADEV at 1s 
tau from a bad oscillator.
The Rigol is not so clean and a PLL shoulder seems to be present just 
above 1kHz.

Next step is to add low noise gain close to the mixer LPF output to get 
more dynamic range and a better VC-OCXO (Morion MV170 (PN -100dBc/Hz at 
1Hz offset) to lower the impact of the reference VC-OCXO
Erik.
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