[time-nuts] DIY Low offset Phase Noise Analyzer

[Erik Kaashoek]

After receiving some opamps and a fairly decent phase noise DOCXO the 
next step of the 1Hz to 20kHz offset Phase Noise Analyzer was implemented.
The simplest method to get the DOCXO locked to the signal from the DUT 
was to also send the DOCXO output as reference to a frequency counter 
and to measure the frequency from the DUT.
Using some course/fine potmeters the DOCXO was tuned to be within 0.01Hz 
of the DUT and this allowed the PLL to lock and after some seconds  the 
frequency difference between the DOCXO and the DUT disappeared. The 
opamps provided 35dB gain to hopefully reduce the noise floor.
First measurement was of a -90dBm/Hz noise source to confirm the -35dB 
gain and the corner frequency of the PLL.
The Phase Noise plots uses 1Hz resolution so both noise and narrow band 
signals use the same power level scale.
As can be seen in attached PN_Noise_-90dBm_Hz.JPG the response is fairly 
flat down to 3Hz and the 35dBm gain was confirmed
The second measurement used a Rigol SG at 10MHz modulated with a PM 
signal at 220Hz at -78dBm which was measured at -42dBm and this also 
roughly confirms the 35dBm gain.
A third measurement uses a AR60 Rb as DUT. The DOCXO used in the Phase 
Noise Analyzer should have a PN at 10Hz offset of -100dBm/Hz and this is 
what is measured so the AR60 Rb and the noise is not the limiting factor 
here.
At 10kHz offset the Phase Noise of the DOCXO should be -150dBm but 
unfortunately either the noise of the ultra low noise opamps or the 
Phase Noise of the AR60 is almost 20dB higher.
As last measurement a really bad GPSDO where the measurement is 
identical to a previous, just shifted up 35dB
Next step is to find way's to reduce the noise level.

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