[time-nuts] Re: DIY Low offset Phase Noise Analyzer

[Erik Kaashoek]

Some more testing done.
The mixer starts to saturate at -3dBm and is fully saturated a -1dBm so 
for every input larger than -1dBm the phase demodulation output is no 
longer depending on the amplitude of the input signal, only on the phase 
modulation.
It took a long time to hunt down the source of all spurs when no DUT was 
connected, every single 10MHz source within 2 meters had to be switched 
off or moved much farther away. Maybe time to start improving shielding, 
including cables.
In the end the noise floor is -135 dBm/Hz between 10 and 100Hz offset, 
dropping down to -150dBm/Hz at 1kHz offset and above (see attached 
PN_baseline_2.JPG, subtract 40dBm from indicated levels) Above 1kHz 
there are still a large amount of spurs but these can easily be ignored. 
Something weird is happening at 20Hz and 25Hz offset. This could be due 
to the temperature control in the used DOXCO
Measuring a not so good OXCO gave good results down to about 5 Hz offset 
(see PN_OCXO_2.JPG )
Locking now happens when the DUT and DOCXO are within 0.5Hz, it does 
take up to 10 seconds so both sources must be stable and not drifting 
due to warming up.
The attached schematic has been updated. (PNA.pdf) showing the balanced 
input of U2 greatly helping to remove any noise from the REF voltage. 
Also shown is the 50 ohm resistor acting as sink for all the high 
frequency signals through the mixer.
The phase noise analyzer is now in a small box and ready for the real world.
Erik.

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