[time-nuts] Re: DIY Low offset Phase Noise Analyzer (Erik Kaashoek)

[Erik Kaashoek]

I've updated the schematic to include the latest additions and added 
some new measurements

Schematic: http://athome.kaashoek.com/time-nuts/PNA/Simple_PNA.pdf

The resistor values (many 18k) are a bit weird but I happen to have a 
big box of 18k resistors.
The value of the low pas filter after the mixer (C2,C3,L1) are probably 
wrong. Calculate yourself for the corner frequency you want.
The elco's in the PI_controller and the input of the Audio_LNA are 
probably going to explode due to reverse polarity.
The output of the REF_Buffer acts as the virtual ground so care was 
taken (almost) not to draw any current, except for the input of the 
Audio_LNA.
The supply of the opamps is not drawn but its from Ground and Vcc (+12V)
I've tested symmetric supply but the combination of the REF output 
voltage from the DOCXO and the REF_Buffer provided the least noise.
The audio_LNA has a gain of 1 for DC and increasing to 100 for for 1Hz 
and above
The R/C values around the PI_Controller have not been optimized but they 
work.
As the Summer OPAMP inverts to 5-10V the Inverter OPAMP brings it back 
to 0-5V for the Vtune of the DOCXO
The LED's provide visual feedback on the tuning. IF both are just on the 
PLL is in lock. It may be better to have two LED's in series at each 
side to increase the dimming.

Some measurements.:
All indicated levels are 40dBc/Hz higher compared to actual.
The noise floor: http://athome.kaashoek.com/time-nuts/PNA/PN_baseline_3.JPG
This is measured without DUT input.

Rigol signal generator generating 10MHz Phase modulated with 60 degrees 
noise at -80dBc/Hz: http://athome.kaashoek.com/time-nuts/PNA/

Rigol signal generator generating 10MHz phase modulated with 0.006 
degrees at 220Hz : 
http://athome.kaashoek.com/time-nuts/PNA/PN_Rigol_3_0.006.JPG
The 220Hz is under the cursor at -27dBc, at 0.006 degrees modulation it 
should be at -88dBc, so there must still be a big mistake somewhere.

AR60 Rubidium reference: 
http://athome.kaashoek.com/time-nuts/PNA/PN_Rb_3.JPG
All seems OK, a bit of 50Hz and harmonics.

OCXO : http://athome.kaashoek.com/time-nuts/PNA/PN_OCXO_3.JPG
very weird spurs between 40 and 50 Hz

The famous cheap Chines TCXO: 
http://athome.kaashoek.com/time-nuts/PNA/PN_TCXO_3.JPG
Not too bad for offsets of 100Hz and higher but at 10Hz and lower its 
20dB worse.

A home designed/build arduino GPSDO: 
http://athome.kaashoek.com/time-nuts/PNA/PN_GPSDO_3.JPG
The GPSDO has a good ADEV but is clearly very noisy!

I also measured a Marconi 2022 signal generator and it was possible to 
lock but the phase noise was terrible with strong factional PLL spurs.
I also tried to measure the phase noise of an old Philips analog 10Hz to 
12MHz signal generator but it was impossible to get a lock because the 
generator output is jumping around several Hz at 10MHz output.

The noise floor of the simple PNA leaves a lot to improve (from 
-140dBc/Hz at 10kHz to -180dBc/Hz with better OCXO, LNA and correlation) 
but it proved to be able to do a first assessment of some not too good 
oscillator performance.

Feedback welcome as these are my first baby steps on phase noise nuttery.
Erik.