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.
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