Hi Erik,
On 7/10/22 17:52, Erik Kaashoek via time-nuts wrote:
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.
I get 22,5 kHz which isn't completely off the charts. Sure helps to eat
the 20 MHz and higher, as well as stray 10 MHz. For the 20 MHz it will
in ideal have -180 dB damping, but in practice it will leak over but
probably not too bad.
The elco's in the PI_controller and the input of the Audio_LNA are
probably going to explode due to reverse polarity.
You want the resistor and capacitor to be in series and not in parallel
in that negative feedback.
As you put a resistor in parallel you will drain the state of the
capacitor and loose performance.
You can choose to either locate a 1 uF non-polar cap, or shift the
values a bit to get into plastic caps such as polypropylene. 100 nF and
220 nF should be easy enough to get hold off. You could even put a pair
of 470 nF in parallel.
A generic note: Most if not all op-amps tends to operate better in terms
of offset behavior as they see about the same resistance DC on both +
and - inputs.
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
You could do away with the Summer and Inverter op-amps if you fed the
TUNING into the + input of the inverter. By skewing the PI-controller
balance the output will be suitably offset. The benefit will be that you
avoid noise contribution from two op-amps and their resistors.
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.
I would advice moving those LEDs off-board. Let that run on separate
"dirty" power. I love the direct observation aspect, but I fear it just
add noise to the measurement.
Keep up the good work!
Cheers,
Magnus
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.
_______________________________________________
time-nuts mailing list -- [email protected]
To unsubscribe send an email to [email protected]
_______________________________________________
time-nuts mailing list -- [email protected]
To unsubscribe send an email to [email protected]
