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

[Bob kb8tq]

Hi

There’s really no need to use the Vref out of the OCXO at all. 
Since many devices don’t have one, you will need a “replacement” 
at some point. Simply pulling the “set reference” off of a cleaned
up output of your main supply(s) is typically how it is done. 

The most basic reason to not hard wire a specific device into the
circuit is to allow a A to B / B to C / C to A swap process to be done. 
That is about the only way to get close to working out the numbers
on this or that reference. Without that data, you are flying blind as
you get close to the limits of the reference. 

Given the characteristics of the mixer and the other stuff involved,
with a roughly +7 dbm input, anything past -174 dbc / Hz is suspect. 
-180 dbc / Hz is significantly better than what you likely can do with
this approach. Indeed, it also is quite a bit better than what you will
find your signal sources doing so that’s not a major constraint. 

Yes there is a wonderful “bet a beer” / after work conversation to be
had about the ultimate phase noise of a +7 dbm signal. More or less,
the bet is won by postulating a 1 ohm source impedance. For real 
world sources …. not so much ….

Bob

> On Jul 10, 2022, at 7:52 AM, Erik Kaashoek via time-nuts <time-nuts at lists.febo.com> 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.
> 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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