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

[Magnus Danielson]

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.
>
>
>
>
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