[time-nuts] Re: First PN measurement results at 1 Hz to 20 kHz from carrier

[Magnus Danielson]

Hi Erik,

On 2022-06-27 11:43, Erik Kaashoek via time-nuts wrote:
> Magnus, Bob,
> When the mixer is operating in the linear region for the DUT input 
> (0dBm or lower), would it be possible to use a calibrated noise 
> sources  to do an extra verification of the noise level measurement?

Yes. NIST build such calibrators for exactly that purpose.

> Of course with a noise source you get 3dB as both sidebands fold.
Naturally.
> Verification steps:
> Verify the DUT output level is correctly brought to 0dB (using 
> attenuators) using a calibrated spectrum analyzer
> Connect the DUT to the phase measurement setup and set the reference 
> to a 500Hz offset to get a beat note and verify the beat note is 
> registered at 0dB, change the DUT level some dB up and down to confirm 
> its in a linear region.
> Measure the per Hz output power of a noise source using a calibrated 
> spectrum analyzer and a noise marker set to 10MHz.
> Connect the noise source to the phase measurement setup and check if 
> the noise level is measured at level measured by the spectrum analyzer 
> + 3dB
> This should work if the RBW of the phase measurement is indeed set to 
> 1Hz.

500 Hz is kind of arbitrary number you chose there. At the very least it 
should be verified, but I would assume there is an underlying goal which 
made you choose 500 Hz and that should be specified.

The RBW is not the noise BW. You need to correct the bin-bandwidth with 
the specific noise bandwidth correction for the window-filter you use. 
There is a neat article [1] on it which I also contributed into IEEE 1139.

>
> Another verification option may be to use the phase modulation of a 
> signal generator. This can not check the effective noise bandwidth of 
> the FFT but it can check linearity over the whole range.
> The output of the mixer is terminated with 50ohm so a factor of 10 in 
> voltage should give a 20dB power step.
> When operation in the linear range the phase noise measurement setup 
> should measure 20dB less with every factor 10 reduction in phase 
> modulation depth where 90 degrees is equal to 100% modulation depth so 
> equal to the signal you get when measuring a beat note.
> When measuring with modulation depth of 90,9,0.9,0.09 and 0.009 
> degrees the measured level should step from 0,-20,-40, -60 to -80dB

For higher modulation depth it's very hard to do linear modulation well. 
There is a different variant you can use, to inject a signal to form a 
side-band signal next to the carrier. By creating a specific amplitude 
compared to the carrier, and for a particular offset, it's trivial 
exercise to know the AM and PM noise level. A single sideband sine will 
divide equally to AM and PM, thus reducing 3 dB. Thus, setting a 
side-band at 27 dB bellow the carrier, makes a -30 dBc phase modulation.

[1] 
https://pure.mpg.de/pubman/faces/ViewItemOverviewPage.jsp?itemId=item_152164

Cheers,
Magnus

>
> Any feedback?
> Erik.
>
> On 26-6-2022 20:52, Magnus Danielson via time-nuts wrote:
>> Hi Erik!
>>
>> Great progress! Sure interesting to look at them phase-noise plots, 
>> right? It's a really good tool in addition to the stability of ADEV 
>> and friends.
>>
>> As I recall it, the ADE-1 is not documented to be isolated, but it is 
>> very obvious when you look down the backside of it. However, it has 
>> capacitive coupling and one should consider both common mode 
>> rejection and common mode loading it down for these to work well.
>>
>> Word of caution when it comes to levels, as the windowing filter used 
>> causes shifts in noise-levels, so estimation of noise-levels becomes 
>> a little bit tricky as you try to get the nitty gritty right, but 
>> getting the overall shape view you already gained a lot with the 
>> things you achieved.
>>
>> A technique used to push further down into lower noise-levels is the 
>> cross-correlation technique, where you split the signal into two 
>> channels, each being exactly what you have now, and then rather than 
>> squaring the output of the FFT from each channel, you multiply one 
>> with the completment of the other, then average on those. This allows 
>> you to supress the noise of each reference oscillator. You do not 
>> have to go there from start, as you already make very useful 
>> measurements, but I'm just suggesting what may lie up ahead.
>>
>> Compared to some of the other sources, the Rigol SG does fairly well, 
>> but then again, things can be even more quiet. For the XO you can see 
>> the 15 dB/Oct slope as expected for flicker frequency. Try to locate 
>> the source of the peaks you see and see if you can clean it up. The 
>> XO seems to be a fairly good DUT for doing that.
>>
>> Cheers,
>> Magnus
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