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

[Magnus Danielson]

Hi,

On 2022-06-27 18:06, Bob kb8tq via time-nuts wrote:
> Hi
>
>> On Jun 27, 2022, at 1:43 AM, Erik Kaashoek via time-nuts <time-nuts at lists.febo.com> 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?
>> Of course with a noise source you get 3dB as both sidebands fold.
> The “normal” approach is to put the mixer into saturation. This gives you the
> best noise floor. It also does a bit better at separating AM noise from PM noise
> (since you are trying to measure phase noise …..).

It could be worth mentioning that there are both linear and non-linear 
AM->PM conversion as well as PM->AM conversion. They *should* be 
perfectly separate, but real life circuits is unable to maintain perfect 
symmetry.

Now, the AM noise tends to be stronger than the PM noise, so leakage 
tends to be more severe in that direction, in that it causes more harm. 
Running the signal into clipping is a non-linear way to cut away most of 
the AM and maintain the PM, and such limiter detectors turns out to be 
very good for FM reception (FM is just a variant of PM).

Modern high-speed ADC receivers with arctan CORDIC processing has 
supperiour AM and PM separation and avoids much of those problems. Just 
for reference, does not help here.

A linear filter provides AM->PM conversion when the upper side-band and 
the lower side-band does not have the same gain for the same offset 
frequency. This asymmetry converts AM (even) to PM (odd) as well as PM 
(odd to AM (even), while AM to AM and PM to PM reduces amplitude. Even a 
simple low-pass filter will do this, and to minimize it's effect the 
cut-off frequency should be higher than the carrier frequency so it is 
essentially flat and equal for both LSB and USB. Similarly should a 
resonator be tuned to be on the carrier frequency, or the upper and 
lower slopes will not match well.

Anyway, wanted to take the opportunity to explain a little more.

>
>> 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.
> If you do it this way, you still need to do conversion for the “one radian” reference
> level that is used with phase modulation ( = the reference is *not* one cycle ). Yes
> that’s a bit weird / obscure.
But then again, it's exactly what the standard says. We even rewrote 
that section to clarify it.
>
>
>> 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.
> Audio termination at 50 ohms does not do much for isolation. (again a bit of an obscure
> point). By terminating in 10X the nominal impedance ( so 500 ohms in this case) you get
> another 6 db of gain in the system. Since this is ahead of the preamp, it might improve
> your noise floor.

You may however want to have 50 Ohm termination for RF frequencies 
(sum-frequency as well as leakage of carrier frequency). The NIST T&F 
archive has illustrations of different mixer loading networks such that 
it is high impedance at LF but low impedance at RF. Loading down the LF 
which is detected signal makes no sense, as we amplify the voltage and 
not the power received. We do want to terminate the RF so that it is not 
reflected back into the mixerin a termal unstable fashion. If the RF 
termination can be close enough to the mixer, other impedance than 50 
Ohm can be considered for even more optimal result, but unless one knows 
what one does, I'd say stick with 50 Ohms for that is what the mixer is 
mast likely designed for.

Cheers,
Magnus

>
> Bob
>
>
>> 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
>>
>> 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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