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

[Bob kb8tq]

Hi

Ok, so mixer phase noise calibration:

Set things up with “full blast” inputs to both sides of the mixer. Keep them
at the same level through all tests. This might be +7 on both ports, it might
be +13, it could be +20 dbm. ( yes, mixers *do* get fried this way ….). 

First cal step, you have the radians to volts factor. The “zero db” point for phase 
noise is one radian. It is unlikely that you come up with exactly one volt per
radian. Oddly enough …. it can happen. This is done by looking at a free running
beat note and measuring the slope at a zero crossing. ( 0.1 volts over 10 degrees
maybe …). If you have a pre-amp involved, it needs to be fiddled to keep it out 
of saturation. 

Back in the day of analog devices, you had a bunch of fun with bandwidths
and averaging factors. These days you run a program and it gives you a 1 hertz
normalized number. You are missing a *lot* of fun there …. :)

As previously mentioned, your result is at least 3 db higher than the single 
sideband phase noise. ( Yes, that’s the definition … single sideband). Back 
before the modern era folks used 6 db close in. Not so much anymore.

Finally, you get whatever the combination factor is for your two sources. If 
they are identical, you get another 3 db. 

Spurs are a bit of an issue. Since you have done this or that for a 1 Hz 
normalization, that can mess up the spur levels. They are an absolute number 
rather than bandwidth normalized. For home / basement use, you typically ignore 
this. On a “pro” device, they have fancy software that guesses what is a spur and 
de-normalizes that section.

Some folks do the whole slope measurement thing every time. Others do a 
(unsaturated preamp) beat note and then use a “known” correction factor. Since 
the beat note is not a sine wave, it can not be used directly as a reference. The “right” 
answer is to do the slope every time. If you are measuring a hundred same / same 
devices that day, you go with the beat note. 

Bob

> On Jun 25, 2022, at 10:07 AM, Erik Kaashoek via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> Thanks to all the great help from people on this list I was able to make some progress in doing close-in phase noise measurements.
> The setup consists of a VC-OCXO going into the LO port of  an ADE-1 mixer, The DUT into the RF port, the IF port is low pass filtered and used to steer the VC-OCXO and is send to a high quality 24 bit USB audio capture unit connected to a PC running ARTA.
> The ADE-1 mixer was selected because that all ports are completely isolated from each other, there is no common ground, which helps to reduce ground loop problems a bit.
> The log plots from ARTA confirm a 130dB dynamic range and the resolution bandwidth is supposed to be about 1Hz. Each plot was averaged over 10 measurements
> All input signals where normalized by attenuators to have their carrier at 0dB in the plot.
> Using a generator at variable frequency offset it was confirmed the audio input is flat down to 1Hz.
> Using a generator with phase modulation down to 0.001 degree the sensitivity of the measurement chain was checked. (20dB level reduction with every factor 10 reduction in phase modulation depth)
> It is expected to have at least +/-5dB level inaccuracy.
> The DUTS measured where:
> - a fairly clean XO (PN_XO.JPG)
> - a rather bad GPSDO output (PN_GPSDO.JPG)
> - The not so famous cheap Chinese TCXO (PN_TCXO.KPG)
> - The output of a Rigol SG (PN_Rigol.JPG)
> The XO is the cleanest
> The TCXO shows odd spurs between 10 and 40 Hz and the PN does not drop down as it should (spec states: -135dBc/Hz at 1kHz offset)
> The GPSDO is terrible, this demonstrates you can have a 1e-10 ADEV at 1s tau from a bad oscillator.
> The Rigol is not so clean and a PLL shoulder seems to be present just above 1kHz.
> 
> Next step is to add low noise gain close to the mixer LPF output to get more dynamic range and a better VC-OCXO (Morion MV170 (PN -100dBc/Hz at 1Hz offset) to lower the impact of the reference VC-OCXO
> Erik.
> <PN_XO.JPG><PN_GPSDO.JPG><PN_TCXO.JPG><PN_Rigol.JPG>_______________________________________________
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