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 > <[email protected]> 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>_______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe send an email to [email protected] _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe send an email to [email protected]
