[time-nuts] Re: First PN measurement results at 1 Hz to 20 kHz from carrier
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 > 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. > ___ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe send an email to time-nuts-le...@lists.febo.com ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe send an email to time-nuts-le...@lists.febo.com
[time-nuts] Re: First PN measurement results at 1 Hz to 20 kHz from carrier
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 On 2022-06-25 20:07, Erik Kaashoek via time-nuts 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. ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe send an email to time-nuts-le...@lists.febo.com ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe send an email to time-nuts-le...@lists.febo.com
[time-nuts] Re: Repeatability of stability measurements
Hi Hans-Georg, You're MDEV slope is not that of white noise, but that of a (correlated) systematic damping. You have 1/tau^2 rather than expected 1/tau^1.5. Also, your levels are way off. This steeper slope for systematics is not widely documented by the way, but direct consequence of the math. This is where I slip between the wrapped-phase (w) and unwrapped-phase (p) in TimeLab and figure out what is going wrong. That can be one hint. Looking on your Frequency difference it may be that you have multiple slips. Could it be that you loose data-samples and thus the phase-slope jumps? Here my main concern is the continuity of your data. That gives this kind of severly distorted plot that swamps the real measurement quickly, and is a sure give-away. Cheers, Magnus On 2022-06-25 17:15, Hans-Georg Lehnard via time-nuts wrote: First of all the resolution factor from my first post is a copy error the correct value is 4.88281248-11 (less decimal places). In the attached diagrams the correct factor was used. I generated testfiles with 2046,2047,2048,2049,2050 intervals, loaded into timelab and scaled them with 4.8828124998-11. MDEV shows more noise as real measurements . Another testfile with 2000 intervals and scaled with 5e-11 shows similar results. In the Frequenc difference plot you can see the difference grows stepwise with time. The zoom shows where the 2048 intervals are already in the next time step and the 2000 intervals are not yet. By zooming in you can also see this between the 2046 and 2050 intervals. Possibly an overflow or rounding error ? I think the overlap of this effect with the white noise of the real measurements creates my measured jumps. More noise attenuates this step-like progression. ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe send an email to time-nuts-le...@lists.febo.com ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe send an email to time-nuts-le...@lists.febo.com
[time-nuts] Re: Repeatability of stability measurements
Sorry, this was completely nonsense .. i correct the resolution factor and forgot the "e" so i get 4.88281248-11 as factor and scaled the timelab plots with it. My interpretation is just as stupid. The correct Factor is 4.88281248e-11. Am 2022-06-25 17:15, schrieb Hans-Georg Lehnard via time-nuts: > First of all the resolution factor from my first post is a copy error > the correct value is 4.88281248-11 (less decimal places). In the > attached diagrams the correct factor was used. > > I generated testfiles with 2046,2047,2048,2049,2050 intervals, loaded > into timelab and scaled them with 4.8828124998-11. > > MDEV shows more noise as real measurements . Another testfile with 2000 > intervals and scaled with 5e-11 shows similar results. > > In the Frequenc difference plot you can see the difference grows > stepwise with time. The zoom shows where the 2048 intervals are already > in the next time step and the 2000 intervals are not yet. By zooming in > you can also see this between the 2046 and 2050 intervals. > > Possibly an overflow or rounding error ? > > I think the overlap of this effect with the white noise of the real > measurements creates my measured jumps. More noise attenuates this > step-like progression. > ___ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe send an email to time-nuts-le...@lists.febo.com ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe send an email to time-nuts-le...@lists.febo.com
