Hi Ok, single mixer phase noise basics:
First thing is to womp the mixer up to the point it almost smokes. Putting +7 dbm into both ports on a “7 dbm” mixer is very normal in this case. Watching for the fact that the mixer likely is *not* a 50 ohm load is part of the process ( = pads might help out) as well as understanding that it does not have a monster amount of isolation ( = isolation amps may be needed ). Next one generates a beat note by offsetting the two oscillators a bit. This gives you a nice 360 degree sweep function ( 360 degrees per cycle :) ). From that you can work out the system sensitivity in volts per degree ( or better yet per radian since that’s what you actually want as the “magic number” …. it’s phase modulation so radian is king …). Next you lock the two oscillators together via a DC feed out of the mixer to one or the other of them. You adjust the “lock point” so that it is at zero volts out of the mixer. This puts the two oscillators in quadrature. Yes, there is that messy 2X the input frequency RF output and the inevitable leakage. Those are handled with a lowpass filter. The output of the mixer is now “just noise”. There is no nasty carrier to deal with. There is no messy fold over to wonder about. What you get is the DSB noise ( so both sides of carrier) from the sum of the two oscillators. Output of the mixer goes up if you terminate it in an “high” load. Something like 500 ohms on a 50 ohm mixer or 5K ohms on an RPD-1 is often used. The isolation seems to be ok either way and the added gain / better floor is “free”. Simply put you add 3 db when you look at DSB if it’s uncorrelated, and another 3 db if the oscillators are identical. Your “output” is 6 db higher than the single sideband / single oscillator phase noise. You can argue that close in noise is likely correlated due to it being a modulation on the carrier. The standard convention is to use 3 db. Amplify the noise up and you can measure very low levels of phase noise. Low noise audio op-amps are pretty easy to find spec sheets on. With anything these days finding them on the shelf may be “interesting”. The OP-27 / OP-37 with low resistance in the feedback path go way back for this application. There are a lot of other candidates. The cutoff of the lock signal typically is adjustable to keep it below the lowest point of interest for your noise testing. If that is impractical, there are ways to calibrate and read “inside the loop”. The HP 3048 phase noise analyzer was based on this approach. The original app note most folks started from came from Fluke back in the early 1970’s. I have not (yet) found a good copy of it on the internet. Fun !!! Bob > On Jun 20, 2022, at 9:43 AM, Erik Kaashoek via time-nuts > <time-nuts@lists.febo.com> wrote: > > Bob, Magnus, > Thanks, clear. A counter is for ADEV, not for phase noise. > I made a test setup to learn how to use the mixer/PLL approach. > First using 10MHz from both outputs of a DSS (Rigol DG990) to observe the DC > shift with shifting the phase between the two signal. > Then by modulating one output with FM or PM. > There is a low pass filter after the mixer to get rid of the 10 MHz and its > harmonics but the LPF is measured flat till about 10kHz. > The output signal from the mixer was kept within 10% of the full voltage > swing to stay in the (hopefully) linear range. > Using PM creates a low frequency output from the mixer that is proportional > to the phase shift (region 0-1 degree) and constant in amplitude with change > of frequency. Also when using external modulation from an audio signal > generator created the expected behavior with drive level and no frequency > impact > Using FM with 0.1 Hz frequency deviation the mixer output amplitude decreases > very fast with increasing frequency (range 0.1 to 10 Hz) > Also when using 1 Hz or more frequency deviation. The higher frequency > deviation leads to higher output levels as expected. > Can someone help me understand how this FM signal (0.1 to 1000 Hz modulation > and 0.1 to 1 Hz frequency deviation) translates to the calibration example > mentioned in the document on phase noise measurement as linked by Bob. (0.1 > Hz deviation at 1 kHz rate leading to a sideband (at 1kHz?) level of -86 dBc) > At a 1kHz rate I see (yet) no output from the mixer where at 1Hz there is a > lot of output. Why is this output frequency dependency? > Is this a problem with the signal generator? Or the mixer? > Then I tried to use the modulated signal from the SG PLL locked to a 10MHz > VCO. Results where the same. FM output signal is frequency dependent, PM not. > 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
