[time-nuts] Re: Fixing PN degradation via ADEV measurement

[Bob kb8tq]

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 at 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 at lists.febo.com
> To unsubscribe send an email to time-nuts-leave at lists.febo.com