[time-nuts] Re: DIY Low offset Phase Noise Analyzer (Erik Kaashoek)

[Bob kb8tq]

Hi

Yes, you do need to know the system gain. Since we are talking about
gain at audio, measuring the gain directly is not a crazy thing to do. One
of the things that makes audio spectrum analyzers a nice tool for this that
they eliminate the “variable gain to the sound card” issue. 

Some sound card setups are a lot easier to work with than others. If you
are restricted to the sound input on your motherboard things can get a bit
crazy. It is not unusual for folks to dig up a “pro” (whatever that means 
on a sound card ) card that has better drivers and more access to this and
that.

Given how fast the PC world changes, the board that was a wonderful thing
last time somebody dove in, likely is long out of production by now. The drivers
that made it work so well may have been “improved” and it no longer gives
you the control it once did. This makes for a bit of trial and error to get it all
going.

Bob

> On Jul 7, 2022, at 2:47 AM, Erik Kaashoek <erik at kaashoek.com> wrote:
> 
> Bob, others.
> It has been explained that for the best phase noise level calibration on should use a signal with one radian phase modulation and measure the output voltage.
> The problem with this approach is the unknown gain of the path into the PC. And due to the gain one can not modulate with one radian as this saturates the whole path.
> An alternative method for phase noise level calibration could be to create an oscillator so bad its phase noise can be measured using a spectrum analyzer. To make such a bad oscillator a 10MHz signal was phase modulated with noise. The phase noise became visible on the spectrum analyzer just above 20 degrees of modulation. The phase noise level saturated between 55 and 60 degrees which is consistent with one radian (57 degrees). The spectrum analyzer could measure the phase noise at a flat -80dbc/Hz ( yes Bob, I better use the right dimensions)
> The simple phase noise analyzer also measured the phase noise at -80dBc providing evidence the level calibration was done correctly.
> I also tried to increase the DUT drive into the mixer further above saturation so see if this made any change in the measured level but once above 0dBm I did not observe any change up to +10dBm drive. Any higher levels felt too dangerous.
> There is still a lot of work to be done to further increase accuracy.
> Erik.