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

[Bob kb8tq]

Hi

Like it or not, the mixer is a non-linear load. It also has a frequency
dependence. Even with “saturation” levels, the slope can and does 
change. That’s the short list, as you dive into it, things get even more
complex in terms of “might be” sort of issues. 

How can you be in saturation and have the slope change ( it does sound
unreasonable) ? The fundamental is not changing much (so you are 
in saturation). The harmonics of the fundamental are changing. Since
the output is actually a triangle wave with rounded “corners” there are 
indeed harmonics very much present. 

The flat parts of the triangle wave are a “good thing” in this case. It 
makes the device linear over a bit wider range than a sine wave would
provide. This gets you out of all sorts of nutty analysis concerning the
noise being “to much” to measure with the device. It also relaxes the 
needed accuracy of the DC lock part of things. ( = slope of a sine wave
changes quickly ….). 

You never really get away from the “to much noise” question. The
common definition of phase noise is that it’s more than 60 db below
carrier. That is really just the commonly used limit for  “you may need 
to think about FM sidebands”.  Yes, that’s another rabbit hole to wander 
down ….

Bob

> On Jul 8, 2022, at 8:32 AM, Erik Kaashoek <erik at kaashoek.com> wrote:
> 
> Bob
> This confuses me.
>> The calibration of the system changes ( or can change ) each and every time you swap
>> out signal sources. The levels are not going to be consistent setup to setup. Thus you
>> calibrate each and every time you change out either device.
> Assuming each source is saturating the mixer sufficiently (to be confirmed by measuring the output level of the source into 50 ohm) I do not understand how changing a source can change the calibration. Can you explain what is happening?
> Please keep in mind I'm not after 0.1dBc/Hz accuracy, +/- 5dBc/Hz would already be great.
> Erik.