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

[Bob kb8tq]

Hi

(see below)

> On Jul 7, 2022, at 10:10 PM, Erik Kaashoek <erik at kaashoek.com> wrote:
> 
> Bob,
> You may have explained this before but I still do not understand.
> Does the phase modulation slope at the detector depend on the depth of the phase modulation? I think not.

The “phase modulation” you are looking at when observing the slope with a beat note
is a full 2-pi radians of modulation for every cycle of the beat note. Since that’s guaranteed
with no further effort, it makes a nice standard to use.  There *is* no modulation being done
to either signal in this case. 

> With 57 degrees one should get an output voltage that is to be regarded as the 0dBc level but this can not be measured due to the high gain in the audio path.

Which is why you want a two op amp approach. This also gets you a nice path to use
for the DC feed for lock. 

> When you reduce the modulation depth with a factor 10 the measured output voltage should decrease with 20dB.

Except you didn’t start out modulating either signal. You simply unlocked them and got 
a result that happens to provide 2 pi radians of signal at the output of the mixer. 

> Modern digital signal generators are supposed to provide phase modulation with at least 0.01 degree accuracy.
> So it could be possible to measure the phase detector slope with 0.57 phase modulation depth by measuring what should be -40dBc
> Or, if the gain is very high, less accurate with 0.06 phase modulation.
> Or am I making a mistake in my reasoning?

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. 

Since signal generators are not likely to get you to the same sort of noise levels as a 
very good stand alone source, you very much do not typically want a signal generator 
involved in a real measurement. Yes, there are always exceptions to any blanket 
statement …

Bob

> Erik.
> 
> 
> On 8-7-2022 3:57, Bob kb8tq via time-nuts wrote:
>> Hi
>> 
>> One consideration:
>> 
>> If you do signal injection for calibration, you have the amplitude uncertainties on
>> both the “carrier” and injected signals. The slope at zero on the beat note is likely
>> to be *much* more accurate ( even if gain measurement at audio gets thrown in …)
>> 
>> Bob
>> 
>