[time-nuts] Re: Phase Noise Measurement in Dallas

[Bob kb8tq]

Hi

Just to add a bit to this ….

The most common way (these days) to get from 10 MHz to “something else” is with a PLL.
That PLL will be designed with a noise bandwidth. It is not unusual to see a bandwidth
somewhere in the 10 Hz to 1 KHz range if the PLL is running a crystal oscillator to generate
the output. The better that oscillator is, the smaller the bandwidth that likely will be used. 

Once you are outside that bandwidth, the phase noise of the reference oscillator does not
really matter. All of the noise is coming from the output oscillator. 

Why does this matter? 

Reference devices can have very good close in phase noise, but not so great noise far 
removed from carrier. If you have a PLL at 10 Hz, that’s probably not a big deal. If your
PLL is set up for something a bit crazy ( maybe in the KHz range ….) it can matter quite
a bit. 

Yes, there’s a lot more to it than this. Noise floors on various parts of the system come
into play. Spur generation can be a bigger problem in some systems than broadband
noise. The allowable operating point ( = comparison frequency ) for the phase detector
will most certainly get into the mix on many designs. 

Once you move past phase noise and over to ADEV, there are equally bothersome 
questions. Things like temperature stability of the device ( and of your operating environment)
will come into play. ADEV will give you one set of numbers other “DEV”’s will paint
a different picture. This or that application may be better described by this or that DEV.
There are a lot of papers out there talking about custom DEV’s to better suit this or that
system’s performance requirements. 

So, as noted previously, it’s  best to do a bit of a deep dive into “what’s next” before 
you start picking and sorting out multiple reference oscillators. It is *very* easy to get 
hung up on numbers that, in the end, turn out not to matter very much. 

Bob

> On Oct 26, 2022, at 4:49 PM, Chris Caudle via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> 
> On Wed, October 26, 2022 1:14 pm, Thomas Tammann via time-nuts wrote:
>> I am an audio nut ;-) and we rely nowadays more and more on good
>> oscillators. The main attribute of this application is phase noise, not
>> long term stability.
> 
> Don't forget that 10MHz is not a frequency related to any integer multiple
> of common sampling frequencies, so you will also want to check the phase
> noise of whatever mechanism you are using to convert to e.g. 11.2896MHz or
> 12.2880MHz
> 
> -- 
> Chris Caudle
> _______________________________________________
> time-nuts mailing list -- time-nuts at lists.febo.com
> To unsubscribe send an email to time-nuts-leave at lists.febo.com