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

[Thomas Tammann]

Very cool, thanks so much Bob

As much as I understand your answer ;-) it makes all sense.

The challenge is really, that making my own clock to attach to my switch is way out of my league. Hence, I buy what I can and I try to understand the quality of the piece I bought. 

Lets say there are two “identical” clocks, all I care really is the end result and how they compare to each other. And, as much as I understand, the phase noise is a good indicator.

I bough this clock on ALibaba with the promise that phase noise at 1HZ is 114dB…and I want to verify ;-)

Again, I appreciate very much how this community helps a noob ;-)

Cheers
Tom W5TAQ



> On Oct 27, 2022, at 7:49 AM, Bob kb8tq via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> 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
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