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

[Bob kb8tq]

Hi

Having measured a number of similar ( = same part number ) OCXO’s….
It’s a pretty good bet that the phase noise is in the -110 to -115 range at
1 Hz offset. 

Without the “deep dive” into this or that, there is no real way of knowing
what impact a 1 Hz phase noise or 10 second ADEV (or any) number will have.
It could be several orders of magnitude better than something else in the 
system.

As you divide frequency, the phase noise gets better. It should improve by
20 log N where N is the divider. Divide 10 MHz to 1 MHz with a chip and
phase noise gets 20 db better. Your -114 number is now -134. In an ideal
world with perfect parts, it would just keep getting better the more you divide.

The world is never that simple.  Chips are never noiseless. They always have 
noise floors on the output stages and various other noise sources. Once you 
hit this or that limit, the chip is what you worry about and not the reference source. 
In a PLL, this can pop up pretty fast.

While we talk about ADEV ( or any DEV ) and phase noise being independent 
measures, they really aren’t. Back in the day, gear that measured phase
noise typically became impractical below 1 Hz ( = it took a very long time
to get data). Folks switched over to an ADEV measure mainly because the
process was more practical. These days, an ADEV at 0.001 seconds is doable.
If you have enough time, phase noise at 0.01 Hz can be done ( = it still takes
a long time ….).  Oddly enough, while phase noise improves by 20 log N when
you divide, ADEV does not change.

As Magnus points out, you may be able to start at the “far end” of the system
and come up with estimates of what might matter.  It’s always better to spend
hard earned cash on whatever the weak link is :)

Bob


> On Oct 27, 2022, at 9:58 AM, Thomas Tammann <t at tammann.com> wrote:
> 
> 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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