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

[Bob kb8tq]

Hi

( see below )

> On Oct 27, 2022, at 2:30 PM, Thomas Tammann via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> Crystek vs my Symmetricom OCXO:
> The producer of my switch says that the Crystek actually measures much better in real than on paper. He talks to the manufacturer and has insight knowledge.
> The Symmetricom may or may NOT improve over the Crystek depending on the individual specimen and implementation.
> 
> Hence, my goal is to measure: I hope I would see 
> 114 at 1HZ and 140db at 10Hz. Not only would it confirm what I hear ;-) but also justifies the money I put down for a “preselected” (binned) specimen. Also, if I buy another clock with different phase noise and ADEV, I can hear for myself how much this makes a difference in real world audio pleasure vs upgrading eg a pre-amp….

Per their data, the Crystek Crystek CCHD-575 at 125 MHz

https://www.crystek.com/crystal/spec-sheets/clock/CCHD-575.pdf

Comes in at -89 dbc / Hz at 10 Hz offset. That’s not a guess, that’s their
actual measured data on their test gear. The shot shown on page 5 of the
spec *is* the output of a phase noise test set. 

As mentioned previously, phase noise goes as 20 log N when you multiply
or divide the frequency. 20 log of 12.5 is 22 db. An ideal divider would put
the 575 at 89 + 22 = -111 dbc / Hz divided to 10 MHz. ( = the highest possible
PLL comparison frequency ). 

If the PLL circuits contribute little or no noise, a reference at about -120 dbc / Hz
should be able to noticeably improve the close in noise. If the PLL has a noise
floor above -120 dbc / Hz ( at 10 Hz …. ) , then a “better” reference will not do much. 

The Symmetricom part you have most certainly is much better than -120 dbc / Hz
at 10 Hz offset. That’s based on testing a number of the same parts. 

While I’m sure that Crystek is proud of their parts, there are basic reasons 
why a VHF VCXO isn’t going to do as well as a low frequency overtone SC 
based part. The much higher Q on the SC is one, there are others. 

Floks have mesured the actual performance of the Crystek parts. They
have not observed them to be massively better than spec close in. A couple 
db better … sure. Tens of db better close in …. nope. 

Welcome to crystal oscillator design. Some of us on this list spent > 40 years
as engineers working in that field. 

Bob

> 
> But yes, in the meantime I learned a lot from you guys, thank you!!
> 
> Tom
> 
> PS:
> If you wanna read how nerdy audio guys are ;-)
> https://audiophilestyle.com/forums/topic/59419-master-clock-for-your-etherregen/ <https://audiophilestyle.com/forums/topic/59419-master-clock-for-your-etherregen/>
> (A few guys would make great members of this mailing list ;-)
> 
> This guy is seriously interested to measure in oder to understand thinks he can hear but not explain
> https://www-open--end--music-com.translate.goog/forum/privatforen/thomas-michael-rudolph-tmr/651284-messungen-von-ethernet-infrastruktur-switches-nur-lesen?_x_tr_sl=auto&_x_tr_tl=en&_x_tr_hl=de&_x_tr_pto=wapp <https://www-open--end--music-com.translate.goog/forum/privatforen/thomas-michael-rudolph-tmr/651284-messungen-von-ethernet-infrastruktur-switches-nur-lesen?_x_tr_sl=auto&_x_tr_tl=en&_x_tr_hl=de&_x_tr_pto=wapp>
> 
> 
> 
>> On Oct 27, 2022, at 12:39 PM, Chris Caudle via time-nuts <time-nuts at lists.febo.com> wrote:
>> 
>> On Wed, October 26, 2022 9:11 pm, Thomas Tammann wrote:
>>> I use the OCXO on one of my specialized network switches
>> ...
>>> And yes, there are people out there claiming to measure the difference and
>>> that these differences have an influence on sound. I guess the causality
>>> is still hard to prove.
>> 
>> Yeah, the Ethernet clock is not related at all to the audio clock used in
>> the D/A converter, even for synchronous audio-over-IP designs like Ravenna
>> and Dante.  Consumer music players (as opposed to professional audio
>> production equipment) do not even have a synchronized clock, they
>> free-run.  Zero link between the Ethernet clock in the switch and audio
>> quality. The quality of the clock to the digital-to-analog converter does
>> influence quality, but again there is absolutely zero link between the
>> Ethernet clock and the audio clock.
>> 
>>> Now the maker of this switch claims that an external clock has to have a
>>> phase noise better than 125dB at 10Hz to make a difference.
>> 
>> According to that link you provided it does have a Crystek crystal
>> oscillator, which is relatively low noise.
>> You can see the specs here:
>> https://www.crystek.com/crystal/spec-sheets/clock/CCHD-575.pdf
>> 
>>> I got likely scamed with my clock from China claiming 140dB @ 10Hz
>> 
>> If it is working well an ovenized SC cut oscillator should be able to
>> reach that.
>> 
>>> hence, yes I want to measure it ;-) and I really just need to know the
>>> phase noise (and Allen dev) at the actual output, no any converter. I hope
>>> that makes all sense.
>> 
>> What you are attempting to do is understandable.  The entire premise
>> behind it doesn't make sense from a technical standpoint, but the starting
>> point is relatively straight forward, compare the phase noise spec of your
>> surplus OCXO to the Crystek spec to make sure your new oscillator is
>> better than the oscillator internal to the device.
>> 
>> -- 
>> Chris C
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