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

[Bob kb8tq]

Hi

Moving back a bit: What *can* you expect an external reference to
improve on a clock system?

Looking for phase noise improvement typically isn’t what you are after. 
The phase noise is determined by what’s already inside the box. The impact
of the reference will not be very apparent as you look at a full up phase
noise plot of the output clock.

What the reference *will* do is to improve the long term stability of the clock
signal. The devices that are already inside the box are not great for drift / aging /
temperature / warmup / wander / ( or whatever term you might wish to use ….). 
There are a number of things you could look for an improvement in. All are 
based on the assumption that the device currently is based on the Crystek part
or possibly a TCXO. ( If a TCXO is present, I have not seen mention of it ). 

1) Temperature stability should improve by several orders of magnitude. Even in 
a normal room setting this improvement should be apparent and very measurable. 

2) Aging should be *much* better with the OCXO. Yes, the OCXO should be kept 
on power all the time. Compared to the Crystek part it should be orders of magnitude
better. 

3) Short and medium term ( tau of 1 second to 1,000 seconds ) stability should be
much better with the OCXO than with the stand alone box. Any of the typical DEV
measurements should show this. Each one may highlight things in a slightly different
way. 

4) Turn on stability with the OCXO always on power and the box cycled, should be 
much better. Effectively the “warm up” process is short circuited when running in 
this mode. 

How much any or all of these matter in the actual system is up to the user. However,
these are the areas that drive folks to put a reference on a clock generation device. If
they *do* matter, then they are the things that probably need to be characterized on
this or that reference ( and on the system).

Bob

> On Oct 27, 2022, at 1:39 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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