John Swenson tutoring paper: "Considerations regarding usage of external reference clocks" (EtherREGEN and other): Sine vs. square, impedance, cables—and a money saving surprise at the end.

[James Stephens]

46 minutes ago, LowMidHigh said:

 

Very helpful, John!

 

And yes, I can see how the increments on the LV-77S is lower than the other cables.

Guess I've just pointed me at the right direction...

 

Hi there,

 

Canare LV-77S does have the lowest attenuation with frequency of the listed cables here:

http://www.canare.com/ProductItemDisplay.aspx?productItemID=74

 

I have been using this up to now for a square wave cable.

 

Of course the info shared today changes things .. lol I might just be buying an AfterDark sine wave clock now!

 

Will read and re-read a few more times first though ....

 

For square wave cable hunters though, Belden 1694A does look to be even better (lower) by the metric of attenuation vs frequency:

 

https://catalog.belden.com/techdata/EN/1694A_techdata.pdf

 

Best,

James

[James Stephens]

29 minutes ago, MartinT said:

 

Trust your ears, it's not just about measurements.

 

Of course.

 

Best,

James

 

[James Stephens]

Are there any other clocks, besides I imagine the Mutec Ref 10 and Ref 10 SE 120 (?), for which we might expect the square wave output into an ER to surpass its sine wave output (+ of course the Mini-Circuits filter)?

 

Thanks!

[James Stephens]

45 minutes ago, Superdad said:

 

[When you accidentally posted your question in AfterDark’s “Filter group buy” thread I answered there. But since you caught that the question is more appropriate here, I’ll just repost my long answer below...]

 

Hi James:

While I understand what you are asking, the phrasing of the latter part of your question does not really fit. You seem to imply that there are first-rate clocks that offer both square and sine waves from the same box. That’s not the case with the Mutec (square only) nor any of the better boxes from Asia (mostly sine) that I am aware of.

 

As John explained in his paper, it takes a lot of care to both produce a really good square wave AND to preserve that quality (via cabling) all the way to the target device. Not that it can’t be done.  And OXCO modules (the circuits in the cans along with the crystal) are available with square wave output. But how that is implemented and handled varies greatly.

Also, some companies (and I have no idea about what AfterDark does to offer square wave in some models) use sine>square circuits between the OCXO module and the output.  There exist a couple of chips that do this extremely well, but then there are the rest which are rather poor at it (and then the truly terrible squarer mess that is in the BG7TBL boxes).

So with inexpensive square wave output clocks you really never know what you will get. Yet with sine wave clocks you need to be very careful to filter to a very clean wave—best done at the endpoint target device end.

 

Back to your question—which likely was more meant to be something like:
“Can a really low-phase noise sine wave output clock—terminated with the Mini-Circuits filter—outperform a first-rate square wave box such as a Mutec REF10 SE20 (assuming $$$ also spent on a cable for that square wave)?

 

I certainly can’t answer that and I think John would decline to as well without making extremely careful (and rather difficult to set up) measurements.  And even the characteristics of the receiver in the endpoint target would likely have significant affect on such a comparison.

 

I write all the above primarily so that we can all begin thinking about these things from the proper angles. It’s confusing stuff! 

 

Hi Alex,

 

Thanks very much.

 

There are of course sine + square wave output choices for BG7TBL and some AfterDark clocks but I forgot there wasn't a sine wave output for the Mutec clocks. 🤭

 

Ok here goes again lol:

 

My sense having read the clock paper is that square wave outputs are to be avoided except perhaps in the case of the Mutec clocks .. and that is what I was driving at in my question .... although I was trying to simultaneously enquire whether there might be any other clocks besides the Mutec that might have a good enough square wave output that it is worth consideration.

 

.. of course even then I get it that preserving that square wave all they way to the target device has its challenges.

 

Thanks again to you and John for everything!

 

Best,

James

 

[James Stephens]

 

Specs for the Belden 4794R are here: https://catalog.belden.com/techdata/EN/4794R_techdata.pdf

 

It has even better attenuation vs frequency characteristics than Belden 1694A and 4694R.

 

Martin just triggered my memory that Alex shared some info on where to get this cable offering back in October:

 

No telling what will sound best in your system with your ears.

 

Best,

James

 

[James Stephens]

1 minute ago, soares said:

Yes I have. Thank you so much Martin!

Gosh... 😂

Cheers Jorge

And which is in turn sine wave, 50 Ohm .... no?

[James Stephens]

28 minutes ago, soares said:

75 Ohms

Hi there,

 

Ok. For the BG7TBL I purchased from the same selller it stated clearly in the listing that is was 75 Ohm square Wave clock but I don’t see that listed for yours which made me think it might be a sine wave clock.

 

The filter is for sine wave clocks only. A square wave is comprised of a fundamental frequency plus its odd-order harmonics; the filter will attenuate those harmonics and the effect wil be to damage the wave. If it was even a square to start with, it certainly won’t be afterwards.

 

Best,

James

[James Stephens]

 

7 hours ago, DarqueKnight said:

The filter did not just damage the square wave, it changed it back to a sine wave!

 

 

 

Absolutely. The filter specs show that insertion loss rises above 40dB around 22MHz. Every single odd-order harmonic falls above that so is attenuated at least 40dB (i.e at least 100x from a voltage perspective) whereas the fundamental remains untouched.

 

Thanks for working these up and sharing.