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.

[Mihaylov]

On 3/28/2021 at 12:10 AM, Superdad said:

However, John did this with the BG7TBL that I sent him and found quite a lot to criticize in that piece with regards to the squarer circuit. In fact, even the sine from the OXCO itself goes through the lousy squarer circuit and then a poor filter I think. Maybe he will speak of what he found as I think it might be educational. Certainly that box was not designed with audiophiles in mind. Would have been better just to run the OXCO module in the thing straight to a BNC jack.

Here are examples of various BG7TBL's products:

 

 

 

 

 

Mini-Circuits (from datasheet):

 

 

It can be seen that simple LCL low pass filters are responsible for creating a square wave signal at the output, exactly as in the Mini-Circuits and Crystek filters (circled in yellow). Blue circled where there is no LCL filter at the output and the sine wave signal at this output accordingly. So how do the recommended Mini-Circuits filters differ fundamentally from LCL filters in BG7TBL's products? Only more steep filter?

 

 

 

[Mihaylov]

Hi John! 

You are absolutely correct. I didn't quite work it out at first but now I get it all out. Thank you for your Clock Considerations article!

[Mihaylov]

On 3/27/2021 at 10:51 PM, Superdad said:

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

Can you specify the names of those two chips?

[Mihaylov]

On 4/22/2021 at 4:38 PM, Superdad said:

John’s examination of the BG7TBL’s circuits revealed, it does not at all qualify as having a “really good sine-square converter.”  It is actually quite poor in that regard. On the opposite side of the solar system in comparison to a Mutec REF10 for square wave performance.

What criterion (indicator) do you use to assess the quality of the conversion of sine to square? On what basis do you draw this conclusion about the quality of the conversion of sine-square to BG7TBL and Mutec? Based on product schematics analysis, measurement analysis (which?) or something else? Have any measurements of BG7TBL been taken in order to infer extremely low quality of square wave at the output of the clock?

[Mihaylov]

5 hours ago, Superdad said:

also made measurements at various places in the circuit.

Too bad he didn't publish the results of his measurements. :( 

[Mihaylov]

23 hours ago, Superdad said:

Someone else here published the nasty-looking square wave output of it as well.

If you're referring to this message, the square wave distortion is caused by impedance mismatch: the output clock impedance and cable impedance is 75 ohm, and the oscilloscope input resistance is 50 ohm.

 

 

Mutec shows the same bad curve when impedances are mismatched.

 

 

 

[Mihaylov]

Do I understand correctly that in order to convert the 75 ohm version of ER to 50 ohm, is it enough to replace the BNC connector with a 50 ohm connector and replace the R86 resistor with 50 ohms?