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MCLK at 24.576MHz: how bad is the signal integrity?

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hitech95:
Hi,
I've desiged an audio board a year ago, but until recently due to chip shortage I was not able to assemble it |O.
 I've finally received all the parts but it is not working as intended.

I've assebled one but the digital Amplifier IC have a flag about a bad MCLK, BCLK, LRCLK clock.  :horse:



That said with a logic analyzer cature everything seems fine, so I've asked a friend to take a capture with a scope:
(Yes the photos are what they are... he is a SW guy like me and borrowed the scope at work during launch break)

MCLK:

BCLK:

LRCLK:


These clocks are generated by a PCM9211, it have a 24.576Mhz OSC and with internal dividers it generate the various clocks.


The quartz clock seems fine to me, but the MCLK is really noisy.  :--
On the board I have impedance matched traces set at 50Ohm, it is a 4 layer board with GND planes in the middle.

The amplifier IC and the PCM9211 (Digital MUX and ADC) are only 2cm apart. so no long traces that might introduce a lot of delay.

Any idea on why this is happening? Any suggestion on what I could try to solve this? :-//

fourfathom:
Can you show us the layout?
It's hard to tell what's going on, since the ringing shown on the scope could easily be due to scope probe ground lead placement, but assuming that the signal is really as shown here are some comments:

* 2cm is quite short, and I wouldn't expect a lot of ringing -- but depending on the actual trace length and edge rate I suppose it's possible.

* There's nothing wrong with 50 Ohm trace impedance, but it's unlikely that you have either 50 Ohm driver Z or 50 Ohm input Z so you still need to take steps to get a good clock signal.  We commonly put a series resistor at the driver output to help match the trace Z, slow down the edge, and absorb reflections from the virtually unterminated end of the trace (a typical input pin Z is a very high R and small C to ground).  There are other ways to get good signal shape, but this method usually works well.  The series resistor method gives a full swing at the output.

T3sl4co1l:
FYI, imagebb doesn't allow embed images.  (They may show on your end due to browser cache.)

Seconding the above.

Tim

fourfathom:

--- Quote from: T3sl4co1l on January 21, 2022, 01:47:03 am ---FYI, imagebb doesn't allow embed images.  (They may show on your end due to browser cache.)
--- End quote ---

FWIW, I saw the image thumbnails.  When clicked they opened up in a new "ibb" tab.  Using Chrome.

T3sl4co1l:
Up-to-date Chrome here too.

Interesting.  On closer inspection, they flash for one frame, then disappear.  An ibb script loads but doesn't seem to be tracking.  (Maybe. Now I don't see it?)  It's uBlock cosmetic filter that's doing it but there's NO rule for those objects, and the logger agrees.

Tim

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