P2 decoupling - as seen on Hsync signal

Tubular

I noticed some RGB video signal leaking into the Hsync signal, while testing HD VGA these past couple of days.

At first I thought it might be IR drop related, then I noticed it was worse with text and lines (=high freq transitions), because the graphics tests with solid graphic blocks didn't have the same issue.

So, I'm guessing its decoupling cap related (open to other ideas though). I think one thing to try is to replace one of the two VIO caps in a block of 8 pins (= 1 LDO regulator) with a much smaller value, eg 47nF and see if it helps. So there would be 4u7 || 47nF.

This may be also affecting analog sigma delta operation

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Here's the scope shot of HSync (yellow trace) when there is text or lots of random lines on the screen






And here's the scope shot of HSync when there is solid blocks of color on the screen. Note there's only a spike where active video starts and stops.






In both these the blue trace is a zoomed in, ac coupled 1v8 rail. It also could do with some decoupling, or improving cro grounding

Rayman

I noticed that too...

Tubular

Great, Rayman. Was hoping it wasn't just my setup.

One thing I was wondering, with my setup there's about a 20~25cm long section of ribbon cable from P2-Eval header to VGA socket. From there it goes through a 1.5m decent quality coax vga cable to the monitor. But it occurs to me there could also be capacitive coupling in the ribbon cable, and I should solder something more shielded to eliminate this possibility

Phil Pilgrim (PhiPi)

It could also be the way your scope probes are connected -- particularly the ground leads. Can you describe your setup?

-Phil

Tubular

Yes, that could certainly be part of it. I'll setup again and get a photo, that'll help explain

Rayman

my setup looked like this.
Definitely possible that some noise is due to lack of ground plane to VGA header

cgracey

It seems to me that HSYNC integrity wouldn't be compromised, since it's some kind of level-sense, only, on the monitor side, and there's no RGB data disturbing HSYNC when it rises and falls (which might have induced a slight phase shift on HSYNC). The noise on HYSNC when it's low wouldn't register as anything to the monitor, which is looking for >1.4V, or so.

Tubular

Thats right, the monitor isn't complaining. This is more about using video as a tool to look at whats happening with HF currents etc
I was thinking we should construct a test screen which has vertical bars at different spacings/frequencies

Mark_T

That looks very much like capacitive crosstalk - does it change if the connector/lead is removed?

Tubular

I'm going to shorten the ribbon and see where we can get to

Tubular

I shortened the ribbon adapter from 23 to 5cm and that helped things a lot.

There's still a small residual amount of crosstalk when the cable is unplugged altogether, but it looks like it is indeed mostly crosstalk. Thanks guys

kwinn

I would bet that an adapter from the pin header on the P2 to a VGA connector that had alternating GND/Signal wires in a short ribbon cable would get rid of most of that crosstalk.

MIchael_Michalski

Tubular wrote: »

I shortened the ribbon adapter from 23 to 5cm and that helped things a lot.

There's still a small residual amount of crosstalk when the cable is unplugged altogether, but it looks like it is indeed mostly crosstalk. Thanks guys

You could try disconnecting the vga and making a connector that plugs directly into the board with termination resistors and see if it goes away completely.

Tubular

Good ideas

I'm keen to see how much an intermediate ground reduces crosstalk. Will give these ideas a try tomorrow