For reference this is a cheap analog RCA capture card I got from amazon a while back. Today I broke the enclosure on accident and as a studying EE who does some PCB design, I was curious about this.
I am assuming these are GND stitching vias, but was curious why there are so many? Did they go overboard or is this crucial to have for good signal?
Yes, those are ground stitching vias. Yes, there are more there than strictly necessary.
Don't use this board as an example of how to do ground stitching.
There are several spots where the designer should have done something else.
Have a look here:
Between the text "C1" and "C2" is a little "island" of ground plane that was left over from the solid ground pour after placing parts and routing traces.
That "island" should have been removed. Those things can radiate high frequencies generated by currents through the ground plane.
The designer tacked the "islands" to another ground plane, so they (probably) won't radiate anything.
It would have been better to remove the little islands than to stitch them. With stitching vias you can be pretty sure the islands won't radiate - but you have to pay for the vias on each board.
If you remove the islands, then they won't radiate at all - and you don't have to pay for vias or remember to stitch that area if you change the layout.
There's another island by R2, and a larger one by R4.
Yes, they're ground-stitching vias. You might also see similar patterns for thermal conduction, but that doesn't seem to be the case here.
Did they go overboard or is this crucial to have for good signal?
There are probably more vias there than are strictly needed, but a few extra vias is cheaper than a board re-spin.
This would capture RGB 75 Ohm signals with perhaps 10MHz or more bandwidth if high end capture . But SNR must be > 50dB for high Quality analog video. This means using 1V standard video the goal for noise ingress is <1mV. Thus using coax like shielding with lots of vias between ground tracks an ground planes and surface signals is a good way for <30MHz yet not for microwave as the via inductance is significant. So more important is to tie track ground tracks to ground plane to reduce induction of stray H fields as E fields won’t be a problem, but long current loops can be a problem with motors or DCDC converters nearby for example. A proper design would compute impedances and crosstalk of noise signals < 10MHz for aliasing at pixel rate . 1mV is -60dB from 1V signal.
Overkill ? Depends on EMI sources and crosstalk criteria for noise from adjacent cards. Not so much for 4GHz as high current <10MHz
I doubt this video capture card was the designer’s first rodeo, so don’t get thrown off the horse by his design and assume he had a good reason for it, if it is a name brand card.
