[Arm-netbook] HDMI High-Frequency Layout: Recommendations Vias Moveable?

[Richard Wilbur]

On Nov 24, 2017, at 23:15, Luke Kenneth Casson Leighton <lkcl at lkcl.net> wrote:

Thank you for the picture.

> ok so this shows what's going on: already those GND vias have been
> moved significantly away from the pads associated with them (the ones
> for the 5V0) and those are MAJOR power (2A) so it's really rather
> important to have them close.

Agreed!

> now,  what i _could_ do is turn the one connected to IPSOUT round by
> 90 degrees (anti-clockwise) and move it upwards, making use of the GND
> vias near the HX2P/N diff-pair vias.  then there would be room to do
> the same thing with the 5V0 power in one as well.

Sounds good.

> one thing: in speaking with mike from the factory, he's been tracking
> the price of capacitors now, which have showed a FIVE TIMES increase
> in price over the past six months.  0.1uF, 10uF and 100uF capacitors
> in particular, the prices have gone INSANE.

[.…]

Sorry about the demand fluctuation creating such huge price fluctuations.

> so in the meantime what i think i will do is, put in double batches of
> *smaller* capacitors, with unusual values (three 33uF instead of one
> 100uF), which is, from what i've read, a better way to ensure stable
> power *anyway*.
> 
> as in, two 4.7uF capacitors stabilise power much better than one 10uF
> capacitor because of the increased current capacity of two caps -
> something like that, anyway.

Great idea!  You are correct in that the real capacitor which we can mount on a circuit board has an equivalent circuit which includes other components to the impedance besides just capacitance.  Namely ESR (Equivalent Series Resistance) and inductance.  The parasitic resistance dominates low-frequency response while the parasitic inductance dominates high-frequency response.  If you have the room, both of these can be combatted by doing just what you have described!

> bottom line: if i shuffle in two 0603 4.7uF caps as a replacement for
> the 0805 10uF, it reduces the cost on the BOM based on current
> pricing, improves power stability, and also means that those VIAs in
> the middle there can entirely be removed.

Excellent solution to both problems!  Id est, "Good show, mate!"