With the 1000baseT1 boards mostly alive, but no progress on decoding until I get the new coupler board, I'm shifting gears to the trigger crossbar.
Building the back side tonight. Probably won't have time to do the front until tomorrow.
I need to figure out a solution for working with larger boards. This thing is maxing out my current stencil fixture. And my dreams are only getting bigger from here.
Ok, kid is in bed and higher priority stuff dealt with.
The back side shouldn't take too long to populate, it's mostly 0402 0.47 uF bypass caps. There's a few other parts but it goes quick when most are the same value.
@thomask77 In specialized applications when you are targeting narrowband noise at a specific, known frequency? Yes, you want to match the cap (including mounting parasitics) to have the lowest possible impedance at that frequency which may mean going with a smaller value.
For general purpose broadband use targeting a wide range of frequencies, not so much. ESR/ESL tend to be close to constant for a given package size and construction (e.g. all Murata GRM series 0402s are likely pretty close) with only C varying. So more C will improve decoupling effectiveness if package size is kept constant.
Also keep in mind the difference between voltage *rating* ("the cap won't blow at this voltage") and the *practically usable* max voltage accounting for the C/V curve. Many higher valued MLCCs derate to 20% or less of datasheet capacitance at Vmax.
Higher C in smaller package size means higher electric field strength and a sharper C/V falloff. So a 470n will lose performance faster than a 100 as voltage climbs.
@thomask77 Based on research I did several years ago, and agreeing with Xilinx recommendations at the time, 470n 0402 was a good sweet spot for typical lowish voltage digital logic work in terms of fitting as much C as possible into an 0402 footprint (which fits well across adjacent vias in a 1mm BGA field) without C/V degrading to an unacceptable level.
It's possible that dielectric tech may have improved since and you can get higher values in the same package with decent performance, but I still have several thousand of this particular Samsung part on the shelf so I'll continue to design around it until I run out.
@thomask77 Oh, and also consider package thickness. A lower profile package in the same footprint will have thinner spaced dielectric layers (because less volume to work with) and thus probably worse C/V characteristics.
(Unless they're the same plate stack padded out to a standard thickness and you're just bolting non-functional dielectric to either side, in which case they'd perform ~identically)
Andrew Zonenberg
thomask77