[sdiy] Aaron's second try at making a PCB

[Aaron Lanterman]

Thank you to the many, many folks who wrote with feedback on my first  
attempt to design a PCB, both on-list and off-list!

One consistent piece of advice I got was that if I was going to send  
this to a fab house anyway, I might as well just do double-sided since  
even if I ask for a single-sided board, they'll do a double-sided  
process and charge me accordingly, so single-sided doesn't really save  
any money, and double-sided would let me compress the board.
In addition, I was thinking that with the experience got on the first  
draft, I could do a much better job if I tried it again from scratch  
anyway.

So, I scrapped round one and started over, thinking with two sides  
from the start. Boy, does that make things easier!

The board is now 3" x 2.15". I wanted to round it down to 2" and shave  
off 0.15" just to have a round number of inches, but I don't think  
that can comfortably happen.

The mounting holes have a drill of .1102" (I dunno if there's a  
standard for that or not?)

I used a trace size of 0.016", except for the power lines, which are  
0.024", and the big fat power traces going to the 2.2R resistors,  
which are 0.05". In retrospect I'm not sure why I used 0.016" for the  
lines, since Eagle I think defaults to 0.01" - I think at some point I  
accidentally set it to 0.016" and kept going and then used that on  
almost everything.

This time, I got the various pads for off-board connections lined up  
nicely along the edge. The "pots" aren't trim pots - they're meant to  
be pads for off-board connections. I created my own symbol labeled  
"wiper," "down," and "up," and switched the pins around according to  
what made the layout nicer.

This version also has two bypass caps for each chip, as suggested by  
several parties.

Here is it without the grounds connected - you can see the airwires  
for the ground:

users.ece.gatech.edu/~lanterma/aarons_board_second_try.png

Here's what a copper pour for the ground on the pink layer looks like  
(blue layer hidden for clarity - note I had been intending to use pink  
for the ground, mostly):

users.ece.gatech.edu/~lanterma/aarons_board_pinklayer.png

Here's what a copper pour for the ground on the blue layer looks like  
(pink layer hidden for clarity - most of the signal is blue, so this  
doesn't work as well.)

users.ece.gatech.edu/~lanterma/aarons_board_bluelayer.png

The pour was done with 0.01" width, 0.024" spacing, and 0.024"  
isolation. I'm not sure if those are good parameters to pick or not.  
Grant mentioned "thermal relief," and I see little gaps around the  
ground pads, but I'm not sure if I have enough.

I got a lot of contradictory advice on the use (or not) of ground  
copper pours. I kind of like doing the copper pour since it's easy and  
fast to do. (I suppose I could even have a ground copper pour on both  
layers, but I'm not sure if that's a better or worse idea than just  
doing the copper pour on the pink layer).

If it seems that a copper pour is a bad idea, I could hand-run some  
ground traces. You can see on the yellow airwires where they need to  
go - I'm not sure what the optimal ground routing strategy is.

The input is in the upper left corner, and the mixed output in the  
lower left. The individual outs of the four bandpass filters are along  
the right and lower sides, with
the outputs next to their pots for the mixer. There is a long pink  
signal run from the pot on the right side of the board all the way to  
a 33K ohm in the mixer section. I don't think there's anyway to avoid  
running a long signal line somewhere, but at least this doesn't snake  
all the way around the board like in my last design.

The leftmost op amp, upper section, buffers the input, and the lower  
section is the mixer. The rest of the op amps are lowpass or highpass  
filters, with the output of
some filters feeding others.

As with last time, comments are most welcome!

I can feel myself slowly getting better at this.

- Aaron

P.S. Many things aren't labeled right now, or labeled messily - I'm  
gonna make sure I'm happy with the placement and routing first, and  
then go back and clean up the labeling.