Circuit Design
We modified our circuit design from the circuit in the RFID design reference from Microchip. We removed several of the chips they used to do pre-processing of the bitstream, and implemented everything in code. We assembled a breadboard, which we used to do testing for converting everything for 5V. Once we had the entire circuit running on the breadboard, we layed out the schematic in Capture CIS. This allowed us to create a PCB design in Layout Plus.
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PCB
The printed circuit board went through several revisions. We were trying to keep the board small, comparable in size to existing readers, so we bought a small project box, and designed everything to fit, so we used many surface mount components, although we still had to use through hole parts for some things.
Our first revision was the first experience creating a 2 layer circuit board on the brand-new circuit mill, using conductive epoxy. This was quite a process. We also discovered that we hadn't received the solder paste, so we hand soldered the traces, used flux to hold the parts on, then baked them in our Zelflow oven. This board had several problems, including the fact that most of the parts that had to be installed in a certain direction (diodes, polarized caps, etc.) were installed backwards.
In the second revision we did a better job, although we discovered that the pinout for our USB plug was wrong, and that soldering a component into a through-hole would melt the conductive epoxy, making the connection stronger than the copper traces. Despite all the problems, this board did end up working.
Our third (and final) revision was our most successful. The components were reorganized by hand to make better use of the space on the board, and the through plating was done by hand instead of using the automatic dispenser. The miswired USB connector was corrected, and everything worked as planned. The only problem is that the tuning is difficult because it changes when the box is closed and the circuitry changes the inductance of the coil. Our read range is still comparable to the existing readers, so we're satisfied with the result. An image of the board layout it available here.
