Min Xu min.xu@min-info.net wrote:

Can you tell me what the intent of the C1 capacitor is for?

It's a bypass cap between VCC and GND, physically placed as close as possible to the C1 (VCC) contact of the SIM socket. The venerable GSM 11.11 spec recommends placing a 100 nF ceramic capacitor "as close as possible to the contacting elements", to absorb current consumption spikes on the part of the SIM - the same spec allows the card to draw spikes of up to 200 mA for up to 400 ns! The design of all classic historical GSM phones (including TI reference designs which I copy in FreeCalypso) follows this recommendation of putting a bypass cap right next to their SIM socket C1 contact - but with the introduction of SIMtrace FPC cable between the phone's SIM socket and the actual SIM, now that phone-provided bypass cap is a cable stretch away, hence I felt it prudent to provide another bypass cap on the sim-fpc-pasv adapter PCB itself.

Previously when I probe the signals I just solder fly wires on to the SIM socket on the FPC,

Given that it costs the same to fab one PCB or 5 of them, I have 4 extra PCBs which I would be happy to give away if anyone else needs to do this kind of purely passive sniffing or probing. Just bare PCBs, though, I don't have extra socket parts. The source is published too, of course:

https://www.freecalypso.org/hg/fc-small-hw/

M~