It’s possible to modify the SWP Transceiver front-end circuit presented earlier to a full functional SWP Tracer/Sniffer.
This is almost the same circuit as the SWP transceiver but I moved some parts around to make the signal flow a bit clearer.
I don’t drive the SWP_TX signal myself anymore. Instead the SWP_TX becomes an output which is directly connected to a SWP master (aka NFC controller chip):
The SWP slave / SWP sim part remains the same, and so does the TX/RX signal splitter circuit.
The trick to get it working is to feed-back the extracted RX signal as a current-sink on the SWP master side.
Here it’s done with a fast BFS20 transistor as a switch.
R1 defines the current. At a SWP signaling voltage of 1.8V the 1.8kOhm roughly sinks 1mA of current. Due to VCE(sat) of Q1 the actual current is slightly lower, but that’s okay. The specification allows us to down to 600µA. We’re at the safe side here.
R2 is the usual base resistor with C1 acting as a speed-up capacitor to improve the switching speed.
The IO voltages are 1.8V for SWP_TX and 3.3V for SWP_RX.
You can now connect SWP_TX and SWP_RX to a logic-analyzer/micro-controller and trace away.
For this circuit you can’t replace the parts with slower devices. The signal already takes a complete round-trip through the opamp and comparator. The overall propagation delay should be small enough not to cause any confusion on the SWP master side.