An input signal is filtered to remove aliases and a switched capacitor network undersamples this signal to provide a downconverted, near-baseband signal. The discharge vessel encloses a discharge volume containing an ionizable gas filling including at least a metal halide, two current lead-through conductors connected to the respective electrode, and a seal having sealing material through which at least one of the respective current lead-through conductors issues to the exterior of the discharge vessel. The pitch of the spiral cut can be varied to facilitate a gradual transition in flexibility along the catheter. The center side member is set larger in strength than the side sill.