A switched passive input network (SPIN) for 433 MHz ISM band wakeup receivers with on-off keying (OOK) is discussed. It is based on a surface acoustic wave (SAW) resonator which is operated in a switched mode. The signal energy accumulated in the SAW resonator during a charge period appears as a voltage pulse, exhibiting up to 24 dB (measured) voltage gain referred to the open-circuit antenna port voltage. Input signal and noise are narrowband band-pass filtered with 364 kHz bandwidth (measured), which is favourable compared to conventional input matching networks in the crowded 433 MHz ISM band. In this paper, we present an analytical expression for the transient voltage at the switched passive input network for arbitrary parallel resistances. The passive voltage gain is calculated and compared to simulation results and measurements of a trial system.