The band pass filters shown between the HFA3624 and the HFA3724 labeled as BPF1a and BPF1b on Figure 1 are centered at IF and filter the spread wideband waveform before demodulation on the receive side and before the final upconversion on the transmit side. One might think that the TX filter can be avoided but it is required to meet the sidelobe suppression specifications according to FCC requirements.