AN9614

Using the PRISM® Chip Set for Low Data Rate Applications Application Note January 1997 AN9614.1 Authors: Carl Andren and John Fakatselis Introduction The PRISM chip set has been optimized to address high data rate applications with up to 4 MBPS data rates. The PRISM can also be utilized for low data rate applications. To implement low data rate applications (below 250 KBPS) the designer needs to address design considerations in the following areas: ™ Description A. External IF Filtering 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. For the high rate configuration of the PRISM, a recommended implementation is to use SAW BPFs centered at 280MHz with a BW of about 17MHz. This is assuming an 11MHz chip rate (thus 22MHz spread null to null bandwidth). A recommended device that meets these requirements is the ToyoCom TQS-432. If a low data rate configuration is implemented then substitute IF filters need to be identified that will filter to the channel bandwidth of the spread waveform at the lower chip rate. The designer can use any IF center frequency within the HFA3724 range. The designer must be sure, though, that the identified filter meets the transmission spectral mask requirements for FCC for the 2.4MHz ISM band. SAW filters A. Selection of external filtering supporting the PRISM components. B. Limitations on filter cut off frequencies of the HFA3724 internal Low Pass Filters. C. Selection of appropriate carrier and clock oscillators to achieve the desired performance, given the HSP3824 internal Acquisition and Tracking loop integration constraints. The system designer should also evaluate the option where the radio maintains its high data rate configuration but transmits the data using infrequent high data rate burst packets. Where the system requires that the radio operate at low rates (