AN9614

AN9614 INTERSIL | Alldatasheet

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AN9614.1 Using the PRISM® Chip Set for Low Data Rate Applications 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: A. Selection of external filtering supporting the PRISM com- ponents. 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 inter- nal 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 (<250 KBPS), the designer must address the areas highlighted on the PRISM block diagram shown in Figure 1.

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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 QUADDRATURE HFA3925 HFA3724 HSP3824 DATA TO MACCTRL FILTER CUTOFF I/Q LO VCO I ADC DATA IO TX/RCV VCO HFA3524 HFA3624 RF/IF 28SSOP Q MODEM RF POWER AMP DE- MOD MOD/ ENCODE DE- SPREAD SPREAD HFA3424 LNA PRISM PCMCIA Reference Radio Block Diagram FIGURE 1. PRISM™ CHIP SET BLOCK DIAGRAM PRISM and PRISM logo are trademarks of Intersil Corporation.

specifications and a custom design may be required. the final filtering stage of the complex spread waveform. four cut off frequencies that can be selected for these LPFs. bandwidth to accommodate oscillator frequency tolerances. the 120kHz offset (~1/8th of 1 MBPS).

250 KBPS, the same offset loss occurs with a frequency

1/8th chip over the 128 symbol integration interval. effected by lower data rates. FIGURE 2. PRISM™ CORRELATION PERFORMANCE vs

All Intersil semiconductor products are manufactured, assembled and tested underISO9000 quality systems certification. Intersil semiconductor products are sold by description only. Intersil Corporation reserves the right to make changes in circuit design and/or specifications at any time with- out notice. Accordingly, the reader is cautioned to verify that data sheets are current before placing orders. Information furnished by Intersil is believed to be accurate and reliable. However, no responsibility is assumed by Intersil or its subsidiaries for its use; nor for any infringements of patents or other rights of third parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of Intersil or its subsidiaries. For information regarding Intersil Corporation and its products, see web sitehttp://www.intersil.com HIGH RATE BURST TRANSMISSIONS WITH LOW AVERAGE RATE Generally, the incentive to use lower data rates is to achieve a given range with the minimum amount of power. We can show that this is also achievable by using the radio in its high data rate design configuration. The PRISM is a packet radio communications device and, as such, can send the data in a short burst with open environment ranges up to 5 miles. This has significant potential for power savings and reduction in interference. In the high data rate configuration the design considerations mentioned above are no longer of concern. The system approach is to accept the 1 MBPS data rate of the radio as long as the achievable range is acceptable, and use it in a short burst mode which is consistent with its’ packet nature. With a low power watch crystal, the controller can keep adequate time to operate either a polled or a time allocated scheme. In these modes, the radio is powered off most of the time and only awakens when communications is expected. This station would be awakened periodically to listen for a beacon transmission. The beacon serves to reset the timing and to alert the radio to traffic. If traffic is waiting, the radio is instructed when to listen and for how long. In a polled scheme, the remote radio can respond to the poll with its traffic if it has any. With these techniques, the average power consumption of the radio can be reduced by more than an order of magnitude while meeting all data transfer objectives. Even using the 802.11 network protocols, the low data rate can allow low average power operation. The Media Access Controller (MAC) or network processor can operate the radio in the sleep mode except for the times it needs to receive the beacon signals. The short, fast transmission is good for several reasons. First, if the signal is corrupted for any reason, a retransmission will occur without noticeable delay. Secondly, interference to other spectrum users is of brief duration. Third, and most important, the burst can be sent into small time gaps in the medium, which makes it more effective against certain type of interference in the ISM band. For example, if an 802.11 FH network is operating in the vicinity, it could cause interference with this network. The FH network has, however, a brief guard time when it is hopping and none of its stations are on the air. This time can be used to transmit the burst communications packets. Additionally, the microwave oven has been identified as an interference source of concern within the 2.4GHz ISM band. The oven is a pulsed source with about a 50% duty cycle. The gaps allow messages of about 1000 bytes through at the 1 MBPS rate. In addition, the system can be set at its sleep mode most of the time to achieve low power consumption. It only needs to operate at full power consumption during the transmission of a packet or during the expected window for received packets. The communications range achievable depends on the nature of the environment. A line of sight (LOS) path allows the best range. With 1W and 6dB gain in the antennas, you can readily achieve a 5 mile LOS range. The propagation loss at S-band is less than 0.5dB per mile in heavy rain, so weather is not usually of great concern. Antennas with 6dB gain are for fixed installations with one on one links. Mobile and network installations use omnidirectional antennas with around 0dB gain. Indoors, the range is much reduced by extra losses due to walls and other obstructions. The power is also usually reduced to 100mW for interference and safety concerns. These reduce the available range, but most applications will achieve sufficient range (300 ft.). Antenna diversity is also used in the PRISM design to combat multipath interference. Since the PRISM waveform is wideband by being spread at the chip rate, the 1 MBPS data rate is not a contributor to multipath problems and a lower data rate is of no benefit. So, in general, unless it is required to use low instantaneous data rates to achieve some other purpose, the packet capabilities of PRISM will serve well for these applications in its normal high data rate design configuration. Application Note 9614