4-236
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.
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
™
QUADDRATURE
HFA3925
HFA3724
HSP3824
D
A
T
A T
O MA
C
CTRL
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
ANTSEL
LNA
AND TX/RX
SWITCH
RF2304
BPF1a
BPF1b
SELECT
Q ADC
5TH ORDER
BUTTERWORTH
LOW PASS
FILTERS
QUADRATURE
DEMOD
BASEBAND
MODULATOR
ADC CCA
PROCESSOR
DUAL SYNTHESIZER
OSC
22MHz
CLK
CONTROL
TEST
I/O
LIMITING IF /RSSI
TX/RX
SELECT
M
U
X
M
U
X
Q
I
Application Note January 1997
Authors: Carl Andren and John Fakatselis
http://www.intersil.com or 407-727-9207 | Copyright © Intersil Corporation 1999
PRISM and PRISM logo are trademarks of Intersil Corporation.