AN9614 Datasheet, PDF (1/3 Pages) Intersil Corporation – Using the PRISM® Chip Set for Low Data Rate Applications

Using the PRISM® Chip Set for Low
Data Rate Applications
Application Note
January 1997
AN9614.1
Authors: Carl Andren and John Fakatselis
Introduction
Description
™ 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
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.
following areas:
One might think that the TX filter can be avoided but it is
A. Selection of external filtering supporting the PRISM com-
ponents.
required to meet the sidelobe suppression specifications
according to FCC requirements.
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.
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
PRISM PCMCIA Reference Radio Block Diagram
ANTSEL
TX/RX
SELECT
LNA
RF2304
HFA3424
LNA
HFA3624
RF/IF
28SSOP
HFA3925
RF POWER AMP
AND TX/RX
SWITCH
4-236
BPF1a
FILTER CUTOFF
SELECT
HFA3724
Q MODEM
QUADRATURE
DEMOD
LIMITING IF /RSSI
BPF1b
I/Q LO
I
M
M
U
U
X
X
ADC CCA
I ADC
Q ADC
DE-
SPREAD
HSP3824
BASEBAND
PROCESSOR
DE-
MOD
TX/RCV
DATA
IO
VCO
VCO
HFA3524
DUAL SYNTHESIZER
Q
QUADDRATURE
MODULATOR
OSC
22MHz
5TH ORDER
BUTTERWORTH
LOW PASS
FILTERS
MOD/ CONTROL
SPREAD _{ENCODE TEST}
I/O
CLK
FIGURE 1. PRISM™ CHIP SET BLOCK DIAGRAM
http://www.intersil.com or 407-727-9207 | Copyright © Intersil Corporation 1999
PRISM and PRISM logo are trademarks of Intersil Corporation.