MRF89XAM8A-I Datasheet, PDF (94/140 Pages) Microchip Technology – Ultra Low-Power, Integrated ISM Band Sub-GHz Transceiver

MRF89XA
4.1 RF Transmitter Matching
The optimum load for the RF port at a given frequency
band is listed in Table 4-1. These load values in the
table are expected by the RF port pins to have as an
antenna load for maximum power transfer. For all
antenna applications, an RF choke inductor (L2) must
be included during transmission because the RF out-
puts are of open-collector type.
4.2 Antenna Components
The MRF89XA is single-ended and has an unbalanced
input and output impedance close to 30-j25. Therefore,
it only requires a matching circuit to the SAW filter and
antenna. The C11, C12, and L6 are part of the match-
ing network these components make for the antenna
circuit. L1, C4, and C5 are tuned to provide that
impedance (30+j25) to the RFIO pin. In this case, the
transceiver will be able to transfer all power toward the
antenna. This impedance is called Optimum Load
Impedance. L2 is a RF choke inductor. L3 and L4 are
basically VCO inductors. The details are shown in
Figure 4-1.
TABLE 4-1:
ANTENNA LOAD VALUES
FOR 868 MHz AND 915 MHz
FREQUENCY BANDS
Band
FL1
C5
C4
L1
868 MHz TA0801A 1.8 pF 22 pF 8.2 nH
915 MHz TA0281A 1.8 pF 30 pF 10 nH
Note 1:
The SAW filter can be of EPCOS (B3717
and B3588) or Taisaw (TA0801A and
TA0281A) for 868 MHz and 915 MHz
respectively with matching components
remaining the same as shown in Figure 4-1
and Table 4-1.
4.3 SAW FILTER
FL1 is a SAW filter. While in Transmitting mode, the
SAW filter is used to suppress the harmonics. While in
Receiving mode, the SAW filter is used to reject the
image frequencies and out-of-band interfering signals.
4.3.1 SAW FILTER PLOT
Figure 4-2 and Figure 4-3 illustrates the plots of the
SAW filter used in the application circuit. The plots
shown are representative. For exact specifications,
refer to the SAW Filter manufacturer data sheet.
DS70622C-page 94
Preliminary
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