mrf89xa Microchip Technology Inc., mrf89xa Datasheet - Page 98

• Manufacturer Part Number: mrf89xa
• Description: Ultra-low Power, Integrated Ism Band Sub-ghz Transceiver
• Manufacturer: Microchip Technology Inc.

4.1
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
The MRF89XA is single-ended and has an unbalanced
input/output impedance close to 30-j25. Therefore, it
only requires a matching circuit to the SAW filter and
antenna. C11, C12, and L6, L1, C4, and C5 are tuned
to provide that impedance (30+j25) to the RFIO pin
(pin 31). 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:
DS70622B-page 98
MRF89XA
868 MHz TA0801A
915 MHz TA0281A
Band
RF Transmitter Matching
Antenna Components
FL1
ANTENNA LOAD VALUES
FOR 868 MHz AND 915 MHz
FREQUENCY BANDS
1.8 pF
1.8 pF
C5
22 pF
30 pF
C4
8.2 nH
10 nH
L1
Preliminary
4.3
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
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.
SAW FILTER
SAW FILTER PLOT
© 2010 Microchip Technology Inc.