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MRF49XA
4.2
Antenna Design Considerations
4.3
RF Transmitter Matching
The MRF49XA is designed to drive a differential output,
such as a dipole antenna or a loop antenna. The loop
antenna is ideally suited for applications where
compact size is required. The dipole is typically not a
good option for compact designs due to its inherent
size at resonance, and its space requirements around
the ground plane, to be an efficient antenna. A
monopole antenna can be used, along with a balun, or
by using the matching circuit.
The RF pins are of high impedance and differential
value. The optimum differential load for the RF port at
a given frequency band is shown 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. Antennas that are suited for such
values would be a Loop, Dipole and Folded Dipole. For
all antenna applications, either a bias, choke inductor
or coils must be included during transmission since the
RF outputs are of open-collector type.
TABLE 4-1:
FREQUENCY BAND – ANTENNA ADMITTANCE/IMPEDANCE
MRF49XA
433 MHz
868 MHz
915 MHz
Admittance (ms)
2–j5.9
1.2–j11.9
1.49–j12.8
Impedance ( Ω )
52+j152
7.8+j83
9+j77
Inductance (nH)
62
15.4
13.6
4.4 General PCB Layout Design
The guidelines in this section help the users in
high-frequency PCB layout design.
The printed circuit board is usually comprised of two or
four basic FR4 layers.
The two-layer printed circuit board has mixed
signal/power/RF and common ground routed in both
the layers.
The four-layer printed circuit board is comprised of the
following layers:
? Signal layout
? RF ground
? Power line routing
? Common ground
The four-layer PCB is shown in Figure 4-4 .
FIGURE 4-3:
FIGURE 4-4:
TWO BASIC COPPER FR4 LAYERS
FOUR BASIC COPPER FR4 LAYERS
Signal/Power/RF and
Common Ground
Dielectric Constant = 4.5
Signal/Power/RF and
Common Ground
Signal Layout
Dielectric Constant = 4.5
RF Ground
Dielectric Constant = 4.5
Power Line Routing
Dielectric Constant = 4.5
Ground
DS70590C-page 74
Preliminary
? 2009-2011 Microchip Technology Inc.
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