MRF49XA Datasheet, PDF (13/102 Pages) Microchip Technology – ISM Band Sub-GHz RF Transceiver

2.1 Power and Ground Pins
The power supply bypassing is very essential for better
handling of signal surges and noise in the power line.
The large value decoupling capacitors should be
placed at the PCB power input. The smaller value
decoupling capacitors should be placed at every power
point of the device and at bias points for the RF port.
Poor bypassing leads to conducted interference which
can cause noise and spurious signals to couple into the
RF sections, thereby significantly reducing the
performance.
The VDD pin requires two bypass capacitors to ensure
sufficient bypass and decoupling. However, based on
the selected carrier frequency, the bypass capacitor
values vary. The recommended bypass capacitor
values are listed in Table 2-2 and the type of capacitor
to be used is listed in Table 2-3. The bypass capacitors
are connected to pin 14, as shown in Figure 4-1. The
trace length (VDD pin to bypass capacitors) should be
made as short as possible.
TABLE 2-2:
Band
(MHz)
433
868
915
RECOMMENDED BYPASS
CAPACITORS VALUE
C1(μF)
C2 (nF)
C3 (pF)
2.2
10
220
2.2
10
47
2.2
10
33
TABLE 2-3: RECOMMENDED BYPASS
CAPACITORS
Property
C1
C2
C3
SMD Size
A
Dielectric Tantalum
0603
Ceramic
0603
Ceramic
2.2 RESET Pin
An external hardware Reset of MRF49XA can be per-
formed by asserting the RESET (pin 10) to low. After
releasing the pin, it takes slightly more than 0.25 ms for
the transceiver to be released from the Reset. The pin
is driven with an open-drain output, and hence, it is
pulled down while the device is in POR. The RESET
pin has an internal, weak, on-chip, pull-up resistor. The
device will not accept commands during the Reset
period.
MRF49XA
The device enters the Reset mode if any of the
following events take place:
• Power-on Reset
• Power Glitch Reset
• Software Reset
• RESET Pin
Software Reset can be issued by sending the appropri-
ate control command to the device. The result of the
command is similar to POR, but the duration of the
Reset event is much less, typically 0.25 ms. The Soft-
ware Reset works only when the Sensitive Reset mode
is selected. See Section 3.1 “Reset” for details on
Reset; for connection details, see Figure 4-1.
2.3 Power Amplifier
The Power Amplifier (PA) has an open-collector differen-
tial output and can directly drive different PCB antennas,
like loop or dipole, with a programmable output power
level during signal transmission. However, certain types
of antennas, like monopole, need an additional matching
circuitry. A built-in, automatic antenna tuning circuit is
used to avoid the manual tuning and trimming
procedures during production process; the so called
“hand effect”.
2.4 Low Noise Amplifier
The Low Noise Amplifier (LNA) has approximately
250Ω of differential input impedance which functions
well with the proposed antenna (PCB/Monopole) dur-
ing signal transmission. The LNA, when connected to
the 50Ω device, needs an external matching circuit
(Balun) for correct matching and to minimize the noise
figure of the receiver.
The LNA gain can be selected in four steps for different
gain factors (between 0 and -20 dB relative to the high-
est gain) based on the required RF signal strength.
This gain selection feature is useful in a noisy
environment.
© 2009 Microchip Technology Inc.
Preliminary
DS70590B-page 11