MRF49XAT-I-ST 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
performed 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
appropriate 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
Software 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 PA has an open-collector differential 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 LNA has approximately 250Ω of differential input
impedance which functions well with the proposed
antenna (PCB/Monopole) during 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 dB and -20 dB relative to the
highest gain) based on the required RF signal strength.
This gain selection feature is useful in a noisy
environment.
© 2009-2011 Microchip Technology Inc.
Preliminary
DS70590C-page 13