MRF49XA-ISLASHT datasheet(13/102 Pages) MICROCHIP

13 / 102 page

Preliminary

DS70590C-page 13

MRF49XA

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.

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.

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 250

Ω of differential input

impedance which functions well with the proposed

antenna (PCB/Monopole) during signal transmission.

The LNA, when connected to the 50

Ωdevice, 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.

TABLE 2-2:

RECOMMENDED BYPASS

CAPACITORS VALUE

Band

(MHz)

C1(

μF)

C2 (nF)

C3 (pF)

433

2.2

220

868

2.2

915

2.2

TABLE 2-3:

RECOMMENDED BYPASS

CAPACITORS

Property

SMD Size

0603

Dielectric

Tantalum

Ceramic

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