MRF49XA-ISLASHT datasheet(74/102 Pages) MICROCHIP

74 / 102 page

MRF49XA

DS70590C-page 74

Preliminary

4.2

Antenna Design Considerations

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.

4.3

RF Transmitter Matching

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.

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:

TWO BASIC COPPER FR4 LAYERS

FIGURE 4-4:

FOUR BASIC COPPER FR4 LAYERS

TABLE 4-1:

FREQUENCY BAND – ANTENNA ADMITTANCE/IMPEDANCE

MRF49XA

Admittance (ms)

Impedance (

Ω)

Inductance (nH)

433 MHz

2–j5.9

52+j152

868 MHz

1.2–j11.9

7.8+j83

15.4

915 MHz

1.49–j12.8

9+j77

13.6

Signal/Power/RF and

Common Ground

Dielectric Constant = 4.5

Signal/Power/RF and

Common Ground

Ground

Power Line Routing

RF Ground

Signal Layout

Dielectric Constant = 4.5

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