MRF49XA-ISLASHT datasheet(75/102 Pages) MICROCHIP

75 / 102 page

Preliminary

DS70590C-page 75

MRF49XA

The following guidelines explain the requirements of

the above mentioned layers.

• It is important to keep the original PCB thickness,

since any change will affect antenna performance

(see total thickness of dielectric) or microstrip

lines’ characteristic impedance.

• For good transmit and receive performance, the

trace lengths at RF pins must be kept as short as

possible. Using small, surface mount components

(in 0402/0603 package) yields good performance

and keeps the RF circuit small. RF connections

should be short and direct.

• Except for the antenna layout, avoid sharp

corners since they can act as an antenna. Round

corners will eliminate possible future EMI

problems.

• Digital lines are prone to be very noisy when

handling periodic waveforms and fast

clock/switching rates. Avoid RF signal layout

close to any of the digital lines.

• A VIA filled ground patch underneath the IC

transceiver is mandatory.

• Power supply must be distributed to each pin in a

star topology and low-ESR capacitors must be

placed at each pin for proper decoupling noise.

• Thorough decoupling on each power pin is

beneficial for reducing in-band transceiver noise,

particularly when this noise degrades perfor-

mance. Usually, low value caps (27 pF – 47 pF)

combined with large value caps (100 nF) will

cover a large spectrum of frequency.

• Passive component (inductors) should be in the

high-frequency category and the Self Resonant

Frequency (SRF) should be at least two times

higher than the operating frequency.

• The additional trace length affects the crystal

oscillator by adding parasitic capacitance to the

overall load of the crystal. To minimize this, place

the crystal as close as possible to the RF device.

• Setting short and direct connections between the

components on board minimizes the effects of

“frequency pulling” that might be introduced by

stray capacitance. It even allows the internal load

capacitance of the chip to be more effective in

properly loading the crystal oscillator circuit.

• Long run tracks of clock signal may radiate and

cause interference. This can degrade receiver

performance and add harmonics or unwanted

modulation to the transmitter.

• Keep clock connections as short as possible and

surround the clock trace with an adjacent ground

plane pour. Pouring helps in reducing any

radiation or crosstalk due to long run traces of the

clock signal.

• Low value decoupling capacitors, typically 0.01

μF

– 0.1

μF, should be placed for VDD of the chip and

for bias points of the RF circuit.

• High value decoupling capacitors, typically 2.2

μF

– 10

μF, should be placed at the point where

power is applied to the PCB.

• Power supply bypassing is necessary. Poor

bypassing contributes to conducted interference

which can cause noise and spurious signals to

couple into the RF sections, significantly reducing

performance.

MRF49XA-I/T データシート(PDF) 75 Page - Microchip Technology

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