MAX1479 Datasheet, PDF(8/11 Page) Maxim Integrated Products – 300MHz to 450MHz Low-Power, Crystal-Based +10dBm ASK/FSK Transmitter

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MAX1479 Datasheet, PDF (8/11 Pages) Maxim Integrated Products – 300MHz to 450MHz Low-Power, Crystal-Based +10dBm ASK/FSK Transmitter

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300MHz to 450MHz Low-Power,

Crystal-Based +10dBm ASK/FSK Transmitter

Output Matching to

PC Board Loop Antenna

In most applications, the MAX1479 power-amplifier out-

put has to be impedance matched to a small-loop

antenna. The antenna is usually fabricated out of a cop-

per trace on a PC board in a rectangular, circular, or

square pattern. The antenna has an impedance that

consists of a lossy component and a radiative compo-

nent. To achieve high radiating efficiency, the radiative

component should be as high as possible, while mini-

mizing the lossy component. In addition, the loop

antenna has an inherent loop inductance associated

with it (assuming the antenna is terminated to ground).

For example, in a typical application, the radiative

impedance is less than 0.5â¦, the lossy impedance is

less than 0.7â¦, and the inductance is approximately

50nH to 100nH.

The objective of the matching network is to match the

power-amplifier output to the impedance of the small-

loop antenna. The matching components thus tune out

the loop inductance and transform the low radiative

and resistive parts of the antenna into the much higher

value of the PA output. This gives higher efficiency. The

low radiative and lossy components of the small-loop

antenna result in a higher Q matching network than the

50â¦ network; thus, the harmonics are lower.

Table 3. Component Values for Typical

Application Circuit

COMPONENT

C10

C11

C12

C14

C15

VALUE FOR

fRF = 433MHz

22nH

18nH

6.8pF

10pF

10nF

680pF

6.8pF

220pF

10nF

220pF

100pF

VALUE FOR

fRF = 315MHz

27nH

22nH

15pF

22pF

10nF

680pF

15pF

220pF

10nF

220pF

100pF

Layout Considerations

A properly designed PC board is an essential part of

any RF/microwave circuit. On the power-amplifier out-

put, use controlled-impedance lines and keep them as

short as possible to minimize losses and radiation.

Keeping the traces short reduces parasitic inductance.

Generally, 1in of PC board trace adds about 20nH of

parasitic inductance. Parasitic inductance can have a

dramatic effect on the effective inductance. For exam-

ple, a 0.5in trace connecting a 100nH inductor adds an

extra 10nH of inductance, or 10%.

To reduce the parasitic inductance, use wider traces

and a solid ground or power plane below the signal

traces. Using a solid ground plane can reduce the par-

asitic inductance from approximately 20nH/in to 7nH/in.

Also, use low-inductance connections to ground on all

GND pins and place decoupling capacitors close to all

VDD connections.

Chip Information

TRANSISTOR COUNT: 2369

PROCESS: CMOS

8 _______________________________________________________________________________________

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