MAX1479ATE+T【PDF 8/10 页】IC TRANSM ASK/FSK 16-TQFN

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Sheet目录55

4页 5页 6页 7页 [8页]9页 10页

300MHz to 450MHz Low-Power,

Crystal-Based +10dBm ASK/FSK Transmitter

8 _______________________________________________________________________________________

Output Matching to

PCB 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 PCB in a rectangular, circular, or square

pattern. The antenna has an impedance that consists of

a lossy component and a radiative component. To

achieve high radiating efficiency, the radiative compo-

nent should be as high as possible, while minimizing

the lossy component. In addition, the loop antenna has

an inherent loop inductance associated with it (assum-

ing the antenna is terminated to ground). For example,

in a typical application, the radiative impedance is less

than 0.5? the lossy impedance is less than 0.7? 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

50?network; thus, the harmonics are lower.

Layout Considerations

A properly designed PCB is an essential part of any

RF/microwave circuit. On the power-amplifier output,

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 PCB trace adds about 20nH of para-

sitic inductance. Parasitic inductance can have a dra-

matic effect on the effective inductance. For example, 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

connections.

COMPONENT

VALUE FOR

= 433MHz

VALUE FOR

= 315MHz

22nH

27nH

18nH

22nH

6.8pF

15pF

10pF

22pF

10nF

680pF

6.8pF

15pF

220pF

C10

10nF

C11

220pF

C12

220pF

C14

100pF

C15

100 F

Table 3. Component Values for Typical

Application Circuit

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