MAX7057ASE+T【PDF 17/20 页】IC TRANSMITTER ASK/FSK 16-SOIC

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Sheet目录55

13页 14页 15页 16页 [17页]18页 19页 20页

Applications Information

Output Matching to 50&

When matched to a 50& system, the MAX7057s PA is

capable of delivering +9.2dBm of output power at

PAVDD = +2.7V with a broadband match. The output of

the PA is an open-drain transistor, which has internal

selectable shunt tuning capacitors (see the Variable

Capacitor section) for impedance matching. It is con-

nected to PAVDD or ROUT through a pullup inductor

for proper biasing. The internal selectable shunt capac-

itors make it easy for tuning when changing the output

frequency. The pullup inductor from the PA to PAVDD

or ROUT serves three main purposes: resonating the

capacitive PA output, providing biasing for the PA, and

acting as a high-frequency choke to prevent RF energy

from coupling onto the supply voltage. The pi network

between the PA output and the antenna also forms a

lowpass filter that provides attenuation for the higher-

order harmonics.

Output Matching to PCB Loop Antenna

In many applications, the MAX7057 must be imped-

ance-matched to a small-loop antenna. The antenna is

usually fabricated out of a copper 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 component should be as high

as possible, while minimizing the lossy component. In

addition, a loop antenna has an inherent loop induc-

tance associated with it (assuming the antenna is termi-

nated to ground). In a typical application, the

inductance of the loop antenna is approximately 50nH

to 100nH. The radiative and lossy impedances can be

anywhere from a few tenths of an ohm to 5& or 10&.

Layout Considerations

A properly designed PCB is an essential part of any

RF/microwave circuit. At high-frequency inputs and out-

puts, use controlled-impedance lines and keep them as

short as possible to minimize losses and radiation. At

high frequencies, trace lengths that are in the order of

?10 or longer act as antennas, where ?is the wave-

length.

Keeping the traces short also reduces parasitic induc-

tance. Generally, 1in of PCB trace adds about 20nH of

parasitic inductance. The parasitic inductance can

have a dramatic effect on the effective inductance of a

passive component. For example, a 0.5in trace con-

necting to a 100nH inductor adds an extra 10nH of

inductance, or 10%.

To reduce parasitic inductance, use wider traces and a

solid ground or power plane below the signal traces.

Using a solid ground plane can reduce the parasitic

inductance from approximately 20nH/in to 7nH/in. Also,

use low-inductance connections to the ground plane,

and place decoupling capacitors as close as possible

to all V

pins.

300MHz to 450MHz Frequency-Programmable

ASK/FSK Transmitter

______________________________________________________________________________________ 17

BIT

NAME

FUNCTION

NoXTAL

Internal Crystal Oscillator Status. High means oscillator is not in operation.

TxREADY

Transmitter Ready Status. High means PLL is locked and MAX7057 is ready to transmit data.

RESERVED 0

RESERVED. Set to 0 for normal operation.

RESERVED

7-4

fhi/lo[15]fhi/lo[12]

ASK mode: Outputs flo[15:12].

FS K m od e: w hen d atai n p i

n/b i t i s hi g h, outp uts fhi [ 15:12] ; w hen d atai n p i n/b i t i

s l ow

, outp uts fl o[ 15:12] .

Table 17. Status Register (Address: 0x0C)

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