Applications Information

Output Matching to 50

Ω

When matched to a 50

Ω system, the MAX7057’s 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

Ω or 10Ω.

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

λ/10 or longer act as antennas, where λ 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

300MHz to 450MHz Frequency-Programmable

ASK/FSK Transmitter

______________________________________________________________________________________

17

BIT

NAME

FUNCTION

0

NoXTAL

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

1

TxREADY

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

2

RESERVED “0”

RESERVED. Set to 0 for normal operation.

3

X

RESERVED

7-4

fhi/lo[15]–fhi/lo[12]

ASK mode: Outputs flo[15:12].

FSK mode: when datain pin/bit is high, outputs fhi[15:12]; when datain pin/bit is low, outputs flo[15:12].

Table 17. Status Register (Address: 0x0C)