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
DD
pins.
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].
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)