Datasheet

Applications Information
Output Matching to 50Ω
When matched to a 50Ω 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 sec-
tion) for impedance matching. It is connected to PAVDD
or ROUT through a pullup inductor for proper biasing.
The internal selectable shunt capacitors 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, pro-
viding 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 attenua-
tion 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 inductance associated with
it (assuming the antenna is terminated 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 5Ω or 10Ω.
Layout Considerations
A properly designed PCB is an essential part of any RF/
microwave circuit. At high-frequency inputs and outputs,
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 wavelength.
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 connecting 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.
Table 17. Status Register (Address: 0x0C)
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].
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Maxim Integrated │ 17
MAX7057 300MHz to 450MHz Frequency-Programmable
ASK/FSK Transmitter