Datasheet

Applications Information
Output Matching to 50Ω
When matched to a 50Ω system, the MAX7058’s PA is
capable of delivering +10dBm of output power at V
DD
=
+2.7V. The output of the PA is an open-drain transistor,
which has internal selectable shunt tuning capacitors for
impedance matching (see the Variable Capacitor section).
It is connected to V
DD
through a pullup inductor for proper
biasing. The internal selectable shunt capacitors make
it easy for tuning when changing the output frequency.
The pullup inductance from the PAOUT to V
DD
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
into V
DD
. 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 MAX7058 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 impedance that consists of a lossy component and
a radiative component. To achieve high radiating effi-
ciency, the radiative component should be as high
as possible, while minimizing the lossy component. In
addition, the loop antenna has an inherent loop induc-
tance 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 may 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 radi-
ation. At high frequencies, trace lengths that are on
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, one inch 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 VDD pins.
Chip Information
PROCESS: CMOS
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MAX7058 315MHz/390MHz Dual-Frequency ASK Transmitter