Rainbow Electronics MAX7058 - Manual (Page 11)

Applications Information
Output Matching to 50 Ω Ω
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 transis-
tor, which has internal selectable shunt tuning capaci-
tors for impedance matching (see the
Variable
Capacitor section). It is connected to V DD through a
pullup inductor for proper biasing. The internal selec-
table 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, provid-
ing 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 attenuation 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 termi-
nated to ground). In a typical application, the induc-
tance 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
MAX7058
315MHz/390MHz Dual-Frequency
ASK Transmitter
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