MAX1479_09 Datasheet, PDF (8/10 Pages) Maxim Integrated Products – 300MHz to 450MHz Low-Power, Crystal-Based +10dBm ASK/FSK Transmitter

300MHz to 450MHz Low-Power,
Crystal-Based +10dBm ASK/FSK Transmitter
Output Matching to
PCB Loop Antenna
In most applications, the MAX1479 power-amplifier out-
put has to be impedance matched to a small-loop
antenna. The antenna is usually fabricated out of a cop-
per 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 compo-
nent should be as high as possible, while minimizing
the lossy component. In addition, the loop antenna has
an inherent loop inductance associated with it (assum-
ing the antenna is terminated to ground). For example,
in a typical application, the radiative impedance is less
than 0.5Ω, the lossy impedance is less than 0.7Ω, and
the inductance is approximately 50nH to 100nH.
Table 3. Component Values for Typical
Application Circuit
COMPONENT
L1
L3
C1
C2
C3
C4
C6
C8
C10
C11
C12
C14
C15
VALUE FOR
f_RF = 433MHz
22nH
18nH
6.8pF
10pF
10nF
680pF
6.8pF
220pF
10nF
220pF
220pF
100pF
100pF
VALUE FOR
f_RF = 315MHz
27nH
22nH
15pF
22pF
10nF
680pF
15pF
220pF
10nF
220pF
220pF
100pF
100pF
The objective of the matching network is to match the
power-amplifier output to the impedance of the small-
loop antenna. The matching components thus tune out
the loop inductance and transform the low radiative
and resistive parts of the antenna into the much higher
value of the PA output. This gives higher efficiency. The
low radiative and lossy components of the small-loop
antenna result in a higher Q matching network than the
50Ω network; thus, the harmonics are lower.
Layout Considerations
A properly designed PCB is an essential part of any
RF/microwave circuit. On the power-amplifier output,
use controlled-impedance lines and keep them as short
as possible to minimize losses and radiation.
Keeping the traces short reduces parasitic inductance.
Generally, 1in of PCB trace adds about 20nH of para-
sitic inductance. Parasitic inductance can have a dra-
matic effect on the effective inductance. For example, a
0.5in trace connecting a 100nH inductor adds an extra
10nH of inductance, or 10%.
To reduce the parasitic inductance, use wider traces
and a solid ground or power plane below the signal
traces. Using a solid ground plane can reduce the par-
asitic inductance from approximately 20nH/in to 7nH/in.
Also, use low-inductance connections to ground on all
GND pins and place decoupling capacitors close to all
V_DD connections.
8 _______________________________________________________________________________________