MAX1479 Maxim, MAX1479 Datasheet - Page 8
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MAX1479
Manufacturer Part Number
MAX1479
Description
300MHz to 450MHz Low-Power / Crystal-Based +10dBm ASK/FSK Transmitter
Manufacturer
Maxim
Datasheet
1.MAX1479.pdf
(11 pages)
300MHz to 450MHz Low-Power,
Crystal-Based +10dBm ASK/FSK Transmitter
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 PC board in a rectangular, circular, or
square pattern. The antenna has an impedance that
consists of a lossy component and a radiative compo-
nent. To achieve high radiating efficiency, the radiative
component should be as high as possible, while mini-
mizing the lossy component. In addition, the loop
antenna has an inherent loop inductance associated
with it (assuming 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.
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.
Table 3. Component Values for Typical
Application Circuit
8
COMPONENT
_______________________________________________________________________________________
C10
C11
C12
C14
C15
C1
C2
C3
C4
C6
C8
L1
L3
f
VALUE FOR
RF
PC Board Loop Antenna
= 433MHz
680pF
220pF
220pF
220pF
100pF
100pF
6.8pF
6.8pF
22nH
18nH
10pF
10nF
10nF
Output Matching to
f
VALUE FOR
RF
= 315MHz
680pF
220pF
220pF
220pF
100pF
100pF
27nH
22nH
15pF
22pF
15pF
10nF
10nF
A properly designed PC board is an essential part of
any RF/microwave circuit. On the power-amplifier out-
put, 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 PC board trace adds about 20nH of
parasitic inductance. Parasitic inductance can have a
dramatic effect on the effective inductance. For exam-
ple, 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
TRANSISTOR COUNT: 2369
PROCESS: CMOS
DD
connections.
Layout Considerations
Chip Information