MAX1472
300MHz-to-450MHz Low-Power,
Crystal-Based ASK Transmitter
7
Crystal Oscillator
The crystal oscillator in the MAX1472 is designed to
present a capacitance of approximately 3.1pF between
the XTAL1 and XTAL2 pins. If a crystal designed to
oscillate with a different load capacitance is used, the
crystal is pulled away from its intended operating fre-
quency, thus introducing an error in the reference fre-
quency. Crystals designed to operate with higher
differential load capacitance always pull the reference
frequency higher. For example, a 9.84375MHz crystal
designed to operate with a 10pF load capacitance
oscillates at 9.84688MHz with the MAX1472, causing
the transmitter to be transmitting at 315.1MHz rather
than 315.0MHz, an error of about 100kHz, or 320ppm.
In actuality, the oscillator pulls every crystal. The crys-
tal’s natural frequency is really below its specified fre-
quency, but when loaded with the specified load
capacitance, the crystal is pulled and oscillates at its
specified frequency. This pulling is already accounted
for in the specification of the load capacitance.
Additional pulling can be calculated if the electrical
parameters of the crystal are known. The frequency
pulling is given by:
where:
f
p
is the amount the crystal frequency is pulled in ppm.
C
m
is the motional capacitance of the crystal.
C
case
is the case capacitance.
C
spec
is the specified load capacitance.
C
load
is the actual load capacitance.
When the crystal is loaded as specified, i.e., C
load
=
C
spec
, the frequency pulling equals zero.
Output Matching to 50
Ω
Ω
When matched to a 50Ω system, the MAX1472 PA is
capable of delivering more than +10dBm of output
power at V
DD
= 2.7V. The output of the PA is an open-
drain transistor that requires external impedance
matching and pullup inductance for proper biasing.
The pullup inductance from PA to V
DD
serves three
main purposes: It resonates the capacitance of the PA
output, provides biasing for the PA, and becomes a
high-frequency choke to reduce the RF energy cou-
pling into V
DD
. The recommended output-matching net-
work topology is shown in the
Typical Application
Circuit
. The matching network transforms the 50Ω load
to a higher impedance at the output of the PA in addi-
tion to forming a bandpass filter that provides attenua-
tion for the higher order harmonics.
Output Matching to PC Board Loop
Antenna
In most applications, the MAX1472 PA output has to be
impedance matched to a small-loop antenna. The
antenna is usually fabricated out of a copper 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 component. To achieve
high radiating efficiency, the radiative component
should be as high as possible, while minimizing 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
PA output to the small loop antenna. The matching
components thus transform the low radiative and resis-
tive parts of the antenna into the much higher value of
the PA output, which gives higher efficiency. The low
radiative and lossy components of the small loop anten-
na result in a higher Q matching network than the 50Ω
network; thus, the harmonics are lower.
