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300MHz to 450MHz High-Efficiency,
Crystal-Based +13dBm ASK Transmitter
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 MAX7044, 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:
Output Matching to
PCB Loop Antenna
In some applications, the MAX7044 power amplifier
output 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 will have an impedance that con-
sists of a lossy component and a radiative component.
To achieve high radiating efficiency, the radiative com-
ponent should be as high as possible, while minimizing
the lossy component. In addition, the loop antenna will
have an inherent loop inductance associated with it
(assuming the antenna is terminated to ground). For
example, in a typical application, the radiative imped-
ance is less than 0.5 Ω , the lossy impedance is less
than 0.7 Ω , and the inductance is approximately 50nH
to 100nH.
The objective of the matching network is to match the
power amplifier output to the small-loop antenna. The
C m ? 1 1 ?
? ? C
C case + C spec ? ?
case + C load
f p =
2
?
x 10 6
matching components thus 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
where:
f p is the amount the crystal frequency is pulled in ppm.
C m is the motional capacitance of the crystal.
C case (or C o ) is the vendor-specified case capacitance
of the crystal.
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
When matched to a 50 Ω system, the MAX7044 PA is
capable of delivering up to +13dBm of output power at
V DD = 2.7V. The output of the PA is an open-drain tran-
sistor that requires external impedance matching and
pullup inductance for proper biasing. The pullup induc-
tance 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 coupling into V DD . The
recommended output-matching network topology is
shown in the Typical Application Circuit. The matching
network transforms the 50 Ω load to approximately
125 Ω at the output of the PA in addition to forming a
bandpass filter that provides attenuation for the higher
order harmonics.
antenna result in a higher Q matching network than the
50 Ω network; thus, the harmonics are lower.
Layout Considerations
A properly designed PCB is an essential part of any
RF/microwave circuit. At the power amplifier output,
use controlled-impedance lines and keep them as short
as possible to minimize losses and radiation. At high
frequencies, trace lengths that are approximately 1/20
the wavelength or longer become antennas. For exam-
ple, a 2in trace at 315MHz can act as an antenna.
Keeping the traces short also reduces parasitic induc-
tance. Generally, 1in of PCB trace adds about 20nH of
parasitic inductance. The parasitic inductance can
have a dramatic 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
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