MAX2742
Single-Chip Global Positioning System
Receiver Front-End
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Detailed Description
LNA/Mixer
The RF input from the GPS antenna is fed through an
LNA with a 24dB gain. The amplified signal is then fed
to a mixer that downconverts the signal (1575.42MHz)
to a quadrature differential IF of 1.023MHz.
IF Stage
The quadrature IF signals pass through the IF filter,
which rejects the out-of-band spurs by more than 60dB
and the image noise by 18dB (typ). After the image
reject filter, the signal is converted from quadrature to
differential. The filtered IF signal is then amplified by the
AGC block, which sets the VGA output signal level to a
predetermined value through the application using 50dB
of dynamic range. The internal offset-cancellation mech-
anism generates a highpass characteristic for the IF
section with a 1dB corner frequency of about 100kHz.
IF Output Selection
The sampled outputs of the GPS signal are available in
a single-ended or differential format. The IFSEL pin
controls the output format.
Synthesizer
An on-chip VCO provides quadrature differential LO
signals to the downconverting mixer and controls the
frequency. An on-board TCXO generates the reference
frequency. The integrated synthesizer includes the
VCO, TCXO buffer, main frequency divider, phase-fre-
quency detector, and charge pump. It uses an off-chip
PLL loop filter and TCXO. Connect the output of the
TCXO to XTALIN1 and XTALIN2 through a coupling
capacitor.
The main division ratio for the synthesizer is 684. With this
division ratio, a low-side injection LO can be synthesized
with an 18.414MHz TXCO.
Applications Information
Layout Issues
A properly designed PC board is an essential part of any
RF/microwave circuit. On all high-frequency inputs and
outputs, use controlled impedance lines and keep them
as short as possible to minimize losses and radiation.
Keeping the traces short also reduces parasitic induc-
tance. To further reduce the parasitic inductance, use
wider traces and a solid ground or power plane below
the signal traces. Also, place decoupling capacitors as
close to the supply pins as possible. For proper power
dissipation and operation, connect the metal exposed
paddle solidly to the ground plane of the PC board.
