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Single-Chip Global Positioning System
Receiver Front-End
Detailed Description
The MAX2745 complete single-chip GPS front-end
downconverter utilizes many innovative and leading-
edge RF CMOS design techniques. This high-perfor-
mance, state-of-the-art device consumes extremely low
power and eliminates the need for costly SAW and
bulky discrete IF filters. The MAX2745 incorporates a
fully integrated LNA/mixer, IF section, digital sampler,
and local oscillator synthesizer. The MAX2745 also pro-
vides voltage supply for an external LNA. In addition,
the MAX2745 features a voltage booster that can
extend the power-supply voltage to as low as 1.6V.
The MAX2745 supports both 3rd overtone crystal
designs in 32MHz and fundamental frequency crystal
design in 16MHz, as well as a TCXO implementation. In
addition, the MAX2745 incorporates an on-chip tempera-
ture sensor and crystal oscillator with trimmed capaci-
tors, resulting in a very accurate frequency and
additional system cost savings.
The MAX2745 also incorporates a 2MHz bandwidth
bandpass filter to reduce off-band noise. The band-
width and the center frequency of the bandpass filter
are guaranteed by an advanced tuning scheme.
The MAX2745 is offered in a space-saving 48-pin TQFP
package. The operational temperature range is speci-
fied from -40 ° C to +85 ° C. The power consumption of
this device is as low as 41mW at 2.4V.
LNA/Mixer
The RF input signal from the GPS antenna is fed
through an LNA with a gain of 20dB. The amplified sig-
nal is then fed to a mixer that downconverts the signal
(1575.42MHz) to a quadrature differential IF of
4.092MHz.
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 17dB (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
56dB of dynamic range.
IF Output Selection
The sampled outputs of the GPS signal are available in
a single-ended or limited-differential format. The ODS
pin controls the output format.
Synthesizer
An on-chip VCO provides quadrature differential LO
signals to the downconverting mixer. An on-chip crystal
oscillator driver generates the reference frequency. The
integrated synthesizer includes the VCO, crystal oscil-
lator driver, main frequency divider, phase-frequency
detector, and charge pump. It uses an off-chip PLL
loop filter and crystal. If a TCXO is used, connect the
output of the TCXO to XTALIN.
The main division ratio for the synthesizer is 96 or 48.
With this division ratio, a low-side injection LO can be
used with a 16.368MHz or 32.736MHz crystal. Note that
if a 3rd overtone crystal is used for the 32.736MHz, an
off-chip notch filter to reject the fundamental frequency
is required. The notch filter can be built with one induc-
tor and one capacitor.
Control Block
This block is used mainly for testing purposes and to
set the internal trimmed capacitor. This block is pro-
grammed through the VSport.
VSport Serial Data Interface
The serial interface within the MAX2745 is used by the
baseband to communicate control signals and is also
used for internal testing of the device. (This serial port
is described in a separate document, which is available
upon request.)
Trimmed VCXO Frequency
An on-chip, trimmed capacitor with a temperature coeffi-
cient of better than 30ppm/ ° C is used to compensate for
the component variation of the reference frequency
associated circuitry. The value of the capacitor is pro-
grammed in 512 binary steps and ranges from a mini-
mum of 80fF to a maximum of 40pF, ± 20%. Pro-
gramming is accomplished with the VSport serial inter-
face. The no load capacitance on the crystal out follows
the equation:
C(no load) = ([Register Value] x 40/512) pF
Upon power-up, the capacitor value is set to the mini-
mum value. This enables the MAX2745 to work with an
off-chip, trimmed capacitor. If the baseband has the
capability of programming the radio, the off-chip,
trimmed capacitor can be omitted by setting the inter-
nal trimmed capacitor through the VSPORT.
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