MAX2745 데이터 시트보기 (PDF) - Maxim Integrated

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MAX2745

Single-Chip Global Positioning System Receiver Front-End

Maxim Integrated 

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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