MAX7034(2008) 查看數據表（PDF） - Maxim Integrated

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MAX7034
(Rev.:2008)

315MHz/434MHz ASK Superheterodyne Receiver

Maxim Integrated 

315MHz/434MHz ASK Superheterodyne

Receiver

on the tank filter center frequency. The total parasitic

capacitance is generally between 4pF and 6pF.

Mixer

A unique feature of the MAX7034 is the integrated

image rejection of the mixer. This device eliminates the

need for a costly front-end SAW filter for most applica-

tions. Advantages of not using a SAW filter are

increased sensitivity, simplified antenna matching, less

board space, and lower cost.

The mixer cell is a pair of double balanced mixers that

perform an IQ downconversion of the RF input to the

10.7MHz IF from a low-side injected LO (i.e., fLO = fRF -

fIF). The image-rejection circuit then combines these

signals to achieve 44dB of image rejection. Low-side

injection is required due to the on-chip image-rejection

architecture. The IF output is driven by a source follow-

er biased to create a driving-point impedance of 330Ω;

this provides a good match to the off-chip 330Ω ceram-

ic IF filter.

The IRSEL pin is a logic input that selects one of the

three possible image-rejection frequencies. When VIRSEL

= 0V, the image rejection is tuned to 315MHz. VIRSEL =

DVDD/2 tunes the image rejection to 375MHz, and

VIRSEL = DVDD tunes the image rejection to 434MHz.

The IRSEL pin is internally set to DVDD/2 (image rejection

at 375MHz) when it is left unconnected, thereby eliminat-

ing the need for an external DVDD/2 voltage.

Phase-Locked Loop

The PLL block contains a phase detector, charge

pump, integrated loop filter, VCO, asynchronous 64x

clock divider, and crystal oscillator driver. Besides the

crystal, this PLL does not require any external compo-

nents. The VCO generates a low-side LO. The relation-

ship between the RF, IF, and reference frequencies is

given by:

fREF

=

fRF - fIF

32 × M

where:

M = 1 (VXTALSEL = DVDD) or 2 (VXTALSEL = 0V)

To allow the smallest possible IF bandwidth (for best sen-

sitivity), minimize the tolerance of the reference crystal.

Intermediate Frequency and RSSI

The IF section presents a differential 330Ω load to pro-

vide matching for the off-chip ceramic filter. The six

internal AC-coupled limiting amplifiers produce an

overall gain of approximately 65dB, with a bandpass-

filter-type response centered near the 10.7MHz IF fre-

quency with a 3dB bandwidth of approximately 10MHz.

The RSSI circuit demodulates the IF by producing a DC

output proportional to the log of the IF signal level, with

a slope of approximately 14.2mV/dB.

Applications Information

Crystal Oscillator

The crystal oscillator in the MAX7034 is designed to

present a capacitance of approximately 3pF between

the XTAL1 and XTAL2. If a crystal designed to oscillate

with a different load capacitance is used, the crystal is

pulled away from its intended operating frequency,

introducing an error in the reference frequency.

Crystals designed to operate with higher differential

load capacitance always pull the reference frequency

higher. For example, a 4.7547MHz crystal designed to

operate with a 10pF load capacitance oscillates at

4.7563MHz with the MAX7034, causing the receiver to

be tuned to 315.1MHz rather than 315.0MHz, an error

of about 100kHz, or 320ppm. It is very important to

use a crystal with a load capacitance that is equal to

the capacitance of the MAX7034 crystal oscillator

plus PCB parasitics.

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:

fP

=

CM

2

⎛

⎝⎜ CCASE

1

+ CLOAD

-

CCASE

1

⎞

+ CSPEC ⎠⎟

× 106

where:

fP is the amount the crystal frequency pulled in ppm.

CM is the motional capacitance of the crystal.

CCASE is the case capacitance.

CSPEC is the specified load capacitance.

CLOAD is the actual load capacitance.

When the crystal is loaded as specified (i.e., CLOAD =

CSPEC), the frequency pulling equals zero.

It is possible to use an external reference oscillator in

place of a crystal to drive the VCO. AC-couple the exter-

nal oscillator to XTAL2 with a 1000pF capacitor. Drive

XTAL2 with a signal level of approximately 500mVP-P.

AC-couple XTAL1 to ground with a 1000pF capacitor.

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