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MAX7036
(Rev.:2010)

300MHz to 450MHz ASK Receiver with Internal IF Filter

Maxim Integrated 

300MHz to 450MHz ASK Receiver

with Internal IF Filter

Detailed Description

The MAX7036 CMOS RF receiver, and a few external

components, provide the complete receiver chain from

the antenna to the digital output data. Depending on

signal power and component selection, data rates as

high as 33kbps Manchester (66kbps NRZ) can be

achieved.

The MAX7036 is designed to receive binary ASK/OOK

data modulated in the 300MHz to 450MHz frequency

range. ASK modulation uses a difference in amplitude

of the carrier to represent digital data.

Voltage Regulator

For operation with a single 3.0V to 3.6V supply voltage,

connect AVDD, DVDD, and VDD to the supply voltage.

For operation with a single 4.5V to 5.5V supply voltage,

connect VDD to the supply voltage. An on-chip voltage

regulator drives the AVDD pin to approximately 3.2V.

For proper operation, connect DVDD and AVDD togeth-

er. Bypass VDD and AVDD to GND with 0.1μF capaci-

tors placed as close as possible to the device. Bypass

DVDD to GND with a 0.01μF capacitor (see the Typical

Application Circuit).

Low-Noise Amplifier

The LNA is an nMOS cascode amplifier. The LNA and

mixer have a combined 55dB voltage gain. The gain

and noise figures are dependent on both the antenna-

matching network at the LNA input and the LC tank net-

work between the LNA output and the mixer inputs.

L2 and C1 comprise the LC tank filter connected to

LNAOUT (see the Typical Application Circuit). L2 also

serves as a bias inductor to LNAOUT. Bypass the

power-supply side of L2 to GND with a capacitor that

provides a low-impedance path at the RF carrier fre-

quency (e.g., 220pF). Select L2 and C1 to resonate at

the desired RF input frequency. The resonant frequen-

cy is given by:

fRF = 2π

1

LTOTAL × CTOTAL

where LTOTAL = L2 + LPARASITICS and CTOTAL = C1 +

CPARASITICS.

LPARASITICS and CPARASITICS include inductance and

capacitance of the PCB traces, package pins, mixer

input impedance, LNA output impedance, etc. At high

frequencies, these parasitics can have a dramatic

effect on the tank filter center frequency and must not

be ignored. The total parasitic capacitance is generally

4pF to 6pF. Adjust L2 and C1 accordingly to achieve

the desired tank center frequency.

Automatic Gain Control (AGC)

The AGC circuit monitors the RSSI output. The AGC

switches to its low-gain state when the RSSI output

reaches 2.2V. The AGC gain reduction is typically

29dB, corresponding to an RSSI voltage drop of

435mV. The LNA resumes high-gain mode when the

RSSI level drops back below 1.67V for 13ms for

315MHz and 10ms for 433MHz operation. The AGC has

a hysteresis of 5dB. With this AGC function, the

MAX7036 can reliably produce an ASK output for RF

input levels up to 0dBm, with modulation depth of

30dB.

Mixer

The mixer cell is a double-balanced mixer that performs

a downconversion of the RF input to a typical IF of

200kHz from either a high-side or a low-side injected LO.

The mixer output drives the input of the on-chip IF filter.

Phase-Locked Loop (PLL)

The PLL block contains a phase detector, charge pump,

integrated loop filter, VCO, asynchronous clock dividers,

and crystal-oscillator driver. Besides the crystal, this PLL

does not require any external components. The VCO

generates the LO. The relationship between the RF, IF,

and crystal reference frequencies is given by:

f XTAL

=

fLO

32

where fLO = fRF ± fIF

Received-Signal-Strength Indicator (RSSI)

The RSSI circuit provides a DC output proportional to

the logarithm of the input power level. RSSI output volt-

age has a slope of about 14.5mV/dB (of input

power).The RSSI monotonic dynamic range exceeds

80dB. This includes the 30dB of AGC.

Applications Information

Crystal Oscillator

The crystal (XTAL) oscillator in the MAX7036 is

designed to present a capacitance of approximately

4pF between XTAL1 and XTAL2. In most cases, this

corresponds to a 6pF load capacitance applied to the

external crystal when typical PCB parasitics are added.

The MAX7036 is designed to operate with a typical

10pF load capacitance crystal. It is very important to

use a crystal with a load capacitance equal to the

capacitance of the MAX7036 crystal oscillator plus

PCB parasitics. If a crystal designed to oscillate with a

different load capacitance is used, the crystal is pulled

away from its stated operating frequency, introducing

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