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MAX7036

300MHz to 450MHz ASK Receiver with Internal IF Filter

Maxim Integrated 

MAX7036

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 sig-

nal 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 together.

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 frequency (e.g.,

220pF). Select L2 and C1 to resonate at the desired RF

input frequency. The resonant frequency is given by:

fRF = 2π

1

L TOTAL × C TOTAL

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 frequen-

cies, 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 pro-

duce 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 gen-

erates the LO. The relationship between the RF, IF, and

crystal reference frequencies is given by:

f

XTAL

=

f LO

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

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