The off-chip inductive degeneration is achieved by

connecting an inductor from LNASRC to AGND. This

inductor sets the real part of the input impedance at

LNAIN, allowing for a more flexible input impedance

match, such as a typical PC board trace antenna. A

nominal value for this inductor with a 50Ω input imped-

ance is 15nH, but is affected by PC board trace.

The LC tank filter connected to LNAOUT comprises L3

and C2 (see the Typical Application Circuit). Select L3

and C2 to resonate at the desired RF input frequency.

The resonant frequency is given by:

where:

capacitance of the PC board traces, package pins,

mixer input impedance, LNA output impedance, etc.

These parasitics at high frequencies cannot be

ignored, and can have a dramatic effect on the tank fil-

ter center frequency. Lab experimentation should be

done to optimize the center frequency of the tank.

Automatic Gain Control

When the AC pin is low, the automatic gain-control

(AGC) circuit monitors the RSSI output. As the RSSI

output reaches 1.98V, which corresponds to RF input

level of -62dBm, the AGC switches on the LNA gain

reduction resistor. The resistor reduces the LNA gain

by 35dB, thereby reducing the RSSI output by about

500mV. The LNA resumes high-gain mode when the

RSSI level drops back below 1.39V (approximately

-70dBm at RF input) for 1ms. The AGC has a hysteresis

of 8dB. With the AGC function, the MAX7033 can reli-

ably produce an ASK output for RF input levels up to

0dBm with modulation depth of 18dB.

When the AC pin is high and SHDN goes high, the

AGC circuit is disabled and the LNA is always in high-

gain mode. The AGC function can be resumed by

bringing the AC pin low when SHDN is high.

The MAX7033 features an AGC lock function that is

asserted when the level at the AC pin transitions from

low to high while SHDN is high. Locking the AGC locks

the AGC lock function can be enabled or disabled as

long as the SHDN pin is high. Changing the state of AC

when SHDN is low has no effect.

Mixer

A unique feature of the MAX7033 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

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

f

LC

RF

TOTAL

TOTAL

=

×

1

2π

315MHz/433MHz ASK Superheterodyne

Receiver with AGC Lock

10

______________________________________________________________________________________

AGC

LOCK

AGC

UNLOCK

AGC

LOCK

AGC

UNLOCK

NO

EFFECT

NO

EFFECT

SHDN

PIN

AC PIN

NO

EFFECT

AGC ENABLED

AGC

ENABLED

AGC

DISABLED

AGC

DISABLED