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MAX7033

315MHz/433MHz ASK Superheterodyne Receiver with AGC Lock

Maxim Integrated 

MAX7033

315MHz/433MHz ASK Superheterodyne

Receiver with AGC Lock

The off-chip inductive degeneration is achieved by con-

necting an inductor from LNASRC to AGND. This inductor

sets the real part of the input impedance at LNAIN, allow-

ing for a more flexible input impedance match, such as a

typical PCB trace antenna. A nominal value for this induc-

tor with a 50Ω input impedance is 15nH, but is affected

by PCB 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:

fRF = 2 π

1

L TOTAL × C TOTAL

where:

LTOTAL = L3 + LPARASITICS.

CTOTAL = C2 + CPARASITICS.

LPARASITICS and CPARASITICS include inductance and

capacitance of the PCB 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 filter 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 reliably produce an ASK out-

put 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 LNA in the current gain state. As shown in Figure 1,

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 applications.

Advantages of not using a SAW filter are increased sen-

sitivity, 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 sig-

nals to achieve 44dB of image rejection. Low-side injection

VIH

SHDN

PIN

VIL

VIH

AC PIN

VIL

AGC

LOCK

AGC

UNLOCK

AGC

LOCK

AGC ENABLED

AGC

UNLOCK

Figure 1. AGC Lock Activation Cycles

NO

EFFECT

NO

EFFECT

AGC

DISABLED

AGC

ENABLED

NO

EFFECT

AGC

DISABLED

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