MAX7036
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 V
DD
to the supply voltage.
For operation with a single 4.5V to 5.5V supply voltage,
connect V
DD
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 V
DD
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:
where L
TOTAL
= L2 + L
PARASITICS
and C
TOTAL
= C1 +
C
PARASITICS
.
L
PARASITICS
and C
PARASITICS
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:
where f
LO
= f
RF
±f
IF
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
