MAX7042ATJ+T Maxim Integrated Products, MAX7042ATJ+T Datasheet - Page 10

• Manufacturer Part Number: MAX7042ATJ+T
• Description: RF Receiver IC RECEIVER FSK -433.92MHz Low-Power
• Manufacturer: Maxim Integrated Products
• Type: Receiverr

Specifications of MAX7042ATJ+T

• Package / Case: TQFN-32 EP
• Operating Frequency: 433.92 MHz
• Operating Supply Voltage: 2.5 V, 3.3 V, 5 V
• Maximum Operating Temperature: + 125 C
• Minimum Operating Temperature: - 40 C
• Mounting Style: SMD/SMT
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Table 1. Frequency Selection Table
308MHz/315MHz/418MHz/433.92MHz
Low-Power, FSK Superheterodyne Receiver
The MAX7042 CMOS superheterodyne receiver and a
few external components provide a complete FSK
receive chain from the antenna to the digital output
data. FSK uses the difference in frequency of the carri-
er to represent a logic 0 and logic 1. Depending on sig-
nal power and component selection, data rates as high
as 66kbps NRZ can be achieved.
The MAX7042 can be tuned to one of four frequencies
using the 2 frequency-select bits FSEL1 and FSEL2:
308, 315, 418, and 433.92MHz, as shown in Table 1.
The LO frequencies are 32 times the reference crystal
frequencies of 9.29063, 9.50939, 12.72813, and
13.22563MHz. The selected crystal frequency is used
to calibrate the FSK detector PLL so that it operates at
the middle of the 10.7MHz IF.
The LNA is a cascode amplifier with off-chip inductive
degeneration. The gain and the noise figure are depen-
dent on both the antenna matching network at the LNA
input and the LC tank network between the LNA output
and the mixer input.
The MAX7042 allows for user programmability of the
LNA bias current. Input LNASEL programs 1x to 2x
bias currents in increments of 0.6mA from 0.6mA to
1.2mA. Setting LNASEL to logic-low programs the LNA
to consume 1x bias current and setting LNASEL to
logic-high programs the LNA to consume 2x bias cur-
rent. Larger bias currents yield better sensitivity and
gain at the expense of current drain.
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 match to a low-input
10
______________________________________________________________________________________
FSEL2
0
0
1
1
Low-Noise Amplifier (LNA)
Detailed Description
FSEL1
0
1
0
1
Frequency Selection
FREQUENCY
433.92
(MHz)
308
315
418
impedance such as printed circuit board (PCB) trace
antenna. A nominal value of this inductor for a 50Ω input
impedance is 3.9nH at 315MHz and 0nH (short) at
433.92MHz, but is affected by the PCB trace. See the
Typical Operating Characteristics for the relationship
between the inductance and input impedance.
The LC tank filter connected to LNAOUT consists of L2
and C9 (see the Typical Application Circuit). Select L2
and C9 to resonate at the desired RF input frequency.
The resonant frequency is given by:
where L
C
L
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 fre-
quency. Lab experimentation is required to optimize the
center frequency of the tank. The parasitic capacitance
is generally 5pF to 7pF.
There are two ways to verify experimentally that the res-
onant frequency of the tank is centered at the desired
RF frequency:
1) Drive the crystal oscillator externally and sweep both
2) Use a network analyzer to measure the resonance.
PARASITICS
PARASITICS
the RF frequency and the LO frequency (FXTAL x
32) to keep the IF at 10.7MHz while monitoring the
RSSI voltage (pin 4). There is a peak in the RSSI
voltage at resonance. The external source must be
AC-coupled into XTAL1 and the XTAL2 pin must
have an AC bypass to ground. The recommended
drive power is -10dBm.
The port 1 power from the network analyzer is input
to the receiver, and this power must be -30dBm or
less. A coaxial stub with the center conductor
exposed (commonly called an RF “sniffer” is used to
monitor the tank power and serves as the port 2
input to the network analyzer. The sniffer should be
placed in close proximity to, but not actually touch-
ing, the tank inductor.
TOTAL
f
.
and C
= L2 + L
=
2π
PARASITICS
L
TOTAL
PARASITICS
1
x C
include inductance and
TOTAL
and C
TOTAL
= C9 +