Rainbow Electronics MAX7042 - Manual (Page 10)

MAX7042
308MHz/315MHz/418MHz/433.92MHz
Low-Power, FSK Superheterodyne Receiver
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Detailed Description
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.
Frequency Selection
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.
Low-Noise Amplifier (LNA)
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
impedance such as a PC board trace antenna. A nominal
value of this inductor for a 50
Ω input impedance is 3.9nH
at 315MHz and 0nH (short) at 433.92MHz, but is affected
by the PC board trace. See the Typical Operating
Characteristics for the relationship between the induc-
tance 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 TOTAL = L2 + L PARASITICS and C TOTAL = C9 +
C PARASITICS .
L PARASITICS and C PARASITICS include inductance and
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 filter center
frequency. Lab experimentation is required to optimize
the center frequency of the tank. The parasitic capaci-
tance 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
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.
2) Use a network analyzer to measure the resonance.
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.
f
L x C TOTAL TOTAL
=
1
2 π
Table 1. Frequency Selection Table
FSEL2 FSEL1
FREQUENCY
(MHz)
0 0 308
0 1 315
1 0 418
1 1 433.92