MAX7034
315MHz/434MHz ASK Superheterodyne
Receiver
9
L
PARASITICS
and C
PARASITICS
include inductance and
capacitance of the PCB traces, package pins, mixer
input impedance, etc. These parasitics at high frequen-
cies cannot be ignored, and can have a dramatic effect
on the tank filter center frequency. The total parasitic
capacitance is generally between 4pF and 6pF.
Mixer
A unique feature of the MAX7034 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
10.7MHz IF from a low-side injected LO (i.e., f
LO
= f
RF
-
f
IF
). The image-rejection circuit then combines these
signals to achieve 44dB of image rejection. Low-side
injection is required due to the on-chip image-rejection
architecture. The IF output is driven by a source follow-
er biased to create a driving-point impedance of 330Ω;
this provides a good match to the off-chip 330Ω ceram-
ic IF filter.
The IRSEL pin is a logic input that selects one of the
three possible image-rejection frequencies. When V
IRSEL
= 0V, the image rejection is tuned to 315MHz. V
IRSEL
=
V
DVDD
/2 tunes the image rejection to 375MHz, and
V
IRSEL
= V
DVDD
tunes the image rejection to 434MHz.
The IRSEL pin is internally set to V
DVDD
/2 (image rejec-
tion at 375MHz) when it is left unconnected, thereby
eliminating the need for an external V
DVDD
/2 voltage.
Phase-Locked Loop
The PLL block contains a phase detector, charge
pump, integrated loop filter, VCO, asynchronous 64x
clock divider, and crystal oscillator driver. Besides the
crystal, this PLL does not require any external compo-
nents. The VCO generates a low-side LO. The relation-
ship between the RF, IF, and crystal frequencies is
given by:
where:
M = 1 (V
XTALSEL
= V
DVDD
) or 2 (V
XTALSEL
= 0V)
To allow the smallest possible IF bandwidth (for best sen-
sitivity), minimize the tolerance of the reference crystal.
Intermediate Frequency and RSSI
The IF section presents a differential 330Ω load to pro-
vide matching for the off-chip ceramic filter. The six
internal AC-coupled limiting amplifiers produce an
overall gain of approximately 65dB, with a bandpass-
filter-type response centered near the 10.7MHz IF fre-
quency with a 3dB bandwidth of approximately 10MHz.
The RSSI circuit demodulates the IF by producing a DC
output proportional to the log of the IF signal level, with
a slope of approximately 14.2mV/dB.
Applications Information
Crystal Oscillator
The crystal oscillator in the MAX7034 is designed to
present a capacitance of approximately 3pF between
the XTAL1 and XTAL2. If a crystal designed to oscillate
with a different load capacitance is used, the crystal is
pulled away from its intended operating frequency,
introducing an error in the reference frequency.
Crystals designed to operate with higher differential
load capacitance always pull the reference frequency
higher. For example, a 4.7547MHz crystal designed to
operate with a 10pF load capacitance oscillates at
4.7563MHz with the MAX7034, causing the receiver to
be tuned to 315.1MHz rather than 315.0MHz, an error
of about 100kHz, or 320ppm. It is very important to
use a crystal with a load capacitance that is equal to
the capacitance of the MAX7034 crystal oscillator
plus PCB parasitics.
In actuality, the oscillator pulls every crystal. The crys-
tal’s natural frequency is really below its specified fre-
quency, but when loaded with the specified load
capacitance, the crystal is pulled and oscillates at its
specified frequency. This pulling is already accounted
for in the specification of the load capacitance.
Additional pulling can be calculated if the electrical
parameters of the crystal are known. The frequency
pulling is given by:
where:
f
P
is the amount the crystal frequency pulled in ppm.
C
M
is the motional capacitance of the crystal.
C
CASE
is the case capacitance.
C
SPEC
is the specified load capacitance.
C
LOAD
is the actual load capacitance.
When the crystal is loaded as specified (i.e., C
LOAD
=
C
SPEC
), the frequency pulling equals zero.
