Datasheet
TOTAL TOTAL
1
2L C
ƒ=
π×
where:
TOTAL PARASITICS
TOTAL PARASITICS
L L1 L
C C9 C
= +
= +
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 should be done to optimize the cen-
ter frequency of the tank.
Mixer
A unique feature of the MAX1470 is the integrated image
rejection of the mixer. This device was designed to elimi-
nate the need for a costly front-end SAW filter for many
applications. The advantage of not using a SAW filter is
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 315MHz RF input
to the 10.7MHz IF with low-side injection (i.e., f
LO
= f
RF
- f
IF
). The image rejection circuit then combines these
signals to achieve ~50dB of image rejection over the full
temperature range. Low-side injection is required due to
the on-chip image-rejection architecture. The IF output
is driven by a source-follower, biased to create a driving
impedance of 330Ω to interface with an off-chip 330Ω
ceramic IF filter. The voltage conversion gain driving a
330Ω load is approximately 13dB.
Phase-Lock Loop
The PLL block contains a phase detector, charge pump/
integrated loop filter, VCO, asynchronous 64x clock
divider, and crystal oscillator. This PLL does not require
any external components. The quadrature VCO is cen-
tered at the nominal LO frequency of 304.3MHz. For an
input RF frequency of 315MHz, a reference frequency of
4.7547MHz is needed for a 10.7MHz IF frequency (low-
side injection is required). The relationship between the
RF, IF, and reference frequencies is given by:
( )
REF RF IF
f f f / 64= −
To allow the smallest possible IF bandwidth (for best sen-
sitivity), the tolerance of the reference must be minimized.
Intermediate Frequency
The IF section presents a differential 330Ω load to pro-
vide matching for the off-chip ceramic filter. The internal
five AC-coupled limiting amplifiers produce an overall
gain of approximately 65dB, with a bandpass-filter-type
response centered near the 10.7MHz IF frequency with
a 3dB bandwidth of approximately 11.5MHz. The RSSI
circuit demodulates the IF to baseband by producing a
DC output proportional to the log of the IF signal level with
a slope of approximately 15mV/dB (see Typical Operating
Characteristics).
Applications Information
Crystal Oscillator
The XTAL oscillator in the MAX1470 is designed to pres-
ent a capacitance of approximately 3pF between XTAL1
and XTAL2. If a crystal designed to oscillate with a differ-
ent load capacitance is used, the crystal is pulled away
from its stated 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 MAX1470, causing the
receiver to be tuned to 315.1MHz rather than 315.0MHz,
an error of about 100kHz, or 320ppm.
In actuality, the oscillator pulls every crystal. The crystal’s
natural frequency is really below its specified frequency,
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:
6
m
p
case load case spec
C
11
10
2CCCC
ƒ= − ×
++
where:
f
p
is the amount the crystal frequency is 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.
MAX1470 315MHz Low-Power, +3V Superheterodyne
Receiver
www.maximintegrated.com
Maxim Integrated
│
7