315MHz Low-Power, +3V Superheterodyne
2π LTOTAL × CTOTAL
LTOTAL = L1+ LPARASITICS
CTOTAL = C9 + CPARASITICS
LPARASITICS and CPARASITICS 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 opti-
mize the center frequency of the tank.
A unique feature of the MAX1470 is the integrated
image rejection of the mixer. This device was designed
to eliminate 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 match-
ing, 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., fLO = fRF
- fIF). 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 dri-
ving a 330Ω load is approximately 13dB.
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 centered at the nominal LO frequency of 304.3MHz.
For an input RF frequency of 315MHz, a reference fre-
quency of 4.7547MHz is needed for a 10.7MHz IF fre-
quency (low-side injection is required). The relationship
between the RF, IF, and reference frequencies is given
fREF = (fRF - fIF) / 64
To allow the smallest possible IF bandwidth (for best
sensitivity), the tolerance of the reference must be mini-
The IF section presents a differential 330Ω load to pro-
vide matching for the off-chip ceramic filter. The inter-
nal five AC-coupled limiting amplifiers produce an
overall gain of approximately 65dB, with a bandpass-fil-
ter-type response centered near the 10.7MHz IF fre-
quency with a 3dB bandwidth of approximately
11.5MHz. The RSSI circuit demodulates the IF to base-
band 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).
The XTAL oscillator in the MAX1470 is designed to pre-
sent a capacitance of approximately 3pF between
XTAL1 and XTAL2. If a crystal designed to oscillate
with a different load capacitance is used, the crystal is
pulled away from its stated operating frequency, intro-
ducing an error in the reference frequency. Crystals
designed to operate with higher differential load capac-
itance 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 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:
fp is the amount the crystal frequency is pulled in ppm.
Cm is the motional capacitance of the crystal.
Ccase is the case capacitance.
Cspec is the specified load capacitance.
Cload is the actual load capacitance.