Rainbow Electronics MAX1471 - Manual (Page 11)

Intermediate Frequency (IF)
The IF section presents a differential 330
Ω load to pro-
vide matching for the off-chip ceramic filter. It contains
five AC-coupled limiting amplifiers with a bandpass-fil-
ter-type response centered near the 10.7MHz IF fre-
quency with a 3dB bandwidth of approximately 10MHz.
For ASK data, 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 approxi-
mately 16mV/dB. For FSK, the limiter output is fed into a
PLL to demodulate the IF.
FSK Demodulator
The FSK demodulator uses an integrated 10.7MHz PLL
that tracks the input RF modulation and determines the
difference between frequencies as logic-level ones and
zeros. The PLL is illustrated in Figure 1. The input to the
PLL comes from the output of the IF limiting amplifiers.
The PLL control voltage responds to changes in the fre-
quency of the input signal with a nominal gain of
2.2mV/kHz. For example, an FSK peak-to-peak devia-
tion of 50kHz generates a 110mV
P-P
signal on the con-
trol line. This control line is then filtered and sliced by
the FSK baseband circuitry.
The FSK demodulator PLL requires calibration to over-
come variations in process, voltage, and temperature.
For more information on calibrating the FSK demodula-
tor, see the Calibration section. The maximum calibra-
tion time is 120µs. In DRX mode, the FSK demodulator
calibration occurs automatically just before the IC
enters sleep mode.
Crystal Oscillator
The XTAL oscillator in the MAX1471 is used to generate
the local oscillator (LO) for mixing with the received sig-
nal. The XTAL oscillator frequency sets the received
signal frequency as:
f RECEIVE = (f XTAL x 32) +10.7MHz
The received image frequency at:
f IMAGE = (f XTAL x 32) -10.7MHz
is suppressed by the integrated quadrature image-
rejection circuitry.
For an input RF frequency of 315MHz, a reference fre-
quency of 9.509MHz is needed for a 10.7MHz IF fre-
quency (low-side injection is required). For an input RF
frequency of 433.92MHz, a reference frequency of
13.2256MHz is required.
The XTAL oscillator in the MAX1471 is designed to pre-
sent 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 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.
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.
f
C
C C C C
p
m
case load case spec
6 =
+
−
+





 ×
2
1 1
10
MAX1471
315MHz/434MHz Low-Power, 3V/5V
ASK/FSK Superheterodyne Receiver
______________________________________________________________________________________ 11
Figure 1. FSK Demodulator PLL Block Diagram
LOOP
FILTER
10.7MHz VCO
2.2mV/kHz
CHARGE
PUMP
PHASE
DETECTOR
IF
LIMITING
AMPS
TO FSK BASEBAND FILTER
AND DATA SLICER