MAX1471EVKIT-433 Maxim Integrated Products, MAX1471EVKIT-433 Datasheet - Page 11

• Manufacturer Part Number: MAX1471EVKIT-433
• Description: EVAL KIT FOR MAX1471 433MHZ
• Manufacturer: Maxim Integrated Products
• Type: Receiver, ASK/FSKr

Specifications of MAX1471EVKIT-433

• Frequency: 433MHz
• For Use With/related Products: MAX1471 ~ 433 MHz
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

The IF section presents a differential 330Ω 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.
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
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.
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:
The received image frequency at:
Figure 1. FSK Demodulator PLL Block Diagram
f
RECEIVE
f
IMAGE
LIMITING
AMPS
______________________________________________________________________________________
IF
Intermediate Frequency (IF)
= (f
= (f
XTAL
XTAL
x 32) -10.7MHz
ASK/FSK Superheterodyne Receiver
x 32) +10.7MHz
315MHz/434MHz Low-Power, 3V/5V
FSK Demodulator
Crystal Oscillator
P-P
signal on the con-
DETECTOR
PHASE
CHARGE
PUMP
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
C
C
C
C
When the crystal is loaded as specified, i.e., C
C
p
m
case
spec
load
spec
is the amount the crystal frequency pulled in ppm.
is the motional capacitance of the crystal.
FILTER
f
LOOP
p
, the frequency pulling equals zero.
is the actual load capacitance.
is the case capacitance.
is the specified load capacitance.
=
C
2
m
⎛
⎜
⎝
C
case
10.7MHz VCO
2.2mV/kHz
1
+
C
load
TO FSK BASEBAND FILTER
AND DATA SLICER
−
C
case
1
+
C
spec
⎞
⎟ ×
⎠
10
load
6
11
=