MAX1473 Datasheet, PDF (10/17 Pages) Maxim Integrated Products – 315MHz/433MHz ASK Superheterodyne Receiver with Extended Dynamic Range

315MHz/433MHz ASK Superheterodyne
Receiver with Extended Dynamic Range
Intermediate Frequency/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 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 by pro-
ducing a DC output proportional to the log of the IF sig-
nal level, with a slope of approximately 14.2mV/dB (see
Typical Operating Characteristics).
The AGC circuit monitors the RSSI output. When the
RSSI output reaches 2.05V, which corresponds to an
RF input level of approximately -57dBm, the AGC
switches on the LNA gain reduction resistor. The resis-
tor reduces the LNA gain by 35dB, thereby reducing
the RSSI output by about 500mV. The LNA resumes
high-gain mode when the RSSI level drops back below
1.45V (approximately -65dBm at RF input) for 150ms.
The AGC has a hysteresis of ~8dB. With the AGC func-
tion, the MAX1473 can reliably produce an ASK output
for RF input levels up to 0dBm with modulation depth of
18dB.
Applications Information
Crystal Oscillator
The XTAL oscillator in the MAX1473 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. For
example, a 4.7547MHz crystal designed to operate
with a 10pF load capacitance oscillates at 4.7563MHz
with the MAX1473, 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.
Table 1. Component Values for Typical Application Circuit
COMPONENT
L1
L2
L3
C1
C2
C3
C4
C5
C6
C7
C8
C9
C10
C11
C12
C13
R1
X1
X2
VALUE FOR 433MHz RF
56nH
15nH
15nH
100pF
2.7pF
100pF
100pF
1500pF
220pF
470pF
0.47µF
220pF
0.01µF
0.01µF
15pF
15pF
5kΩ
6.6128MHz or 13.2256MHz
10.7MHz ceramic filter
VALUE FOR 315MHz RF
110nH
15nH
27nH
100pF
4.7pF
100pF
100pF
1500pF
220pF
470pF
0.47µF
220pF
0.01µF
0.01µF
15pF
15pF
5kΩ
4.7547MHz or 9.5094MHz
10.7MHz ceramic filter
DESCRIPTION
Toko LL1608-FH
Murata LQP11A
Murata LQP11A
5%
± 0.1pF
5%
5%
10%
5%
5%
20%
10%
20%
20%
Depends on XTAL
Depends on XTAL
5%
—
Murata SFECV10.7 series
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