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MAX1473
315MHz/433MHz ASK Superheterodyne
Receiver with Extended Dynamic Range
Phase-Locked Loop
The PLL block contains a phase detector, charge
pump/integrated loop filter, VCO, asynchronous 64x
clock divider, and crystal oscillator driver. Besides the
crystal, this PLL does not require any external compo-
nents. The VCO generates a low-side local oscillator
(LO). The relationship between the RF, IF, and crystal
reference frequencies is given by:
f XTAL = (f RF - f IF )/(32 M)
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:
C m ?
?
? × 10 6
?
2 ? ? C case + C load case + C spec ? ?
where:
M = 1 (V XTALSEL = V DD ) or 2 (V XTALSEL = 0V)
To allow the smallest possible IF bandwidth (for best sen-
f p =
1 1
-
C
sitivity), the tolerance of the reference must be minimized.
Intermediate Frequency/RSSI
The IF section presents a differential 330 ? 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
the 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 resistor reduces
the LNA gain by 35dB, thereby reducing the RSSI out-
put by about 500mV. The LNA resumes high-gain mode
when the RSSI level drops back below 1.45V (approxi-
mately -65dBm at RF input) for 150ms. The AGC has a
hysteresis of ~8dB. With the AGC function, 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
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.
Data Filter
The data filter is implemented as a 2nd-order lowpass
Sallen-Key filter. The pole locations are set by the com-
bination of two on-chip resistors and two external
capacitors. Adjusting the value of the external capaci-
tors changes the corner frequency to optimize for dif-
ferent data rates. The corner frequency should be set
to approximately 1.5 times the fastest expected data
rate from the transmitter. Keeping the corner frequency
near the data rate rejects any noise at higher frequen-
cies, resulting in an increase in receiver sensitivity.
The configuration shown in Figure 1 can create a
Butterworth or Bessel response. The Butterworth filter
offers a very flat amplitude response in the passband
and a rolloff rate of 40dB/decade for the two-pole filter.
The Bessel filter has a linear phase response, which
works well for filtering digital data. To calculate the
value of C7 and C6, use the following equations along
with the coefficients in Table 1:
Table 1. Coefficents to Calculate C7 and C6
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
FILTER TYPE
Butterworth (Q = 0.707)
Bessel (Q = 0.577)
a
1.414
1.3617
b
1.000
0.618
with the MAX1473, causing the receiver to be tuned to
315.1MHz rather than 315.0MHz, an error of about
100kHz, or 320ppm.
10
Maxim Integrated
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