MAX7033 データシートの表示（PDF） - Maxim Integrated
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MAX7033 Datasheet PDF : 16 Pages
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MAX7033
315MHz/433MHz ASK Superheterodyne
Receiver with AGC Lock
Intermediate Frequency and RSSI
The IF section presents a differential 330Ω load to provide
matching for the off-chip ceramic filter. The six internal
AC-coupled limiting amplifiers produce an overall gain of
approximately 65dB, with a bandpass-filter-type response
centered near the 10.7MHz IF frequency 1 with a 3dB
bandwidth of approximately 10MHz. The RSSI circuit
demodulates the IF by producing a DC output propor-
tional to the log of the IF signal level, with a slope of
approximately 14.2mV/dB (see the Typical Operating
Characteristics).
Applications Information
Crystal Oscillator
The crystal oscillator in the MAX7033 is designed to
present 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, introducing an
error in the reference frequency. Crystals designed to
operate with higher differential load capacitance 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 MAX7033,
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 crystal’s
natural frequency is really below its specified frequency,
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
CM
2
C CASE
1
+
CLOAD
-
C CASE
1
+
C SPEC
×10 6
where:
fP is the amount the crystal frequency 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.
When the crystal is loaded as specified, i.e., CLOAD =
CSPEC, the frequency pulling equals zero.
It is possible to use an external reference oscillator in
place of a crystal to drive the VCO. AC-couple the exter-
nal oscillator to XTAL2 with a 1000pF capacitor. Drive
XTAL2 with a signal level of approximately -10dBm.
AC-couple XTAL1 to ground with a 1000pF capacitor.
Data Filter
The data filter is implemented as a 2nd-order lowpass
Sallen-Key filter. The pole locations are set by the
combination of two on-chip resistors and two external
capacitors. Adjusting the value of the external capacitors
changes the corner frequency to optimize for different
data rates. The corner frequency should be set to approxi-
mately 1.5 times the fastest expected data rate from the
transmitter. Keeping the corner frequency near the data
rate rejects any noise at higher frequencies, resulting in
an increase in receiver sensitivity.
The configuration shown in Figure 2 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 C5
and C6, use the following equations, along with the coef-
ficients in Table 2:
C
5
=
a(100
b
k)(π)(f
C
)
C
6
=
a
4(100 k)(π)(f C
)
where fC is the desired 3dB corner frequency.
Table 2. Coefficents to Calculate C5 and C6
FILTER TYPE
Butterworth (Q = 0.707)
Bessel (Q = 0.577)
a
1.414
1.3617
b
1.000
0.618
MAX7033
RSSI
RDF2
100kΩ
RDF1
100kΩ
19
21
22
DFO
OPP
DFFB
C6
C5
Figure 2. Sallen-Key Lowpass Data Filter
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