MAX1473_11 Datasheet, PDF (11/15 Pages) Maxim Integrated Products – 315MHz/433MHz ASK Superheterodyne Receiver with Extended Dynamic Range

315MHz/433MHz ASK Superheterodyne
Receiver with Extended Dynamic Range
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:
f_p
= ^Cm
2
⎛
⎜
⎝⎜ ^Ccase
1
+
C_load
-
C_case
1
+
C_spec
⎞
⎠⎟⎟
× 10⁶
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 C5 and C6, use the following equations along
with the coefficients in Table 2:
( )( )( ) ^{C5 =}
b
a 100k π f_c
( )( )( ) ^{C6 =}
a
4 100k π f_c
where f_C is the desired 3dB corner frequency.
For example, choose a Butterworth filter response with
a corner frequency of 5kHz:
( )( )( )( ) ^{C5 =}
1.000
≈ 450pF
1.414 100kΩ 3.14 5kHz
Choosing standard capacitor values changes C5 to
470pF and C6 to 220pF, as shown in the Typical
Application Circuit.
Data Slicer
The purpose of the data slicer is to take the analog out-
put of the data filter and convert it to a digital signal.
This is achieved by using a comparator and comparing
the analog input to a threshold voltage. One input is
supplied by the data filter output. Both comparator
inputs are accessible off chip to allow for different
methods of generating the slicing threshold, which is
applied to the second comparator input.
The suggested data slicer configuration uses a resistor
(R1) connected between DSN and DSP with a capaci-
tor (C4) from DSN to DGND (Figure 2). This configura-
tion averages the analog output of the filter and sets the
threshold to approximately 50% of that amplitude. With
this configuration, the threshold automatically adjusts
as the analog signal varies, minimizing the possibility
for errors in the digital data. The sizes of R1 and C4
affect how fast the threshold tracks to the analog ampli-
tude. Be sure to keep the corner frequency of the RC
circuit much lower than the lowest expected data rate.
MAX1473
R_DF2
100kΩ
RSSI
R_DF1
100kΩ
19
21
22
DFO
OPP
DFFB
C6
C5
Figure 1. Sallen-Key Lowpass Data Filter
Table 2. Coefficents to Calculate C5 and C6
FILTER TYPE
Butterworth (Q = 0.707)
a
1.414
b
1.000
Bessel (Q = 0.577)
1.3617
0.618
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