315MHz/433MHz ASK Superheterodyne
Receiver with AGC Lock
For example, to choose a Butterworth filter response
with a corner frequency of 5kHz:
C 5 =
1 . 000
( 1 . 414 )( 100 k Ω )( 3 . 14 )( 5 kHz )
≈ 450 pF
MAX7033
C 6 =
(
1 . 414
4 )( 100 k Ω )( 3 . 14 )( 5 kHz )
≈ 225 pF
DATA
SLICER
Choosing standard capacitor values changes C5 to
470pF and C6 to 220pF, as shown in the Typical
25
DATAOUT
20
DSN
23
DSP
19
DFO
Application Circuit.
Data Slicer
The data slicer takes the analog output of the data filter
and converts it to a digital signal. This is achieved by
using a comparator and comparing the analog input to
C4
R1
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 com-
parator 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 3). This configura-
tion averages the analog output of the filter and sets the
Figure 3. Generating Data Slicer Threshold
MAX7033
DATA
SLICER
threshold to approximately 50% of that amplitude. With
25
23
20
19
this configuration, the threshold automatically adjusts
DATAOUT
DSP
DSN
R4
DFO
as the analog signal varies, minimizing the possibility
for errors in the digital data. The values of R1 and C4
affect how fast the threshold tracks to the analog ampli-
R1
R3
tude. Be sure to keep the corner frequency of the RC
R2
*OPTIONAL
C4
circuit much lower than the lowest expected data rate.
Note that a long string of zeros or ones can cause the
threshold to drift. This configuration works best if a cod-
ing scheme, such as Manchester coding, which has an
equal number of zeros and ones, is used.
To prevent continuous toggling of DATAOUT in the
absence of an RF signal due to noise, add hysteresis to
the data slicer as shown in Figure 4.
Peak Detector
The peak-detector output (PDOUT), in conjunction with
an external RC filter, creates a DC output voltage equal
to the peak value of the data signal. The resistor pro-
Figure 4. Generating Data Slicer Hysteresis
MAX7033
DATA
SLICER
vides a path for the capacitor to discharge, allowing the
peak detector to dynamically follow peak changes of
the data-filter output voltage. For faster data slicer
25
DATAOUT
20
DSN
23
DSP
25k Ω
19
DFO
26
PDOUT
response, use the circuit shown in Figure 5.
47nF
Figure 5. Using PDOUT for Faster Startup
______________________________________________________________________________________
13
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MAX7033EVKIT-315 EVAL KIT FOR MAX7033 315MHZ
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