LM1872 National Semiconductor, LM1872 Datasheet - Page 7

• Manufacturer Part Number: LM1872
• Description: Radio Control Receiver/Decoder
• Manufacturer: National Semiconductor

Circuit Description
Thus either 3 4 5 or 6 channels are transmitted to repre-
sent the four possible codes that two digital channels repre-
sent The receiver intrinsically counts channels with its de-
coder flip-flops by responding to the negative edges of the
demodulated RF waveform of which there is always one
more than the number of channels The two LSBs of the
binary count are read latched and fed to the output drivers
which comprise digital channels A and B
RECEIVER SECTION
The receiver circuit is a simple single conversion design
with AGC which mixes down to 455 kHz and provides
R1
R2
R3
C1
C2
C3
C4
C5
C6
C7
C8
C9
C10
LO bypass optional
LO tank C2
Ant input tank C3
V
Motor decoupling
Sync timer C6
Mixer decouple 0 01 F
AGC
IF bypass optional
V
Motor decoupling
Sync timer R2
Mixer decoupling
BIAS
a
bypass 0 01 F
bypass
e
e
e
e
24 pF
43 pF
0 7 R2
t’
0 7 C6
t’
SYNC
SYNC
e
e
s
39 pF
24 pF
C10
27 MHz
49 MHz
(Continued)
FIGURE 5 Typical Application Circuit for 27 MHz or 49 MHz
R2
C6
s
C7
s
s
s
49 MHz
0 5 F
470k
27 MHz
0 1 F
s
0 1 F
C11
L1
T1
T2
T3
X1
D1
455 kHz mixer transformer
455 kHz IF transformer
Ant input transformer
3rd overtone parallel-mode crystal
Electrostatic discharge (ESD) protection
LO bypass
LO coil
Toko 10k type (KXNA-4434 DZ) 9T 0 8 H
Toko 10k type (KEN-4028 DZ) 6T 0 4 H
L1 could be made a fixed coil if desired
Toko 10 EZC type (RMC-202313 NO) Qu
Pin 1–2 131T pin 2–3 33T
Pin 1–3 164T pin 4–6 5T
Toko 10 EZC type (RMC-402503 NO) Qu
Pin 1–2 98T pin 2–3 66T
Pin 1–3 164T pin 4–6 8T
Toko 10k type (KXNA-4434 DZ) 3T sec
Toko 10k type (KEN-4028 DZ) 1 T sec
7
58 dB of gain using the suggested transformers in Figure 5
The active digital detector provides an additional 30 dB gain
over a silicon diode resulting in an overall system gain of
88 dB More or less gain can be obtained by using different
transformers The frequency range of operation extends
from 50 kHz to 72 MHz encompassing a wide range of allo-
cated frequency bands
The short (1 to 2 ) vertical whip antenna that is typically
used has a very low radiation resistance (0 5
approximately 3 pF to 5 pF of capacitance This antenna is
coupled to the mixer through a high Q tank consisting of C3
e
e
9T pri of 0 8 H
6T pri of 0 4 H
49 MHz
110
110
27 MHz
1250 Feehanville Drive
Mount Prospect IL 60056
[phone]
Toko America
TL H 7912 – 11
27 MHz
49 MHz
to 4 ) and