LM1872

Manufacturer Part NumberLM1872
DescriptionRadio Control Receiver/Decoder
ManufacturerNational Semiconductor
LM1872 datasheet
 


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Circuit Description
The following discussion is best understood by referring to
Figures 2 3 4 and 5
SYSTEM ENCODING AND DECODING SCHEME
For the transfer of analog information the LM1871 LM1872
system uses conventional pulse width modulation (PWM) In
applying this technique the RF carrier is interrupted for
short fixed intervals (t
in Figure 2 ) with each interval fol-
M
lowed by variable width pulses (t
) so as to define multiple
CH
variable time spans (t
t
) occurring in serial fashion
a
M
CH
Synchronization is accomplished by allowing one of the
transmitted variable pulse widths (t
) to exceed the du-
SYNC
ration (t’
) of a receiver-based timer thus allowing the
SYNC
receiver to recognize this pulse for synchronization purpos-
es Taken in sequence this collection of pulses constitutes
a single frame period (t
)
F
LM1871
Pin Conditions
Pin 5 (CH A)
Pin 6 (CH B)
OPEN
OPEN
GND
OPEN
OPEN
GND
GND
GND
FIGURE 3 Digital Channel Encoding and Decoding via Pulse Count Modulation
The LM1871 transmitter is equipped to transmit up to six
channels which the companion LM1872 receiver uses to
derive 2 analog and 2 digital channels The receiver de-
codes the demodulated RF waveform from the transmitter
by negative edge triggering a cascade of three binary divid-
ers called the A B and C toggle flip-flops ( Figure 4 ) By
‘‘examining’’ all three flip-flop outputs simultaneously up to
6 unique channel time intervals could be identified and re-
covered Only the first two channels are actually decoded
however and outputted by the receiver the rest being used
for identification of two digital (ON OFF) channels In pass-
ing digital information a pulse count modulation scheme is
used whereby different quantities of channel pulses are
transmitted by varying the number of fixed width channels
following the two variable width analog channels 1 and 2
(see Figure 3 )
FIGURE 2 RX Timing Waveforms
TX
Transmitted Waveform
Pulse Count
TL H 7912 – 6
TL H 7912 – 7
TL H 7912 – 8
TL H 7912 – 9
5
TL H 7912 – 5
LM1872
RX
Binary
Digital Outputs
CH A
CH B
100
OFF
OFF
101
ON
OFF
110
OFF
ON
111
ON
ON