LM1872

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


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Application Hints
A typical application circuit for either 27 MHz or 49 MHz is
shown in Figure 5 Using the recommended antenna input
networks and driving the circuit through the antenna simula-
tion network of Figure 6 a solid decoded output occurs for
10
V and 12
V input signals at 27 MHz and 49 MHz
respectively
FIGURE 6 Antenna Simulation Network
This sensitivity has been determined empirically to be opti-
mum for toy vehicle applications Less gain will reduce
range unacceptably and more gain will increase susceptibili-
ty to noise However should the application require greater
range (
50m for a land vehicle for example) either the
l
antenna could be lengthened beyond 2 and or receiver
sensitivity could be improved There are a number of ways
to alter the sensitivity of the receiver Decreasing the turns
ratio of input transformer T3 for example will couple more
signal into the mixer at the expense of lower tank Q due to
mixer loading Moving the primary tap on mixer transformer
T1 further from the supply side and or decreasing the pri-
mary to secondary turns ratio will also increase gain For
example just changing T1 from a 32 1 primary to secondary
ratio to a 5 1 turns ratio (Toko
RMC202202) will double
49 MHz sensitivity (6
V vs 12
V) Mixer tank Q will be
affected but overall 3 dB BW will remain largely unchanged
FIGURE 7 PCB Layout Stuffing Diagram and Complete
RX Module for Typical Application Circuit of Figure 5
The primary tap on the IF transformer T2 can also be ad-
justed (further from the supply side) for higher gain but it is
possible to cause the AGC loop to oscillate with this meth-
od
Narrow overall bandwidth is important for good receiver op-
eration The 3 2 kHz 3 dB bandwidth of the circuit in Figure
5 is just wide enough to pass 500 s carrier dropout pulses
t
yet narrow enough to hold down electrical noise and
M
reject potentially interfering adjacent channels In the
49 MHz band the five frequencies available are only 15 kHz
apart Should only two frequencies be used simultaneously
these channels could be chosen 60 kHz apart Should three
frequencies be used the spacing could be no more than
30 kHz At four or five frequencies 15 kHz spacings must be
dealt with making narrow bandwidth highly desirable Even
at 27 MHz where allocated frequencies are 50 kHz apart
TL H 7912– 12
the proliferation of CB stations only 10 kHz away represents
a formidable source of interference The response of the
circuit of Figure 5 is 34 dB and 56 dB down at 15 kHz and
50 kHz away respectively (see characteristic curves)
The sync timer should have a timeout t’
than the longest channel pulse transmitted but shorter than
the shortest sync pulse t
ponent values in Figure 5 t’
well with a transmitted sync pulse t
Numerous bypass capacitors appear in the circuit of Figure
5 not all of which may be necessary for good stability and
performance A low cost approach may eliminate one or
more of the capacitors C1 C9 C10 and C11 The cleaner
and tighter the PCB layout used the more likely is the case
that bypass capacitors can be eliminated In the case of
marginal board stability increasing the size of capacitors
C7 C9 and C10 to 0 1
layout and parts loading diagram shown in Figure 7 is used
the circuit will be quite stable up to 72 MHz
TL H 7912 – 13
9
set longer
SYNC
transmitted Using the com-
SYNC
3 5 ms which works
e
SYNC
5 ms
SYNC t
F may prove helpful If the PCB
TL H 7912– 14