LM1872

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


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Application Hints
(Continued)
The digital channel output devices have significant drive ca-
pability they can typically sink 100 mA and possess a 7
saturation resistance Through their emitters they can
source 100 mA up to 1V above ground for driving grounded
NPNs and SCRs Unfortunately this kind of drive capability
can cause thermally induced chip destruction unless total
power dissipation is limited to less than 1000 mW It is good
practice and highly recommended to allow the digital output
devices to fully saturate at all times (sinking or sourcing) and
to limit the current at saturation to no more than 100 mA
For extra drive the two digital outputs can always be
summed by connecting pin 7 to pin 9
The IF frequency is not constrained to be 455 kHz Opera-
tion is limited on the high end to about 1 MHz due to the
frequency response limitations of the active detector The
low end is limited to about 50 kHz due to the envelope
detector integration time (Figure 4)
RECEIVER ALIGNMENT
The receiver alignment procedure is relatively straightfor-
ward because of an absence of interaction between the ad-
justments First the oscillator is tuned by adjusting L1 while
monitoring the LO signal at pin 2 with a low capacity
( j 10 pF) probe During tuning the amplitude will rise peak
and then abruptly quit Adjust the coil away from the quitting
point and just below the amplitude peak
In order to properly tune T1 T2 and T3 the RF signal must
be provided through the receiver antenna by the specific
transmitter which is to be used with that specific receiver
This is because the crystals which are commonly used with
these systems may have tolerances as loose as
At 49 MHz the resultant
5 kHz deviation could easily put
g
the incoming signal out of the 3 2 kHz receiver IF bandpass
The signal should be coupled through the receiving antenna
to ensure proper loading of the T3 input tank
Alignment is easier with a defeated AGC which is accom-
plished by merely grounding pin 16 The amplitude of the
455 kHz signal at pin 15 is used to guide alignment Care
should be exercised that the signal swing not exceed rough-
ly 400 mVp or diode D2 in Figure 4 will threshold and
clamp the waveform Also note that a standard 10 pF probe
at pin 15 will shift the IF tank frequency an undesirable
2 kHz Unless a lower capacity probe is available it is rec-
ommended that the signal be monitored at the unused sec-
ondary of T2 Although the signal amplitude would be down
by a factor of 8 25 relative to pin 15 up to 50 pF probe
capacitance could be tolerated with negligible frequency
shift
The incoming signal is obtained by removing the antenna
from the transmitter and then locating the transmitter at a
sufficient distance from the receiver to give a convenient
signal level (
400 mVp) at pin 15 T3 T1 and T2 are then
s
tuned for maximum signal
Applications
OPERATION AT 72 MHz
The licensed 72 MHz band is popular among hobby enthusi-
asts for controlling aircraft The higher transmitted power
levels that the FCC allows yield much greater operating
range and the frequency band is uncluttered relative to
27 MHz Elevated frequencies such as 72 MHz are no prob-
lem with the LM1872 The part is stable and will provide
good sensitivity and selectivity at that frequency The appli-
cation circuit in Figure 8 will provide a set of solid decoded
outputs for
2
V of signal at the antenna input which is
k
designed to match the 100
wavelength antenna IF bandwidth is a respectable 3 2 kHz
For good immunity to overload from a very closely (anten-
nas touching) operating high power transmitter the trans-
mitter design should emphasize a high carrier ON OFF ra-
tio Using the LM1871 as a low power exciter to drive one or
more external class C power amplifier stages will result in a
simple acceptable low cost transmitter at 72 MHz
Inasmuch as many hobby applications require more analog
channels than the LM1872 normally provides particular at-
tention should be paid to Figures 10 and 12 which describe
how to expand analog channel capacity up to 4 and 6 chan-
nels respectively
OPERATION WITH AN IR CARRIER
An infra-red (or visible) light data link is a useful alternative
to its RF counterpart Should the application demand that
the radiation not leave the room or that it be directional or
not involve FCC certification then a light carrier should be
given consideration The principal drawbacks to this ap-
0 01%
g
proach include short range (
power consumption There is little that can be done to dra-
matically improve range but short burst-type operation of
the transmitter will still permit battery operation
The information link (Figure 9a) consists of a light carrier
amplitude modulated by a 455 kHz subcarrier The subcarri-
er in turn is modulated by the normal Pulse Width Pulse
Count Scheme produced by the LM1871 encoder A husky
focused LED is used as the transmitter running Class A
100% modulated with an average current drain of 50 mA to
500 mA depending upon range requirements The detector
consists of a large area silicon PN or PIN photodiode for
good sensitivity The LM1872 will directly interface to such a
diode and give very good performance Only a few na-
noamps of photo current from D1 are required to threshold
the detector Ambient light rejection is excellect due to the
very narrow bandwidth ( j 3 kHz) that results from the use
of three high Q 455 kHz transformers T1 T2 and T3 Note
that the LO has been defeated and the mixer runs as a
conventional 455 kHz amplifier Otherwise circuit operation
is the same as if an RF carrier were being received
10
resistive impedance of the
20 ft ) and high transmitter
s