LM1868 National Semiconductor, LM1868 Datasheet - Page 7

• Manufacturer Part Number: LM1868
• Description: Radio System
• Manufacturer: National Semiconductor

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Layout Considerations
AM SECTION
Most problems in an AM radio design are associated with
radiation of undesired signals to the loopstick Depending
on the source this radiation can cause a variety of problems
including tweet poor signal-to-noise and low frequency os-
cillation (motor boating) Although the level of radiation from
the LM1868 is low the overall radio performance can be
degraded by improper PCB layout Listed below are layout
considerations association with common problems
1 Tweet Locate the loopstick as far as possible from de-
tector components C6 C9 R4 and R5 Orient C6 C9 R4
and R5 parallel to the axis of the loopstick Return R8 C6
C9 and C19 to a separate ground run (see Typical Applica-
tion PCB)
2
Twist speaker leads Orient R10 and C20 parallel to the axis
of the loopstick Locate C11 away from the loopstick
In general radiation results from current flowing in a loop In
case 1 this current loop results from decoupling detector
harmonics at pin 17 while in case 2 the current loop results
from decoupling noise at the output of the audio amplifier
and the output of the regulator The level of radiation picked
up by the loopstick is approximately proportional to 1) 1 r
where r is the distance from the center of the loopstick to
the center of the current loop 2) SIN
between the plane of the current loop and the axis of the
loopstick 3) I the current flowing in the loop and 4) A the
cross-sectional area of the current loop
Pickup is kept low by short leads (low A) proper orientation
( j 0 so SIN
loopstick and keeping current levels low
FM SECTION
The pinout of the LM1868 has been chosen to minimize
layout problems however some care in layout is required to
insure stability The input source ground should return to C4
ground Capacitors C13 and C18 form the return path for
signal currents flowing in the quadrature coil They should
connect directly to the proper pins with short PC traces (see
Typical Application PCB) The quadrature coil and input cir-
cuitry should be separated from each other as far as possi-
ble
AUDIO AMPLIFIER
The standard layout considerations for audio amplifiers ap-
ply to the LM1868 that is positive and negative inputs
should be returned to the same ground point and leads to
the high frequency load should be kept short In the case of
the LM1868 this means returning the volume control ground
(R6) to the same ground point as C17 and keeping the
leads to C20 and R10 short
Poor Signal-to-Noise Low Frequency Oscillation
j 0) maximizing distance from sources to
where
is the angle
TL H 7909 – 14
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Circuit Description
AM SECTION
The AM section consists of a mixer stage a separate local
oscillator an IF gain block an envelope detector AGC cir-
cuits for controlling the IF and mixer gains and a switching
circuit which disables the AM section in the FM mode
Signals from the antenna are AC-coupled into pin 7 the
mixer input This stage consists of a common-emitter ampli-
fier driving a differential amp which is switched by the local
oscillator With no mixer AGC the current in the mixer is
330
decreasing the gain and also the input impedance drops
reducing the signal at the input The differential amp con-
nected to pin 8 forms the local oscillator Bias resistors are
arranged to present a negative impedance at pin 8 The
frequency of oscillation is determined by the tank circuit the
peak-to-peak amplitude is approximately 300
impedance at pin 8 in parallel with 8k2
After passing through the ceramic filter the IF signals are
applied to the IF input Signals at pin 11 are amplified by two
AGC controlled common-emitter stages and then applied to
the PNP output stage connected to pin 13 Biasing is ar-
ranged so that the current in the first two stages is set by
the difference between a 250
Darlington device connected to pin 12
When the AGC threshold is exceeded the Darlington device
turns ON steering current away from the IF into ground
reducing the IF gain Current in the IF is monitored by the
mixer AGC circuit When the current in the IF has dropped
to 30
the mixer AGC line begins to draw current This causes the
mixer current and input impedance to drop as previously
described
The IF output is level shifted and then peak detected at
detector cap C1 By loading C1 with only the base current of
the following device detector currents are kept low Drive
from the AGC is taken at pin 14 while the AM detector
output is summed with the FM detector output at pin 17
FM SECTION
The FM section is composed of a 6-stage limiting IF driving
a quadrature detector The IF stages are identical with the
exceptions of the input stage which is run at higher current
to reduce noise and the last stage which is switched OFF
in the AM mode The quadrature detector collectors drive a
level shift arrangement which allows the detector output
load to be connected to the regulated supply
AUDIO AMPLIFIER
The audio amplifier has an internally set voltage gain of 120
The bandwidth of the audio amplifier is reduced in the AM
mode so as to reduce the output noise falling in the AM
band The bandwidth reduction is accomplished by reducing
the current in the input stage
REGULATOR
A series pass regulator provides biasing for the AM and FM
sections Use of a PNP pass device allows the supply to
drop to within a few hundred millivolts of the regulator out-
put and still be in regulation
A as the AGC is applied the mixer current drops
A corresponding to 30 dB gain reduction in the IF
(See Equivalent Schematic)
A current source and the
A times the