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參數(shù)資料
| 型號： | LM1868N |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 接收器 |
| 英文描述： | Radio System |
| 中文描述： | AM/FM, AUDIO SINGLE CHIP RECEIVER, PDIP20 |
| 封裝： | PLASTIC, DIP-20 |
| 文件頁數(shù)： | 7/10頁 |
| 文件大?。?/td> | 251K |
| 代理商： | LM1868N |
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.
Poor Signal-to-Noise/Low Frequency Oscillation:
Twist speaker leads. Orient R10 and C20 parallel to the axis
of the loopstick. Locate C11 away from the loopstick.
TL/H/7909–14
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
3
;
where r is the distance from the center of the loopstick to
the center of the current loop; 2) SIN
i
, where
i
is the angle
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
(
i
j
0 so SIN
i
j
0), maximizing distance from sources to
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.
Circuit Description
(See Equivalent Schematic)
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
m
A; as the AGC is applied, the mixer current drops,
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
m
A times the
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
m
A current source and the
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
m
A, corresponding to 30 dB gain reduction in the IF,
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.
7
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相關代理商/技術參數(shù) |
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| LM1868N/A+ | 制造商:未知廠家 制造商全稱:未知廠家 功能描述:AM/FM Receiver Circuit |
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| LM1870N | 制造商:未知廠家 制造商全稱:未知廠家 功能描述:FM Receiver Circuit |
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