LM1868 [NSC]
Radio System; 无线电系统
| 型号： | LM1868 |
| 厂家： | 
National Semiconductor |
| 描述： | Radio System |
| 文件： | 总10页 (文件大小：253K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
February 1995
LM1868 AM/FM Radio System
General Description
Features
Y
DC selection of AM/FM mode
The combination of the LM1868 and an FM tuner will pro-
vide all the necessary functions for a 0.5 watt AM/FM radio.
Included in the LM 1868 are the audio power amplifier, FM
IF and detector, and the AM converter, IF, and detector.
The device is suitable for both line operated and 9V battery
Y
Regulated supply
Y
Audio amplifier bandwidth decreased in AM mode,
reducing amplifier noise in the AM band
AM converter AGC for excellent overload
characteristics
Y
applications.
Y
Low current internal AM detector for low tweet radiation
Block Diagram
TL/H/7909–1
Order Number LM1868N
See NS Package Number N20A
Note: See table for coil data
C
1995 National Semiconductor Corporation
TL/H/7909
RRD-B30M115/Printed in U. S. A.
Absolute Maximum Ratings
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales
Office/Distributors for availability and specifications.
b
a
55 C to 150 C
Storage Temperature Range
§
0 C to 70 C
§
a
Operating Temperature Range
Lead Temperature (Soldering, 10 sec.)
§
§
260 C
§
Supply Voltage (Pin 19)
Package Dissipation
15V
2.0W
e
Above T
T
25 C, Derate Based on
§
A
e
e
60 C/W
150 C and i
§
§
J(MAX)
JA
e
e
e
9V, R 8X (unless otherwise noted)
L
Electrical Characteristics Test Circuit, T
25 C, V
§
A
S
Parameter
STATIC CHARACTERISTICS e
Supply Current
Conditions
Min
Typ
Max
Units
e
e
0
0, e
FM
AM
AM Mode, S1 in Position 1
22
30
4.8
15
mA
V
Regulator Output Voltage (Pin 16)
Operating Voltage Range
3.5
4.5
3.9
DYNAMIC CHARACTERISTICSÐAM MODE
e
e
e
1 kHz, 30% Modulation, S1 in Position 1, P 50 mW unless noted
O
f
1 MHz, f
mod
AM
e
50 mW,
Maximum Sensitivity
Measure e
AM
Maximum Volume
for P
O
8
16
85
mV
dB
e
e
Signal-to-Noise
Detector Output
e
10 mV
40
40
50
60
AM
e
AM
1 mV
mV
Measure at Top of Volume Control
e
e
e
Overload Distortion
e
50 mV, 80% Modulation
2
10
2
%
%
AM
AM
Total Harmonic Distortion (THD)
e
10 mV
1.1
e
e
e
50 mW, S1 in Position 1
O
g
DYNAMIC CHARACTERISTICSÐFM MODE f
10.7 MHz, f
400 Hz, Df
75 kHz, P
FM
mod
b
3 dB Limiting Sensitivity
15
64
45
85
2
mV
e
e
Signal-to-Noise Ratio
Detector Output
e
e
10 mV
50
40
40
dB
FM
e
g
10 mV, Df
22.5 kHz
FM
60
mV
Measure at Top of Volume Control
e
e
AM Rejection
e
10 mV, 30% AM Modulation
10 mV
50
dB
%
FM
FM
Total Harmonic Distortion (THD)
e
1.1
e
e
e
0, S1 in Position 2
FM
DYNAMIC CHARACTERISTICSÐAUDIO AMPLIFIER ONLY f
1 kHz, e
AM
0, e
e
10%, R 8X
L
Power Output
Bandwidth
THD
e
V
V
6V
250
500
325
700
mW
mW
S
e
9V
S
e
e
AM Mode, P
50 mW
50 mW
11
22
kHz
kHz
O
FM Mode, P
O
e
Total Harmonic Distortion (THD)
Voltage Gain
P
O
50 mW, FM Mode
0.2
41
%
dB
Typical Performance Characteristics (Test Circuit) All curves are measured at audio output
Quiescent Supply Current
vs Voltage
FM Limiting Characteristics
FM IF AM Rejection
TL/H/7909–2
2
Typical Performance Characteristics (Continued)
All curves are measured at audio output (Test Circuit)
Gain vs Frequency Audio
Amplifier Only
AM Characteristics
Recovered Audio vs Supply
TL/H/7909–3
Test Circuit
N
TL/H/7909–4
3
Typical Application
4
PC Board Layout
TL/H/7909–6
Component Side
Typical Performance Characteristics Typical Application
All curves are measured at audio output
TL/H/7909–7
TL/H/7909–8
5
IC External Components (Application Circuit)
Typical
Typical
Value
240k
Component
Comments
Component
Comments
Value
100 pF
0.1 mF
C1
Removes tuner LO from IF input
Antenna coupling capacitor
FM IF decoupling capacitors
R9
Set AGC time constant
C2
C19
C7
1 mF
(
(
C4, C5
C6, C9
R5
0.01 mF
10 mF
0.1 mF
0.1 mF
5X
IF coupling
IF coupling
0.005 mF AM smoothing/FM de-emphasis
network, de-emphasis pole is
given by.
C8
1k
(
C20
R10
High frequency load for audio
amplifier, required to stabilize
audio amplifier
1
j
f1
R4 R6
C21
R1
250 mF
Output coupling capacitor
a
2q (C6
C9)
a
R4
R6
6k2
Sets Q of quadrature coil,
determining FM THD and
recovered audio
#
J
C10
C11
C12
C13
C14
C15
R7
10 mF
0.1 mF
10mF
0.1 mF
50 mF
0.1 mF
3k
Regulator decoupling capacitor
Regulator decoupling capacitor
AC coupling to volume control
Power supply decoupling
R2
R3
12k
5k6
IF amplifier bias R
Sets gain of AM IF and Q of AM
IF output tank
Power supply decoupling
R4
10k
Detector load resistor
Volume control
Audio amplifier input coupling
R6
50k
Roll off signals from detector in
the AM band to prevent radiation
C18
0.02 mF
150X
Power supply decoupling
C16
C17
0.001 mF
100 mF
(
R11, R12
Terminates the ceramic filter,
biases FM IF input stage
Power amplifier feedback
decoupling, sets low frequency
supply rejection
D1
1N4148
Optional. Quickens the AGC
response during turn on
R8
16k
AM detector bias resistor
Coil and Tuning Capacitor Specifications
l
@
70 10.7 MHz, L to
C1
AM ANT 140 pF max 5.0 pF min
AM OSC 82 pF max 5.0 pF min
Trimmers 5 pF
FM 20 pF max 4.5 pF min
T1
Q
u
@
TOKO CY2-22124PT
resonate w/82 pF 10.7 MHz
TOKO KAC-K2318 or equivalent
e
L1
640 mH, Q
200
e
AM antenna
u
@
e
(At secondary)
R
P
3k5
F
796 kHz
1 mV/meter induces
approximately 100 mV
open circuit at the secondary
l
@
F
e
L0, L2 360 mH, Q
80
796 kHz
TOKO RWO-6A5105 or
equivalent
TL/H/7909–10
U
l
@
14 455 kHz, L to
T2
Q
u
Toko America
@
resonate w/180 pF 455 kHz
TOKO 159GC-A3785 or
equivalent
1250 Feehanville Drive
Mount Prospect, IL 60056
(312) 297-0070
TL/H/7909–9
e
5 mm
Ý
SWG 20, N
diameter
L4
L5
3(/2T, inner
TL/H/7909–11
e
CF1
TOKO CFU-090D or equivalent
e
5 mm
Ý
SWG 20, N
diameter
3(/2T, inner
l
@
BW 4.8 kHz 455 kHz
e
e
e
e
70
L6
L7
L
0.44 mH, N
4 (/2T, Qu
2 (/2T, inner
5 mm
e
Ý
SWG 20, N
e
diameter
TL/H/7909–12
CF2
10.7 MHz ceramic filter
MURATA SFE 10.7 mA or
equivalent
Murata
2200 Lake Park Drive
Smyrna, GA 30080
(404) 436-1300
T3
Apollo Electronics NS-107C
or equivalent
TL/H/7909–13
6
Layout Considerations
Circuit Description (See Equivalent Schematic)
AM SECTION
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.
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 mA; 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 mA times the
impedance at pin 8 in parallel with 8k2.
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.
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 mA 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 mA, 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.
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
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.
3
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 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.
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.
Pickup is kept low by short leads (low A), proper orientation
j
j
loopstick, and keeping current levels low.
(i
0 so SIN i
0), maximizing distance from sources to
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 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.
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.
7
Equivalent Schematic
8
9
Physical Dimensions inches (millimeters)
Molded Dual-In-Line Package (N)
Order Number LM1868N
NS Package Number N20A
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