NSC LM1868 Radio system Datasheet

LM1868 AM/FM Radio System
General Description
Features
The combination of the LM1868 and an FM tuner will provide 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
applications.
Y
Y
Y
Y
Y
DC selection of AM/FM mode
Regulated supply
Audio amplifier bandwidth decreased in AM mode,
reducing amplifier noise in the AM band
AM converter AGC for excellent overload
characteristics
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
C1995 National Semiconductor Corporation
TL/H/7909
RRD-B30M115/Printed in U. S. A.
LM1868 AM/FM Radio System
February 1995
Absolute Maximum Ratings
Storage Temperature Range
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales
Office/Distributors for availability and specifications.
Supply Voltage (Pin 19)
Package Dissipation
Above TA e 25§ C, Derate Based on
TJ(MAX) e 150§ C and iJA e 60§ C/W
b 55§ C to a 150§ C
Operating Temperature Range
0§ C to a 70§ C
260§ C
Lead Temperature (Soldering, 10 sec.)
15V
2.0W
Electrical Characteristics Test Circuit, TA e 25§ C, VS e 9V, RL e 8X (unless otherwise noted)
Parameter
Conditions
Min
Typ
Max
Units
Regulator Output Voltage (Pin 16)
3.5
22
30
mA
3.9
4.8
Operating Voltage Range
4.5
V
STATIC CHARACTERISTICS eAM e 0, eFM e 0
Supply Current
AM Mode, S1 in Position 1
15
DYNAMIC CHARACTERISTICSÐAM MODE
fAM e 1 MHz, fmod e 1 kHz, 30% Modulation, S1 in Position 1, PO e 50 mW unless noted
Maximum Sensitivity
Measure eAM for PO e 50 mW,
Maximum Volume
8
Signal-to-Noise
eAM e 10 mV
40
50
Detector Output
eAM e 1 mV
Measure at Top of Volume Control
40
60
85
Overload Distortion
eAM e 50 mV, 80% Modulation
2
10
%
Total Harmonic Distortion (THD)
eAM e 10 mV
1.1
2
%
16
mV
dB
mV
DYNAMIC CHARACTERISTICSÐFM MODE fFM e 10.7 MHz, fmod e 400 Hz, Df e g 75 kHz, PO e 50 mW, S1 in Position 1
b 3 dB Limiting Sensitivity
15
Signal-to-Noise Ratio
eFM e 10 mV
50
64
Detector Output
eFM e 10 mV, Df e g 22.5 kHz
Measure at Top of Volume Control
40
60
AM Rejection
eFM e 10 mV, 30% AM Modulation
40
Total Harmonic Distortion (THD)
eFM e 10 mV
45
dB
85
50
1.1
mV
mV
dB
2
%
DYNAMIC CHARACTERISTICSÐAUDIO AMPLIFIER ONLY f e 1 kHz, eAM e 0, eFM e 0, S1 in Position 2
Power Output
THD e 10%, RL 8X
VS e 6V
VS e 9V
250
500
325
700
mW
mW
kHz
kHz
Bandwidth
AM Mode, PO e 50 mW
FM Mode, PO e 50 mW
11
22
Total Harmonic Distortion (THD)
PO e 50 mW, FM Mode
0.2
%
41
dB
Voltage Gain
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)
AM Characteristics
Recovered Audio vs Supply
Gain vs Frequency Audio
Amplifier Only
Power Dissipation vs Power
Output, RL e 8X
Power Dissipation vs Power
Out, RL e 16X
Distortion vs Frequency
Audio Amplifier Only
TL/H/7909 – 3
Test Circuit
Note: See table for coil data
TL/H/7909 – 4
3
4
# Maximum sensitivity: 100 mV/m
# 20 dB quieting sensitivity: 250 mV/m
# Tweet* worst case: 5%
# 30 dB quieting sensitivity: 3.5 mV
# b 3 dB limiting sensitivity: 7 mV
*Tweet is an audio tone produced by the 2nd and 3rd harmonic of the IF
beating against the received signal. It is measured as an equivalent modulation level: i.e., a 30% tweet has the same amplitude at the detector as a
desired signal with 30% modulation.
100 mV/m: 1.5%
AM Performance (525 kHz–1650 kHz)
FM Performance (88 MHz–108 MHz)
TL/H/7909 – 5
Typical Application
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)
Component
C1
C2
C4, C5
C6, C9
R5
Typical
100 pF
0.1 mF
0.01 mF
0.005 mF
1k
(
Removes tuner LO from IF input
Antenna coupling capacitor
FM IF decoupling capacitors
AM smoothing/FM de-emphasis
network, de-emphasis pole is
given by.
1
f1 j
2q (C6 a C9)
C10
C11
C12
C13
C14
C15
R7
C16
C17
R8
10 mF
0.1 mF
10mF
0.1 mF
50 mF
0.1 mF
3k
0.001 mF
100 mF
Component
Comments
Value
# R4
R4
R6
a R6
J
Regulator decoupling capacitor
Regulator decoupling capacitor
AC coupling to volume control
Power supply decoupling
Power supply decoupling
Audio amplifier input coupling
Roll off signals from detector in
the AM band to prevent radiation
( Power amplifier feedback
decoupling, sets low frequency
supply rejection
AM detector bias resistor
16k
Typical
Comments
Value
R9
C19
1 mF
240k
C7
C8
C20
R10
10 mF
0.1 mF
0.1 mF
5X
C21
R1
250 mF
6k2
R2
R3
12k
5k6
R4
R6
C18
R11, R12
10k
50k
0.02 mF
150X
D1
1N4148
(
(
Set AGC time constant
IF coupling
IF coupling
High frequency load for audio
amplifier, required to stabilize
audio amplifier
Output coupling capacitor
Sets Q of quadrature coil,
determining FM THD and
recovered audio
IF amplifier bias R
Sets gain of AM IF and Q of AM
IF output tank
Detector load resistor
Volume control
Power supply decoupling
Terminates the ceramic filter,
biases FM IF input stage
Optional. Quickens the AGC
response during turn on
Coil and Tuning Capacitor Specifications
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
TOKO CY2-22124PT
L1
640 mH, Qu e 200
RP e 3k5 @ F e 796 kHz
(At secondary)
AM antenna
1 mV/meter induces
approximately 100 mV
open circuit at the secondary
L0, L2 360 mH, QU l 80
@
F e 796 kHz
Qu l 70 @ 10.7 MHz, L to
resonate w/82 pF @ 10.7 MHz
TOKO KAC-K2318 or equivalent
T1
TOKO RWO-6A5105 or
equivalent
TL/H/7909 – 10
T2
Toko America
1250 Feehanville Drive
Mount Prospect, IL 60056
(312) 297-0070
Qu l 14 @ 455 kHz, L to
resonate w/180 pF @ 455 kHz
TOKO 159GC-A3785 or
equivalent
TL/H/7909–9
L4
SWG Ý20, N e 3(/2T, inner
diameter e 5 mm
L5
SWG Ý20, N e 3(/2T, inner
diameter e 5 mm
L6
L e 0.44 mH, N e 4 (/2T, Qu e 70
L7
SWG Ý20, N e 2 (/2T, inner
diameter e 5 mm
CF2
10.7 MHz ceramic filter
MURATA SFE 10.7 mA or
equivalent
TL/H/7909 – 11
CF1
TOKO CFU-090D or equivalent
BW l 4.8 kHz @ 455 kHz
TL/H/7909 – 12
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
The AM section consists of a mixer stage, a separate local
oscillator, an IF gain block, an envelope detector, AGC circuits 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 amplifier 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 connected 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.
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 arranged 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.
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.
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 oscillation (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 detector 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 Application 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/r3;
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 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.
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 circuitry should be separated from each other as far as possible.
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 output and still be in regulation.
AUDIO AMPLIFIER
The standard layout considerations for audio amplifiers apply 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
TL/H/7909 – 15
Equivalent Schematic
8
9
LM1868 AM/FM Radio System
Physical Dimensions inches (millimeters)
Molded Dual-In-Line Package (N)
Order Number LM1868N
NS Package Number N20A
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