Texas Instruments | AN-147 Low Cost IC Stereo Receiver | Application notes | Texas Instruments AN-147 Low Cost IC Stereo Receiver Application notes

Texas Instruments AN-147 Low Cost IC Stereo Receiver Application notes
LM2878,LM3089
AN-147 Low Cost IC Stereo Receiver
Literature Number: SNOA640
National Semiconductor
Application Note 147
Jim Sherwin
June 1975
INTRODUCTION
The recent availability of a broad line of truly high-performance consumer integrated circuits makes it possible to construct a high quality, low noise, low distortion and low cost
AM/FM/Stereo receiver. Design emphasis is placed on a
high level of performance, minimum factory adjustments
and low parts count. As such, the receiver has immediate
applications to table-top, high-fidelity, automotive and communications markets.
Provisions are included for the addition of a ceramic phono
unit as well as a tapehead amplifier allowing inclusion of
eight-track or cassette transport systems. Complete tone
control circuitry is provided offering both boost and cut of
Bass and Treble frequencies. Left and right channel Balance, and system Volume complete the manual front-panel
controls.
Panel meters are employed in the FM system for both signal
strength and center tuning, allowing for easy and accurate
tuning. A directly driven LED offers immediate indication of
FM stereo reception.
The complete design requires just five IC’s, restricting the
use of discrete active elements to the preassembled FM
front-end and the single transistor tone control design.
FM AND FM STEREO
A preassembled front-end was selected as the cost-effective approach to minimum parts count and minimum factory
adjustments. This model features an FET input stage providing excellent distortion performance. High selectivity is
obtained through the use of two cascaded ceramic filters
yielding an approximate 6-pole response with less than 12
dB insertion loss.
The LM3089 FM IF System does all the major functions
necessary for FM processing, including a three stage amplifier/limiter and balanced product detector, as well as an
audio preamplifier. A single quadrature coil was used for
ease of ailgnment; yielding recovered audio with THD less
than 0.5%, however a double coil may be used to diminish
THD to 0.1% if required. Carrier level detectors provide delayed AGC, SIGNAL strength meter drive, and adjustable
interstation mute control R11. The internal AFC amplifier
was used to drive the TUNING meter, giving a visual indication of center tuning.
FM stereo demodulation is accomplished by the use of the
LM1800 phase locked loop, thereby eliminating the need for
external coils. Only two adjustments are necessary: R14,
which sets the 19 kHz oscillator, and R17, which corrects
Low Cost IC Stereo Receiver
Low Cost IC
Stereo Receiver
TL/H/7250 – 1
FIGURE 1. IC Receiver
AN-147
C1995 National Semiconductor Corporation
TL/H/7250
RRD-B30M115/Printed in U. S. A.
control amplifier. Balance and Volume controls are included
as shown. Loudness control may be included by using a
tapped Volume pot and the associated bass boost RC network if desired. More elaborate tone controls such as Baxandall feedback are possible, but at a premium in cost.
which corrects for excess phase shift thru the IF stages, and
yields maximum channel separation. Automatic stereo/
monaural switching is built-in, and may be used in lieu of
mechanical switching if desired. The open collector lamp
driver is used to light a LED whenever a stereo station is
encountered. (Further details available from application
note AN-81.)
POWER AMPLIFIER
The stereo power section, consisting of the LM378 dual audio amplifier, delivers 3W/channel with total harmonic distortion (THD) less than 1% and 4W/channel with THD less
than 10%, operating from split supply voltages of g 11V.
Split supplies were chosen to facilitate a minimum parts
count design. This approach allows direct coupling of the
amplifier to the speakers since the output DC level is zero
volts (offset voltage will be less than 25 mV), thereby eliminating the need of large coupling capacitors and their associated degradation of power, distortion and cost. Since the
input bias voltages are zero volts, the need for bias resistors
and the bias-pin supply bypassing capacitor are also eliminated. Input capacitors are omitted and bias current for the
positive input is obtained directly thru the Volume pots since
the tone control circuitry has been designed such that there
is no DC potential applied.
It is important to apply proper supply voltages and adequate
heatsinking in using the LM2878. Note that while the standby and low output power operating points of the power supply are g 15V, the maximum power out point causes the
supply to sag to g 11V therefore reducing package dissipation to acceptable levels.
AM
The AM function of the receiver is done completely with the
LM3820 AM radio system. While designed for 3 section
tuned superheterodyne applications, the LM3820 may be
used with the less expensive 2 section tuned designs by
omitting the RF stage and redefining its function as 2ND IF
stage (see linear brief LB-29). As shown, the LM3820 provides the necessary converter/oscillator, IF, and AGC detector functions, while the external diode D1 does the audio
demodulation. D1 is slightly forward biased thru R2 for improved distortion performance. In addition its resistance is
used in conjunction with C9 to form the first stage of the
required low pass filter; the second stage consists of R3 and
C10.
TAPE
The LM382 dual preamplifier was selected for its minimum
parts count and low noise capability. With a guaranteed
maximum equivalent input noise voltage of 1.2 mVrms (10
kHz BW), it easily amplifies the low level tape signals while
retaining excellent S/N ratio ( E 64 dB below 2 mV input
level). An ion-implanted resistor matrix is supplied on the
chip for self-biasing the output to half-supply, and provides
the resistors necessary to create the NAB equalization
curve; requiring only four external capacitors per channel to
complete the amplifier. For production models this preamplifier would normally be mounted directly on the tape player
to minimize hum and noise pick-up.
POWER SUPPLY
The worst case ripple rejection of 45 dB for the IC’s used
allows for a simple unregulated power supply, however the
discrete front-end and tone control amplifiers require some
regulation to preserve the IC performance. A single zener
diode Z1 was selected to create a a 12V supply for this
function. The split supplies required for the power amplifier
are derived from a conventional full-wave bridge rectifier operating off of the center-tapped secondary of the line transformer.
TONE CONTROLS
A single transistor tone control circuit was designed as the
optimum cost/performance trade-off. The transistor is configured in a shunt-shunt feedback design, allowing gain variations between input functions. This is necessary to prevent
sudden changes in outlput level when different inputs are
selected. With a shunt feedback design the gain is easily
controlled by choice of source impedance per Figure 5.
Approximately 20 dB of boost or cut of Bass and/or Treble
frequencies is possible with the network shown. The turnover frequencies are approximately 500 Hz and 1600 Hz for
bass and treble, respectively. The insertion loss of approximately 27 dB is made up by the gain of the transistor tone
REFERENCES
1. Isbell, T.D. and Mishler, D.S., ‘‘LM1800 Phase Locked
Loop FM Stereo Demodulator’’. National Semiconductor
Application Note AN-81, June 1973.
2. Papanicolaou, E.S. and Mortensen, H.H., ‘‘Low-Cost AMRadio System Using LM1820 And LM386’’. National
Semiconductor Linear Brief LB-29, May 1975.
2
TL/H/7250 – 2
TL/H/7250 – 3
FIGURE 2. FM Sensitivity
FIGURE 3. AM Sensitivity
TL/H/7250 – 4
TL/H/7250 – 5
FIGURE 4. Power Amplifier Frequency Response
and Total Harmonic Distortion
FIGURE 5. AV vs. RSOURCE for
Tone Control Preamplifier State
SPECIFICATIONS
VENDOR DEVICES
FM Tuner: Waller 32SN2F1-30
Coils: T3: AM Osc. Toko RWO-6A6255
T4: IF, 455 kHz Toko RRC-3A6426N
T5: IF, 455 kHz Toko RRC-3A6427A
T6: IF, 455 kHz Toko RZC-1A6425A
T7: Quadrature, 10.7 MHzToko TKXC-33733BS
Ceramic Filters: 10.7 MHz Toko CFS-30AE-10
Selector Switch: IEE/Schadow Type F-4U with
FA201 Mech. Reflecting Indicators
Meters: TUNING: Ý11226, SIGNAL: Ý11222
Mercer Electronicis
FM-MONO
Sensitivity: 2.5 mV for 30 dB quieting
Harmonic Distortion: 0.3%
Hum * Noise: b65 dB
Frequency Response: 50–15 kHz g 3 dB
FM-STEREO
Channel Separation: 45 dB
Harmonic Distortion: 0.4%
AM
Sensitivity: 20 mV for 20 dB S a N/N
Harmonic Distortion: 2%
Hum & Noise: b45 dB
TAPE
Frequency Response: NAB equalized g 2 dB
Harmonic Distortion: 0.3%
Hum & Noise: b64 dB below 2 mV input level
VENDOR LOCATIONS
Waller Corp., Crystal Lake, ILL. (815) 459-6510
Toko (America), Inc. Toko America, Inc.
1250 Feehanville Drive
Mount Prospect, IL 60056
Tel: (312) 297-0070
IEE/Schadow Corp., Eden Prairie Minn.
(612) 944-1820
Mercer Electronics (Simpson Electric Co.),
Elgin, ILL. (312) 379-1130
AMPLIFIER
Power Output: 3W RMS, per channel into 8Xs
at less than 1% THD from 40–30 kHz
Frequency Response: 35–55 kHz g 3 dbn
3
4
Note 4: Change to 0.015 mF for 50 ms de-emphasis.
Note 3: Omit for 50 ms de-emphasis.
Note 2: All unmarked capacitors in microfarads.
Note 1: All unmarked resistors in Xs, 1/4W.
FIGURE 6. Complete Schematic
TL/H/7250 – 6
5
Low Cost IC Stereo Receiver
LIFE SUPPORT POLICY
NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT
DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT OF NATIONAL
SEMICONDUCTOR CORPORATION. As used herein:
AN-147
1. Life support devices or systems are devices or
systems which, (a) are intended for surgical implant
into the body, or (b) support or sustain life, and whose
failure to perform, when properly used in accordance
with instructions for use provided in the labeling, can
be reasonably expected to result in a significant injury
to the user.
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Tel: 1(800) 272-9959
Fax: 1(800) 737-7018
2. A critical component is any component of a life
support device or system whose failure to perform can
be reasonably expected to cause the failure of the life
support device or system, or to affect its safety or
effectiveness.
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National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications.
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