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 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: 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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