3845

3845 AM NOISE BLANKER This noise blanker integrated circuit contains all of the necessary circuitry for adding an extremely efficient (patented) noise-blanking technique to any type of AM tuner or receiver with RF input frequencies (or a first IF) to 30 MHz. The A3845ELW and A3845SLW feature dual audio channels and are intended for AM-stereo or independent sideband applications. A high input impedance, high-gain, broadband RF amplifier permits these devices to be directly connected to the RF stage of a tuner. Internal AGC circuitry ensures that the noise detection threshold remains constant with changes in input signal level. The RF gate response time is sufficiently fast to blank the noise pulse at the output of the mixer before the IF filter. Short blanking times effectively suppress most of the interfering noise. Residual audio noise is removed by an audio sample-and-hold gate. The RF blanking time, audio gate delay time, and audio gate blanking time can all be independently adjusted to suit the particular application. These AM noise blankers are packaged in plastic SOICs and are rated for operation over the a standard temperature range of -20°C to +85°C (suffix ‘SLW’) or an extended temperature range to -40°C (suffix ‘ELW’). Data Sheet 27126B* RF IN RF BYPASS RF BIAS RF AGC AUDIO DELAY AUDIO BLANK TIME (R) NO CONNECT AUDIO BLANK TIME (C) AUDIO OUT1 AUDIO IN 1 1 2 3 4 5 6 7 8 9 10 NC DET VCC NC 20 19 18 17 16 15 SUPPLY NO CONNECT RF GATE LOW RF GATE HIGH GROUND RF BLANK TIME NO CONNECT NOISE DIFFERENTIATOR AUDIO OUT 2 AUDIO IN 2 NC 14 13 12 11 Dwg. PS-003-1A FEATURES I I I I I I I RF Blanking to 30 MHz Single-Channel or Stereo Audio Blanking Adjustable RF and Audio Blanking Time Adjustable Audio Blanking Delay Sample-and-Hold MOS Audio Gates Internal Voltage Regulation Minimum External Components AM and AM-Stereo Automotive Radios CB Transmitter/Receivers Short-Wave Receivers Mobile Communications Equipment ABSOLUTE MAXIMUM RATINGS at TA = +25°C Supply Voltage, VCC . . . . . . . . . . . . . . Package Power Dissipation, PD . . . . . . . . . . . . . . . . . . . . . . . 12 V APPLICATIONS I I I I 1.78 W Operating Temperature Range, TA Suffix ‘ELW’ . . . . . . . . -40°C to +85°C Suffix ‘SLW’ . . . . . . . . -20°C to +85°C Storage Temperature Range, TS . . . . . . . . . . . . . . . . -55°C to +125°C Always order by complete part number: Part Number A3845ELW A3845SLW Function Stereo Noise Blanker, Extended Temp. Range Stereo Noise Blanker, Standard Temp. Range 3845 AM NOISE BLANKER FUNCTIONAL BLOCK DIAGRAM MIXER OUT RF IN RF RF BYPASS dV/dt DET PEAK DET NOISE DIFFERENTIATOR 1 2 3 RF AGC 17 RF GATE HIGH RF BLANK TIME 69 pF 40 Ω IF IN RF BIAS 13 15 18 +4 V RF GATE LOW 100 kΩ VCC 4 GROUND 16 100 kΩ 10 AUDIO IN 1 SUPPLY 20 REG AUDIO DELAY 69 pF AUDIO BLANK TIME 1 kΩ 9 8 AUDIO OUT 1 19 14 7 5 6 NO CONNECTION 11 AUDIO IN 2 1 kΩ 12 AUDIO OUT 2 Dwg. FS-004-1A TEST CIRCUIT NOISE RF 60.4 Ω 0.01 267 Ω 0.005 SUPPLY 0.01 1 93.1 Ω VCC NC DET 20 19 18 17 16 R15 15 RF BYPASS MIXER OUT 187 Ω 2 kΩ 2 0.1 3 0.1 10 µF + R5 R6 4 5 6 7 C8 8 9 10 NC NC 14 13 0.1 12 11 0.1 0.001 AUDIO OUT1 AUDIO IN 1 0.1 0.1 AUDIO OUT2 AUDIO IN 2 Dwg. ES-007-1A Note that the noise-pulse input is attenuated 20 dB by the test circuit. 115 Northeast Cutoff, Box 15036 Worcester, Massachusetts 01615-0036 (508) 853-5000 Copyright © 1988, 2000 Allegro MicroSystems, Inc. 3845 AM NOISE BLANKER ELECTRICAL CHARACTERISTICS over operating temperature range, at VCC = 7.5 V to 11 V, frf = 1 MHz, Noise (fnoise) = 500 Hz Square Wave, faf = 1 kHz, Test Figure. Test Leads 20 20 Limits Test Conditions Operating VRF = 0 Min. 7.5 — Typ. 9.0 15 Max. 12 22 Units V mA Characteristic Supply Voltage Range Quiescent Supply Current RF INPUT AMPLIFIER: Trigger Threshold Modulation Threshold Detector Rise Time RF SWITCH: ON Resistance OFF Resistance Time Delay 1 1 13 Noise Pulse Amplitude for VRF = 0 Noise Pulse Modulation for VRF = 1 mV C13 = 0 45 35 — 100 75 500 240 220 — µV % ns 17-18 17-18 1-17 From Beginning of RF Pulse to Beginning of RF Blanking — — — 30 100 1.5 100 — 5.0 Ω kΩ µs AUDIO SWITCHES: Attenuation Noise Crosstalk Gain Total Harmonic Distortion Input Impedance Output Impedance BLANKING TIMERS: RF Blanking Audio Delay Audio Blanking 17 9 9 R15 = 350 kΩ R5 = 350 kΩ R6 = 110 kΩ, C8 = 0.0012 µF 35 30 210 60 55 250 75 67 400 µs µs µs 10-9, 11-12 9, 12 9, 12 10-9, 11-12 9, 12 10, 11 9, 12 Vaf =700 mV, Vnoise = 0 55 — 40 -1.2 — — — 80 1.5 60 -0.3