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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 -20C to +85C (suffix `SLW') or an extended temperature range to -40C (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 = +25C
Supply Voltage, VCC . . . . . . . . . . . . . . Package Power Dissipation, PD . . . . . . . . . . . . . . . . . . . . . . . 12 V
APPLICATIONS
I I I I
1.78 W
Operating Temperature Range, TA Suffix `ELW' . . . . . . . . -40C to +85C Suffix `SLW' . . . . . . . . -20C to +85C Storage Temperature Range, TS . . . . . . . . . . . . . . . . -55C to +125C
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
IF IN
RF BIAS
13
15
18
+4 V RF GATE LOW
100 k
VCC
4
GROUND
16
100 k
10 AUDIO IN 1
SUPPLY 20
REG
1 k
AUDIO DELAY 69 pF AUDIO BLANK TIME
9 8
AUDIO OUT 1
19
14
7
5
6
NO CONNECTION
11 AUDIO IN 2
1 k
12 AUDIO OUT 2
Dwg. FS-004-1A
TEST CIRCUIT
NOISE
RF
60.4
0.01
267
0.005
SUPPLY
0.01 1
93.1
VCC NC DET
20 19 18 17 16 R15 15 RF BYPASS MIXER OUT
187
2 k
2 0.1 3
0.1
10 F + 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 (c) 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 --
V % ns
17-18 17-18 1-17 From Beginning of RF Pulse to Beginning of RF Blanking
-- -- --
30 100 1.5
100 -- 5.0
k s
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 k R5 = 350 k R6 = 110 k, C8 = 0.0012 F 35 30 210 60 55 250 75 67 400 s s s 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 <0.1 100 1.0 _ 12 -- 0 1.0 -- -- dB mVpp dB dB % k k
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3845 AM NOISE BLANKER
CIRCUIT DESCRIPTION
Previous attempts at suppression of impulse noise in AM receivers have used a variety of approaches ranging from gating the signal OFF at the antenna to simply clipping (limiting) any signal that was larger than the average modulation. Unfortunately, the former can generate as much noise as it removes while the latter only reduces the level of noise impulses and does not remove them. A major problem in attempting to suppress impulse noise in an AM receiver can best be described by looking at the shape of a noise pulse as it passes through a typical tuner as shown in the Figure. Here, a typical 0.5 s pulse is applied to the antenna input. The resulting waveforms are essentially the impulse response of the different selectivity sections as limited only by the dynamic range of the individual sections. Note that the signal remains quite narrow until the IF filter is reached. Because of the relatively narrow bandwidth of the IF filter, the limiting of the IF amplifier, and the filtering effect of the detector, the audio output resulting from the impulse is much wider than the original input pulse and is therefore much more objectionable. One blanking scheme currently in use senses the noise pulse in the IF amplifier and blanks the audio output. This results in a long blanking time and poor performance at the higher frequencies where a short blanking time is needed most. The A3845xLW takes a different approach to the noise suppression problem by sensing the noise pulse in the receiver's RF section and blanking the pulse before it reaches the IF. This requires a noise amplifier with a minimum propagation delay and high-speed gating. Blanking the noise pulse in this way is very effective, but some of the interference can still reach the audio output due to the loss of carrier during the blanking interval. For this purpose, an additional delay, blanking interval, and audio gates are included to further suppress any residual signal. The result is almost 100% suppression of
QUIESCENT DC VOLTAGES
(for circuit design information only)
Typical DC Voltage 3.1 3.1 3.1 0.9 4.8 4.8 0 4.8 4.75 4.0 4.75 4.0 4.9 0 4.8 Reference -- -- 0 VCC
Lead Number 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Function RF In RF Bypass RFBias RF AGC Audio Delay Audio Blank Time (R) No Connection Audio BlankTime (C) Audio Out1 Audio In1 Audio In2 Audio Out2 Noise Differentiator No Connection RF Blank Time Ground RF Gate High RF Gate Low No Connection Supply
impulse noise including that from ignition systems and from sources producing interference at a power line rate such as light dimmers and fluorescent lamps. Referring to the Functional Block Diagram, the RF input stage is a differential amplifier, so that the input impedance is high. The triggering threshold at the RF amplifier input is about 15 V at 1 MHz. This means that a pulsed RF input signal of 15 V will exceed the threshold and trigger the blanker. The external capacitor at the dV/dt detector circuit (C13) is selected so that audio signals do not cause triggering. At high input levels, the threshold is internally set so that an RF burst of 50% modulation triggers the blanker. A resistor in parallel with C15 will increase the detection threshold level. The RF-switching MOSFET (leads 17-18) is controlled by the RF oneshot whose gate time is determined by the value of R15. RF Gate Time (s) = 171 x 10-12 x R15
115 Northeast Cutoff, Box 15036 Worcester, Massachusetts 01615-0036 (508) 853-5000
3845 AM NOISE BLANKER
TYPICAL PULSE RESPONSE
ANTENNA BW = "WIDE" RF 20 kHz IF 12 kHz AUDIO 5 kHz
MIXER
IF LIMITING
0.5 s NOISE PULSE
50 s
500 s
600 s
Dwg. OS-001A
The products described here are manufactured under one or more U.S. patents or U.S. patents pending. Allegro MicroSystems, Inc. reserves the right to make, from time to time, such departures from the detail specifications as may be required to permit improvements in the performance, reliability, or manufacturability of its products. Before placing an order, the user is cautioned to verify that the information being relied upon is current. Allegro products are not authorized for use as critical components in life-support devices or systems without express written approval. The information included herein is believed to be accurate and reliable. However, Allegro MicroSystems, Inc. assumes no responsibility for its use; nor for any infringement of patents or other rights of third parties which may result from its use.
where R15 should be greater than 33 k. Smaller values for C13 will reduce the sensitivity to RF input pulses. The MOSFET turns ON within approximately 1.5 s (shunting the RF signal to ground) after a noise pulse is detected and then turns OFF over a 15 s period after the end of the RF gate time. The ON resistance of the MOSFET is about 30 . The slow turn-OFF prevents any additional transients from being introduced into the receiver by the RF gate. The internal gate circuit also includes charge-balancing circuits so that switching transients are canceled and do not appear at the output. These features ensure transient-free switching even when the RF gate is connected to the low-level input stages of a receiver. Note that the RF gate must be connected to a supply to obtain the minimum ON-resistance of the MOSFET gate. This makes it convenient to connect the RF gate in parallel with the receiver mixer output transformer primary. Blanking in the RF or mixer sections of the receiver removes most of the noise pulse but a small amount still remains due to the hole punched in the carrier. This residual noise is theoretically somewhere between the peak audio and 100% negative modulation but is significantly smaller and narrower than that which the impulse would normally produce without blanking. An audio delay, one-shot, and audio gates are included to eliminate this residual signal.
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3845 AM NOISE BLANKER
TYPICAL APPLICATION
The audio delay is determined by the value of R5: Audio Gate Delay (s) = 157 x 10-12 x R5 where R5 should be greater than 33 k. The amount of delay required will depend on the IF filtering characteristics of the particular receiver design. After the audio delay time, the audio oneshot is triggered. The audio switching MOSFETs (leads 9-10 and leads11-12) are controlled by the audio one-shot whose gate time is determined by the values of R6 and C8: Audio Gate Time (s) = 1.9 x R6 x C8 The MOSFET audio gates also include charge-balancing circuits to eliminate switching transients. A typical application uses the A3845xLW in a C-QUAM(R) AM stereo car-radio tuner with its input from between the RF tuned circuits and the mixer input. Although there is a 1.5 s delay from the beginning of the noise pulse to the start of blanking, this is small compared with the impulse response time of the receiver. It takes almost 10 s for the RF noise burst to reach 70% amplitude at the mixer input. The blanker RF input could be connected to the collector of the discrete RF amplifier, but the bandwidth is much wider there and false triggering from strong adjacent channel signals could occur. The A3845xLW noise blanker can also be used in dual-conversion AM tuners. The blanker RF input would then be connected at the first IF amplifier input and the blanker RF gate connected at the second mixer output. Because the first IF band-width is usually relatively wide, the noise pulses are narrower, and the RF blanking time will be correspondingly less. In this case, it may be necessary to reduce the value of capacitor C13 so that the noise separator does not extend the RF blanking time.
COIL INFORMATION FOR HIGH-PERFORMANCE ETR AM STEREO RECEIVER WITH NOISE BLANKING
Symbol Antenna RF T1 T2 , T 3 T4 T5 L2 100 120 120 2:1 Q N1:N2 1:1.6 10:1 5:1 8.9:1 N1:N3 Toko Part Number 7HN-60064CY RWOS-6A7894AO, L = 178 H 7TRS-A5609AO 7LC-502112N4, CT = 180 pF A7BRS-T1041Z, CT = 1000 pF
TYPICAL RF FREQUENCY RESPONSE
Local Osc. Mixer
0
Detector
RELATIVE SENSITIVITY IN dB
-5
-10
(R) Registered trademark of MOTOROLA, INC.
-15
0.1
1.0
10
RF INPUT FREQUENCY IN MHz
Dwg. GS-006
115 Northeast Cutoff, Box 15036 Worcester, Massachusetts 01615-0036 (508) 853-5000
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ETR AM-STEREO RECEIVER WITH NOISE BLANKING
3845 AM NOISE BLANKER
3845 AM NOISE BLANKER
Dimensions in Inches (for reference only)
20 11 0.0125 0.0091
0.2992 0.2914
0.419 0.394
0.050 0.016 0.020 0.013 1 2 3 0.5118 0.4961 0.050
BSC
0 TO 8
0.0926 0.1043 0.0040 MIN.
Dwg. MA-008-20 in
Dimensions in Millimeters (controlling dimensions)
20 11 0.32 0.23
7.60 7.40
10.65 10.00
1.27 0.40 0.51 0.33 1 2 3 13.00 12.60 1.27
BSC
0 TO 8
2.65 2.35 0.10 MIN.
Dwg. MA-008-20 mm
NOTES: 1. Exact body and lead configuration at vendor's option within limits shown. 2. Lead spacing tolerance is non-cumulative. 3. Supplied in standard sticks/tubes of 37 devices or add "TR" to part number for tape and reel.
115 Northeast Cutoff, Box 15036 Worcester, Massachusetts 01615-0036 (508) 853-5000

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