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 frequen-
cies (or a first IF) to 30 MHz. The A3845ELW and A3845SLW
feature dual audio channels and are intended for AM-stereo or indepen-
dent 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 re-
moved by an audio sample-and-hold gate. The RF blanking time,
audio gate delay time, and audio gate blanking time can all be indepen-
dently 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 -40C
(suffix `ELW').
AM NOISE BLANKER
Data Sheet
27126B*
ABSOLUTE MAXIMUM RATINGS
at T
A
= +25
C
Supply Voltage, V
CC
. . . . . . . . . . . . . . 12 V
Package Power Dissipation,
P
D
. . . . . . . . . . . . . . . . . . . . . . . 1.78 W
Operating Temperature Range, T
A
Suffix `ELW' . . . . . . . . -40
C to +85
C
Suffix `SLW' . . . . . . . . -20
C to +85
C
Storage Temperature Range,
T
S
. . . . . . . . . . . . . . . . -55
C to +125
C
FEATURES
I RF Blanking to 30 MHz
I Single-Channel or Stereo Audio Blanking
I Adjustable RF and Audio Blanking Time
I Adjustable Audio Blanking Delay
I Sample-and-Hold MOS Audio Gates
I Internal Voltage Regulation
I Minimum External Components
APPLICATIONS
I AM and AM-Stereo Automotive Radios
I CB Transmitter/Receivers
I Short-Wave Receivers
I Mobile Communications Equipment
3845
Always order by complete part number:
Part Number
Function
A3845ELW
Stereo Noise Blanker, Extended Temp. Range
A3845SLW
Stereo Noise Blanker, Standard Temp. Range
GROUND
13
14
15
16
17
19
12
18
20
11
1
2
3
8
9
4
5
6
7
10
RF AGC
SUPPLY
CC
V
AUDIO DELAY
RF BIAS
AUDIO BLANK
TIME (R)
AUDIO BLANK
TIME (C)
RF BLANK
TIME
AUDIO OUT
1
NO
CONNECT
NO
CONNECT
NC
NC
Dwg. PS-003-1A
RF GATE
LOW
RF IN
RF BYPASS
AUDIO IN
1
AUDIO OUT
2
AUDIO IN
2
NO
CONNECT
NC
RF GATE
HIGH
DET
NOISE
DIFFERENTIATOR
3845
AM NOISE BLANKER
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
5
15
CC
V
SUPPLY
AUDIO OUT 1
Dwg. FS-004-1A
RF BYPASS
AUDIO IN 1
AUDIO OUT 2
AUDIO IN 2
MIXER
OUT
6
8
17
18
PEAK
DET
REG
1
4
16
3
2
20
10
12
11
9
GROUND
RF AGC
AUDIO DELAY
RF BIAS
AUDIO BLANK TIME
RF BLANK TIME
RF GATE
LOW
NOISE
DIFFERENTIATOR
RF IN
RF GATE
HIGH
RF
13
69 pF
69 pF
dV/dt DET
+4 V
100 k
100 k
IF
IN
40
19
14
7
NO CONNECTION
1 k
1 k
Copyright 1988, 2000 Allegro MicroSystems, Inc.
Note that the noise-pulse input is attenuated 20 dB by the test circuit.
AUDIO OUT
1
AUDIO IN 1
AUDIO OUT2
AUDIO IN2
0.1
0.1
0.1
0.1
0.001
C 8
SUPPLY
Dwg. ES-007-1A
RF
RF BYPASS
0.1
2 k
MIXER OUT
NOISE
+
R15
R 5
R 6
10
F
0.1
267
187
60.4
93.1
0.005
0.01
0.01
13
14
15
16
17
19
12
18
20
11
1
2
3
8
9
4
5
6
7
10
CC
V
NC
NC
NC
DET
FUNCTIONAL BLOCK DIAGRAM
TEST CIRCUIT
3845
AM NOISE BLANKER
www.allegromicro.com
Limits
Characteristic
Leads
Test Conditions
Min.
Typ.
Max.
Units
Supply Voltage Range
20
Operating
7.5
9.0
12
V
Quiescent Supply Current
20
V
RF
= 0
--
15
22
mA
RF INPUT AMPLIFIER:
Trigger Threshold
1
Noise Pulse Amplitude for V
RF
= 0
45
100
240
V
Modulation Threshold
1
Noise Pulse Modulation for V
RF
= 1 mV
35
75
220
%
Detector Rise Time
13
C
13
= 0
--
500
--
ns
RF SWITCH:
ON Resistance
17-18
--
30
100
OFF Resistance
17-18
--
100
--
k
Time Delay
1-17
From Beginning of RF Pulse
--
1.5
5.0
s
to Beginning of RF Blanking
AUDIO SWITCHES:
Attenuation
10-9, 11-12
55
80
_
dB
Noise
9, 12
--
1.5
12
mVpp
Crosstalk
9, 12
40
60
--
dB
Gain
10-9, 11-12
-1.2
-0.3
0
dB
Total Harmonic Distortion
9, 12
V
af
=700 mV, V
noise
= 0
--
<0.1
1.0
%
Input Impedance
10, 11
--
100
--
k
Output Impedance
9, 12
--
1.0
--
k
BLANKING TIMERS:
RF Blanking
17
R
15
= 350 k
35
60
75
s
Audio Delay
9
R
5
= 350 k
30
55
67
s
Audio Blanking
9
R
6
= 110 k
, C
8
= 0.0012
F
210
250
400
s
ELECTRICAL CHARACTERISTICS over operating temperature range, at V
CC
= 7.5 V to
11 V, f
rf
= 1 MHz, Noise (f
noise
) = 500 Hz Square Wave, f
af
= 1 kHz, Test Figure.
Test
3845
AM NOISE BLANKER
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
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 modula-
tion. 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 dy-
namic 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)
impulse noise including that from ignition systems and from sources produc-
ing 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 (C
13
) 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 C
15
will increase the detection threshold
level.
The RF-switching MOSFET (leads 17-18) is controlled by the RF one-
shot whose gate time is determined by the value of R
15
.
RF Gate Time (
s) = 171 x 10
-12
x R
15
Lead Number
Function
DC Voltage
1
RF In
3.1
2
RF Bypass
3.1
3
RFBias
3.1
4
RF AGC
0.9
5
Audio Delay
4.8
6
Audio Blank Time (R)
4.8
7
No Connection
0
8
Audio BlankTime (C)
4.8
9
Audio Out
1
4.75
10
Audio In
1
4.0
11
Audio In
2
4.75
12
Audio Out
2
4.0
13
Noise Differentiator
4.9
14
No Connection
0
15
RF Blank Time
4.8
16
Ground
Reference
17
RF Gate High
--
18
RF Gate Low
--
19
No Connection
0
20
Supply
V
CC
Typical
3845
AM NOISE BLANKER
www.allegromicro.com
TYPICAL PULSE RESPONSE
where R
15
should be greater than 33 k
. Smaller values for C
13
will reduce
the sensitivity to RF input pulses. The MOSFET turns ON within approxi-
mately 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.
Dwg. OS-001A
0.5
s
NOISE
PULSE
50
s
500
s
600
s
IF LIMITING
IF
12 kHz
ANTENNA
BW = "WIDE"
RF
20 kHz
MIXER
AUDIO
5 kHz
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 informa-
tion 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.
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