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 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 -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 OUT
1
AUDIO IN
1
1
2
3
4
5
6
7
8
9
10
NC
DET
V
CC
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 T
A
= +25
°
C
Supply Voltage, V
CC
. . . . . . . . . . . . . .
Package Power Dissipation,
P
D
. . . . . . . . . . . . . . . . . . . . . . .
12 V
APPLICATIONS
I
I
I
I
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
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
V
CC
4
GROUND
16
100 kΩ
10
AUDIO IN 1
SUPPLY
20
REG
1 kΩ
AUDIO DELAY
69 pF
AUDIO BLANK TIME
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
V
CC
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 OUT
1
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 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
Leads
20
20
Limits
Test Conditions
Operating
V
RF
= 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 V
RF
= 0
Noise Pulse Modulation for V
RF
= 1 mV
C
13
= 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
R
15
= 350 kΩ
R
5
= 350 kΩ
R
6
= 110 kΩ, C
8
= 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
V
af
=700 mV, V
noise
= 0
55
—
40
-1.2
—
—
—
80
1.5
60
-0.3
<0.1
100
1.0
_
12
—
0
1.0
—
—
dB
mVpp
dB
dB
%
kΩ
kΩ
www.allegromicro.com
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 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
µs
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)
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
V
CC
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 Out
1
Audio In
1
Audio In
2
Audio Out
2
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 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
µV
at 1 MHz. This means that
a pulsed RF input signal of 15
µV
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 (µs) = 171 x 10
-12
x R
15
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
µs
NOISE
PULSE
50
µs
500
µs
600
µs
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 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.
where R
15
should be greater than 33 kΩ. Smaller values for C
13
will reduce
the sensitivity to RF input pulses. The MOSFET turns ON within approxi-
mately 1.5
µs
(shunting the RF signal to ground) after a noise pulse is
detected and then turns OFF over a 15
µs
period after the end of the RF gate
time. The ON resistance of the MOSFET is about 30
Ω.
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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