3845, katalog

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3845
AM NOISE BLANKER
RF IN
1
V
CC
20
SUPPLY
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
RF BYPASS
2
NC
19
NO
CONNECT
RF BIAS
3
DET
18
RF GATE
LOW
RF AGC
4
17
RF GATE
HIGH
AUDIO DELAY
5
16
GROUND
AUDIO BLANK
TIME (R)
6
15
RF BLANK
TIME
NO
CONNECT
7
NC
NC
14
NO
CONNECT
AUDIO BLANK
TIME (C)
8
13
NOISE
DIFFERENTIATOR
AUDIO OUT
1
9
12
AUDIO OUT
2
AUDIO IN
1
10
11
AUDIO IN
2
Dwg. PS-003-1A
°
C to
+85
C (suffix ‘SLW’) or an extended temperature range to -40
°
C
(suffix ‘ELW’).
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
APPLICATIONS
ABSOLUTE MAXIMUM RATINGS
at T
A
= +25
°
C
AM and AM-Stereo Automotive Radios
CB Transmitter/Receivers
Supply Voltage, V
CC
. . . . . . . . . . . . . .
12 V
Package Power Dissipation,
P
D
. . . . . . . . . . . . . . . . . . . . . . .
1.78 W
Operating Temperature Range, T
A
Suffix ‘ELW’ . . . . . . . .
-40
Short-Wave Receivers
Mobile Communications Equipment
°
C to +85
°
C
Always order by complete part number:
Suffix ‘SLW’ . . . . . . . .
-20
°
C to +85
°
C
Part Number
Function
Storage Temperature Range,
T
S
. . . . . . . . . . . . . . . .
-55
°
C to +125
°
C
A3845ELW
Stereo Noise Blanker, Extended Temp. Range
A3845SLW
Stereo Noise Blanker, Standard Temp. Range
°
FEATURES
3845
AM NOISE BLANKER
FUNCTIONAL BLOCK DIAGRAM
MIXER
OUT
RF IN
dV
/dt DET
17
1
RF
PEAK
DET
RF GATE
HIGH
IF
IN
RF BYPASS
2
RF BIAS
13
15
3
18
V
RF AGC
CC
RF GATE
LOW
+4 V
4
GROUND
16
10
AUDIO IN
1
SUPPLY
20
REG
1 k
Ω
9
AUDIO OUT
1
19
14
7
5
6
8
NO CONNECTION
11
AUDIO IN
2
1 k
Ω
12
AUDIO OUT
2
Dwg. FS-004-1A
TEST CIRCUIT
N
OIS
E
0.005
SUPPLY
R
F
0.01
0.01
1
CC
20
2
NC
19
0.1
3
DET
18
RF BYPASS
0.1
10
µ
F
+
4
17
MIXER OUT
R
5
5
16
R
6
R
15
6
15
7
NC
NC
14
C
8
8
13
0.001
AUDIO OUT
1
0.1
9
12
0.1
AUDIO OUT
2
AUDIO IN
1
0.1
10
11
0.1
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.
V
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
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
µ
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
Ω
OFF Resistance
17-18
—
100
—
k
Ω
Time Delay
1-17
From Beginning of RF Pulse
—
1.5
5.0
µ
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
Ω
Output Impedance
9, 12
—
1.0
—
k
Ω
BLANKING TIMERS:
RF Blanking
17
R
15
= 350 k
Ω
35
60
75
µ
s
Audio Delay
9
R
5
= 350 k
Ω
30
55
67
µ
s
Audio Blanking
9
R
6
= 110 k
Ω
, C
8
= 0.0012
µ
F
210
250
400
µ
s
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.
QUIESCENT DC VOLTAGES
(for circuit design information only)
Typical
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
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
6
Audio Blank Time (R)
4.8
7
No Connection
0
8
Audio BlankTime (C)
4.8
9
Audio Out
1
4.75
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.
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
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.
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.
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.
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
µ
V at 1 MHz. This means that
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.
µ
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
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 (
µ
s) = 171 x 10
-12
x R
15
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
µ
a pulsed RF input signal of 15
 3845
AM NOISE BLANKER
TYPICAL PULSE RESPONSE
ANTENNA
BW
= "W
IDE"
RF
20 kHz
MIXER
IF
12 kHz
AUDIO
5 kHz
IF LIMITING
0.5
µ
s
NOISE
PULSE
50
µ
s
500
µ
s
600
µ
s
Dwg. OS-001A
. Smaller values for C
13
will reduce
the sensitivity to RF input pulses. The MOSFET turns ON within approxi-
mately 1.5
Ω
s (shunting the RF signal to ground) after a noise pulse is
detected and then turns OFF over a 15
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.
s period after the end of the RF gate
time. The ON resistance of the MOSFET is about 30
µ
. 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.
Ω
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.
www.allegromicro.com
where R
15
should be greater than 33 k
µ
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