A3845: AM Noise Blanker

Data Sheet
27126B*
3845
AM NOISE BLANKER
RF IN
RF BYPASS
VCC
1
NC
2
DET
20
SUPPLY
19
NO
CONNECT
18
RF GATE
LOW
RF BIAS
3
RF AGC
4
17
RF GATE
HIGH
AUDIO DELAY
5
16
GROUND
AUDIO BLANK
TIME (R)
6
15
RF BLANK
TIME
NO
CONNECT
7
14
NO
CONNECT
AUDIO BLANK
TIME (C)
8
13
NOISE
DIFFERENTIATOR
AUDIO OUT1
9
12
AUDIO OUT 2
10
11
AUDIO IN 2
AUDIO IN 1
NC
NC
Dwg. PS-003-1A
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.
T
C
U
D
O
Y
L
R
P ON
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.
D
E
E
C
U
N
N RE
I
T
E
N
F
E
O
R
C
S OR
I
D F
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’).
FEATURES
■
■
■
■
■
■
■
ABSOLUTE MAXIMUM RATINGS
at TA = +25°C
Supply Voltage, VCC . . . . . . . . . . . . . .
Package Power Dissipation,
PD . . . . . . . . . . . . . . . . . . . . . . .
—
12 V
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
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
■
■
■
■
AM and AM-Stereo Automotive Radios
CB Transmitter/Receivers
Short-Wave Receivers
Mobile Communications Equipment
Always order by complete part number:
Part Number
Function
A3845ELW
A3845SLW
Stereo Noise Blanker, Extended Temp. Range
Stereo Noise Blanker, Standard Temp. Range
3845
AM NOISE BLANKER
FUNCTIONAL BLOCK DIAGRAM
MIXER
OUT
RF IN
dV/dt DET
1
3
RF AGC
4
15
VCC
18
RF GATE
LOW
+4 V
16
100 kΩ
100 kΩ
GROUND
13
IF
IN
40 Ω
RF BIAS
69 pF
2
RF BLANK TIME
RF BYPASS
17
RF GATE
HIGH
PEAK
DET
NOISE
DIFFERENTIATOR
RF
10
AUDIO IN 1
REG
SUPPLY 20
7
5
6
AUDIO BLANK TIME
14
NO CONNECTION
69 pF
19
AUDIO DELAY
1 kΩ
9
AUDIO OUT 1
8
11 AUDIO IN 2
1 kΩ
12
AUDIO OUT 2
Dwg. FS-004-1A
TEST CIRCUIT
267 Ω
SUPPLY
0.01
0.01
187 Ω
60.4 Ω
93.1 Ω
RF
0.005
2 kΩ
NOISE
1
VCC
20
2
NC
19
0.1
DET
18
RF BYPASS
4
17
MIXER OUT
5
16
6
15
3
10 µF
0.1
+
R5
R6
R15
7
C8
0.1
AUDIO OUT1
AUDIO IN 1
0.1
NC
NC
14
8
13
9
12
10
0.001
0.1
11
AUDIO OUT2
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.
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.
Characteristic
Test
Leads
Limits
Test Conditions
Min.
Typ.
Max.
Units
Supply Voltage Range
20
Operating
7.5
9.0
12
V
Quiescent Supply Current
20
VRF = 0
—
15
22
mA
Trigger Threshold
1
Noise Pulse Amplitude for VRF = 0
45
100
240
µV
Modulation Threshold
1
Noise Pulse Modulation for VRF = 1 mV
35
75
220
%
Detector Rise Time
13
C13 = 0
—
500
—
ns
RF INPUT AMPLIFIER:
RF SWITCH:
ON Resistance
17-18
—
30
100
Ω
OFF Resistance
17-18
—
100
—
kΩ
Time Delay
1-17
—
1.5
5.0
µs
10-9, 11-12
55
80
_
dB
Noise
9, 12
—
1.5
12
mVpp
Crosstalk
9, 12
40
60
—
dB
10-9, 11-12
-1.2
-0.3
0
dB
—
<0.1
1.0
%
From Beginning of RF Pulse
to Beginning of RF Blanking
AUDIO SWITCHES:
Attenuation
Gain
Total Harmonic Distortion
9, 12
Vaf =700 mV, Vnoise = 0
Input Impedance
10, 11
—
100
—
kΩ
Output Impedance
9, 12
—
1.0
—
kΩ
BLANKING TIMERS:
RF Blanking
17
R15 = 350 kΩ
35
60
75
µs
Audio Delay
9
R5 = 350 kΩ
30
55
67
µs
Audio Blanking
9
R6 = 110 kΩ, C8 = 0.0012 µF
210
250
400
µs
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 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 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 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)
Lead Number
Function
Typical
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
8
Audio BlankTime (C)
4.8
9
Audio Out1
4.75
10
11
Audio In1
Audio In2
4.0
4.75
12
Audio Out2
4.0
13
Noise Differentiator
4.9
14
No Connection
15
RF Blank Time
16
Ground
17
RF Gate High
—
18
RF Gate Low
—
19
No Connection
20
Supply
0
0
4.8
Reference
0
VCC
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 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 (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 (µs) = 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
MIXER
AUDIO
5 kHz
IF LIMITING
0.5 µs
NOISE
PULSE
50 µs
500 µs
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.
www.allegromicro.com
600 µs
Dwg. OS-001A
where R15 should be greater than 33 kΩ. Smaller values for C13 will reduce
the sensitivity to RF input pulses. The MOSFET turns ON within approximately 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.
3845
AM NOISE BLANKER
TYPICAL APPLICATION
The audio delay is determined by the value of
R5:
Audio Gate Delay (µs) = 157 x 10-12 x R5
where R5 should be greater than 33 kΩ. 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 (µs) = 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® AM stereo
car-radio tuner with its input from between the RF tuned circuits and the
mixer input. Although there is a 1.5 µs 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 µs 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
TYPICAL RF FREQUENCY
RESPONSE
RELATIVE SENSITIVITY IN dB
0
Q
T1
N1:N2
N1:N3
1:1.6
7HN-60064CY
T2 , T 3
120
10:1
RWOS-6A7894AO,
L = 178 µH
Local Osc.
T4
120
5:1
7TRS-A5609AO
Mixer
T5
8.9:1
7LC-502112N4,
CT = 180 pF
Detector
L2
2:1
100
-5
® Registered trademark of MOTOROLA, INC.
-10
-15
0.1
1.0
Toko Part Number
10
RF INPUT FREQUENCY IN MHz
Dwg. GS-006
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
A7BRS-T1041Z,
CT = 1000 pF
ETR AM-STEREO RECEIVER WITH NOISE BLANKING
3845
AM NOISE BLANKER
www.allegromicro.com
3845
AM NOISE BLANKER
Dimensions in Inches
(for reference only)
20
11
0.0125
0.0091
0.419
0.394
0.2992
0.2914
0.050
0.016
0.020
0.013
1
2
0.050
3
0.5118
0.4961
0° TO 8°
BSC
0.0926
0.1043
Dwg. MA-008-20 in
0.0040 MIN.
Dimensions in Millimeters
(controlling dimensions)
20
11
0.32
0.23
10.65
10.00
7.60
7.40
1.27
0.40
0.51
0.33
1
2
1.27
3
13.00
12.60
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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