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A3845ELW Schematic ( PDF Datasheet ) - Allegro MicroSystems

Teilenummer A3845ELW
Beschreibung AM NOISE BLANKER
Hersteller Allegro MicroSystems
Logo Allegro MicroSystems Logo 




Gesamt 8 Seiten
A3845ELW Datasheet, Funktion
3845
AM NOISE BLANKER
RF IN 1
RF BYPASS 2
RF BIAS 3
RF AGC 4
AUDIO DELAY 5
AUDIO BLANK
TIME (R)
NO
CONNECT
AUDIO BLANK
TIME (C)
6
7
8
AUDIO OUT1 9
NC
AUDIO IN1 10
DET
VCC 20 SUPPLY
NC
19
NO
CONNECT
RF GATE
18 LOW
17 RF GATE
HIGH
16 GROUND
15 RF BLANK
TIME
NC 14 NO
CONNECT
13 NOISE
DIFFERENTIATOR
12 AUDIO OUT2
11 AUDIO IN 2
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 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’).
ABSOLUTE MAXIMUM RATINGS
at TA = +25°C
Supply Voltage, VCC . . . . . . . . . . . . . . 12 V
Package Power Dissipation,
PD . . . . . . . . . . . . . . . . . . . . . . . 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
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
Always order by complete part number:
Part Number Function
A3845ELW
A3845SLW
Stereo Noise Blanker, Extended Temp. Range
Stereo Noise Blanker, Standard Temp. Range






A3845ELW Datasheet, Funktion
3845
AM NOISE BLANKER
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 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 one-
shot 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.
TYPICAL APPLICATION
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 re-
sponse 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 nar-
rower, 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
TYPICAL RF FREQUENCY
RESPONSE
0
-5
Antenna
RF
Symbol
T1
T2, T3
Q
120
Local Osc.
Mixer
T4
T5
120
Detector
L2 100
N1:N2
1:1.6
N1:N3
10:1
5:1
2:1 8.9:1
Toko Part Number
7HN-60064CY
RWOS-6A7894AO,
L = 178 µH
7TRS-A5609AO
7LC-502112N4,
CT = 180 pF
A7BRS-T1041Z,
CT = 1000 pF
® Registered trademark of MOTOROLA, INC.
-10
-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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