Allegro A3845SLW, A3845ELW Datasheet

RF IN

RF BYPASS

RF BIAS

RF AGC

AUDIO DELAY

AUDIO BLANK

TIME (R)

NO

CONNECT

AUDIO BLANK

TIME (C)

AUDIO OUT

AUDIO IN

Data Sheet

27126B*

3845

AM NOISE BLANKER

This noise blanker integrated circuit contains all of the necessary

circuitry for adding an extremely efficient (patented) noise-blanking

CC

NC

NC

SUPPLY

20

NO

19

CONNECT

RF GATE

18

LOW

RF GATE

17

HIGH

GROUND

16

RF BLANK

15

TIME

NO

14

CONNECT

NOISE

13

DIFFERENTIATOR

AUDIO OUT

12

11

AUDIO IN

2

2

Dwg. PS-003-1A

1

2

3

4

5

6

NC

7

8

9

1

10

1

V

DET

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.

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.

ABSOLUTE MAXIMUM RATINGS

at TA = +25°C

Supply Voltage, VCC. . . . . . . . . . . . . . 12 V

Package Power Dissipation,

. . . . . . . . . . . . . . . . . . . . . . . 1.78 W

P

D

Operating Temperature Range, T

Suffix ‘ELW’ . . . . . . . . -40°C to +85°C

Suffix ‘SLW’ . . . . . . . . -20

Storage Temperature Range,

. . . . . . . . . . . . . . . . -55°C to +125°C

T

S

A

°C to +85°C

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

■ 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 Stereo Noise Blanker, Extended Temp. Range

A3845SLW Stereo Noise Blanker, Standard Temp. Range

3845

AM NOISE BLANKER

RF IN

RF

RF BYPASS

SUPPLY

1

2

RF BIAS

3

RF AGC

4

GROUND

16

20

19 14 7

NO CONNECTION

FUNCTIONAL BLOCK DIAGRAM

dV/dt DET

PEAK

DET

13

NOISE

DIFFERENTIATOR

REG

5

69 pF

AUDIO DELAY

6 8

15

RF BLANK TIME

+4 V

100 kΩ

1 kΩ

69 pF

MIXER

100 kΩ

OUT

17

RF GATE

HIGH

40 Ω

18

RF GATE

LOW

AUDIO IN

10

AUDIO OUT

9

AUDIO IN

11

IF
IN

V

CC

1

1

2

NOISE

RF

0.005

60.4 Ω

0.01 0.01

93.1 Ω

187 Ω

AUDIO OUT

AUDIO IN

AUDIO BLANK TIME

1 kΩ

12

AUDIO OUT

2

Dwg. FS-004-1A

TEST CIRCUIT

SUPPLY

267 Ω

V

1

10 µF

+

R

5

R

6

C

8

0.1

2

0.1

3

4

5

6

NC

7

8

9

10

DET

2 kΩ

0.1

1

0.1

1

20

CC

NC

19

R

15

0.001

0.1

RF BYPASS

MIXER OUT

AUDIO OUT

AUDIO IN

Dwg. ES-007-1A

2

2

18

17

16

15

NC

14

13

0.1

12

11

Note that the noise-pulse input is attenuated 20 dB by the test circuit.

115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 [phone]
Copyright © 1988, 2000 Allegro MicroSystems, Inc.

AM NOISE BLANKER

ELECTRICAL CHARACTERISTICS over operating temperature range, at VCC = 7.5 V to
11 V, frf = 1 MHz, Noise (f

) = 500 Hz Square Wave, faf = 1 kHz, Test Figure.

noise

3845

Test

Characteristic Leads 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

RF INPUT AMPLIFIER:

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 SWITCH:

ON Resistance 17-18 — 30 100 Ω

OFF Resistance 17-18 — 100 — kΩ

Time Delay 1-17

AUDIO SWITCHES:

Attenuation 10-9, 11-12 55 80 _ dB

Noise 9, 12 — 1.5 12 mVpp

From Beginning of RF Pulse
to Beginning of RF Blanking

— 1.5 5.0 µs

Limits

Crosstalk 9, 12 40 60 — dB

Gain 10-9, 11-12 -1.2 -0.3 0 dB

Total Harmonic Distortion 9, 12 Vaf =700 mV, V

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

= 0 — <0.1 1.0 %

noise

www.allegromicro.com