3845

AM NOISE BLANKER

115 Northeast Cutoff, Box 15036

Worcester, Massachusetts 01615-0036 (508) 853-5000

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)

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

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

level.

The RF-switching MOSFET (leads 17-18) is controlled by the RF one-

RF Gate Time (

15

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

6

Audio Blank Time (R)

4.8

7

No Connection

0

8

Audio BlankTime (C)

4.8

9

4.75

10

4.0

11

4.75

12

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

Typical