HA12181FP Просмотр технического описания (PDF) - Renesas Electronics

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HA12181FP

AM Radio Noise Reduction System

Renesas Electronics 

HA12181FP

A system block diagram of AM Radio using the HA12181FP is shown in Figure 2 and waveforms at each point in the

system are illustrated in Figure 3. For AM wave with impulse noise from ANT, the pulse spreads its width each time

when the AM wave passes through a selection filter.

The pulse width becomes the order of several hundred microseconds at detector output (Point C).

A radio without a noise canceller produces large noise to the audience. This IC perfectly detects every noise by using

the signals from 1st IFT (Point B) in front of the narrow band filter.

The wave process circuit approximates the voltage linearly at the pulse to reduce the noise in the output.

The principle for wave processing follows. Further investigation make it clear that the pulse width of impulse noise is

constant (several handred microseconds) and independent of the waveform or waveheight.

Therefore the former and later voltage (VA, VB) of the pulse can be found at the same time (T1) by means of the wave

and the delayed one for this time, as shown in the right figure.

Each Point

in the Figure

Waveform including Noise

A

Narrower Pulse Width

and Higher Wave Height

B

C

Wider Pulse

Width and Lower Wave Height

D

Point D

VB

Point C

VA

T1

T2

Noiseless

Figure 3 Waveforms at Each Point in the System

In an actual circuit, the differential voltage between input and output of phase shift circuit is changed to the capacitor

C511 at pin 6.

At the time of T1, when the switch turns to the noise processing mode (the switch positions in Figure 4 are inverted),

the voltage difference (VA – VB) is held in C511.

C509 at pin 10 is changed by the differential voltage between the held voltage and the output voltage at pin 9 (VA):

VA – (VA – VB) = VB.

Rev.2.00 Jun 15, 2005 page 8 of 19

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