HA12181FP Datasheet, PDF (9/21 Pages) Hitachi Semiconductor – AM Radio Noise Reduction System

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
Noiseless
Point D
V_B
Point C
V_A
T₁
T₂
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, Jun. 1997, page 9 of 21