3845 Datasheet, PDF (4/8 Pages) Allegro MicroSystems – AM NOISE BLANKER

3845
AM NOISE BLANKER
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)
Lead Number
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
Function
RF In
RF Bypass
RFBias
RF AGC
Audio Delay
Audio Blank Time (R)
No Connection
Audio BlankTime (C)
Audio Out₁
Audio In₁
Audio In₂
Audio Out₂
Noise Differentiator
No Connection
RF Blank Time
Ground
RF Gate High
RF Gate Low
No Connection
Supply
Typical
DC Voltage
3.1
3.1
3.1
0.9
4.8
4.8
0
4.8
4.75
4.0
4.75
4.0
4.9
0
4.8
Reference
—
—
0
V_CC
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
blanker. The external capacitor at the dV/dt detector circuit (C₁₃) is selected
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
blanker. A resistor in parallel with C₁₅ will increase the detection threshold
level.
The RF-switching MOSFET (leads 17-18) is controlled by the RF one-
shot whose gate time is determined by the value of R₁₅.
RF Gate Time (µs) = 171 x 10^-12 x R₁₅
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