3845 查看數據表（PDF） - Allegro MicroSystems
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3845 Datasheet PDF : 8 Pages
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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 modulation. 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 sup-
press 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 dynamic 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 objection-
able.
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 blank-
ing time is needed most.
The A3845xLW takes a different ap-
proach 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 ampli-
fier 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.
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
VCC
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 impulse noise including that from
ignition systems and from sources producing 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 trigger-
ing 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
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
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