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3845
AM NOISE BLANKER
www.allegromicro.com
TYPICAL PULSE RESPONSE
where R
15
should be greater than 33 k
. Smaller values for C
13
will reduce
the sensitivity to RF input pulses. The MOSFET turns ON within approxi-
mately 1.5
μ
s (shunting the RF signal to ground) after a noise pulse is
detected and then turns OFF over a 15
μ
s period after the end of the RF gate
time. The ON resistance of the MOSFET is about 30
. The slow turn-OFF
prevents any additional transients from being introduced into the receiver by
the RF gate. The internal gate circuit also includes charge-balancing circuits
so that switching transients are canceled and do not appear at the output.
These features ensure transient-free switching even when the RF gate is
connected to the low-level input stages of a receiver. Note that the RF gate
must be connected to a supply to obtain the minimum ON-resistance of the
MOSFET gate. This makes it convenient to connect the RF gate in parallel
with the receiver mixer output transformer primary.
Blanking in the RF or mixer sections of the receiver removes most of the
noise pulse but a small amount still remains due to the hole punched in the
carrier. This residual noise is theoretically somewhere between the peak
audio and 100% negative modulation but is significantly smaller and narrower
than that which the impulse would normally produce without blanking. An
audio delay, one-shot, and audio gates are included to eliminate this residual
signal.
Dwg. OS-001A
0.5
μ
s
NOISE
PULSE
50
μ
s
500
μ
s
600
μ
s
IF LIMITING
IF
12 kHz
ANTENNA
BW = "WIDE"
RF
20 kHz
MIXER
AUDIO
5 kHz
The products described here are manufactured
under one or more U.S. patents or U.S. patents
pending.
Allegro MicroSystems, Inc. reserves the right to
make, from time to time, such departures from the
detail specifications as may be required to permit
improvements in the performance, reliability, or
manufacturability of its products. Before placing an
order, the user is cautioned to verify that the informa-
tion being relied upon is current.
Allegro products are not authorized for use as
critical components in life-support devices or systems
without express written approval.
The information included herein is believed to be
accurate and reliable. However, Allegro
MicroSystems, Inc. assumes no responsibility for its
use; nor for any infringement of patents or other rights
of third parties which may result from its use.