3845 Просмотр технического описания (PDF) - Allegro MicroSystems
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3845 Datasheet PDF : 8 Pages
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3845
AM NOISE BLANKER
TYPICAL PULSE RESPONSE
ANTENNA
BW = "WIDE"
RF
20 kHz
MIXER
IF
12 kHz
AUDIO
5 kHz
IF LIMITING
0.5 µs
NOISE
PULSE
50 µs
500 µs
600 µs
Dwg. OS-001A
where R15 should be greater than 33 kΩ. Smaller values for C13 will
reduce the sensitivity to RF input pulses. The MOSFET turns ON
within approximately 1.5 µs (shunting the RF signal to ground) after a
noise pulse is detected and then turns OFF over a 15 µs period after
the end of the RF gate time. The ON resistance of the MOSFET is
about 30 Ω. 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 signifi-
cantly smaller and narrower than that which the impulse would nor-
mally produce without blanking. An audio delay, one-shot, and audio
gates are included to eliminate this residual signal.
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