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
The audio delay is determined by the
value of R5:
Audio Gate Delay (µs) = 157 x 10-12 x R5
where R5 should be greater than 33 kΩ. The
amount of delay required will depend on the
IF filtering characteristics of the particular
receiver design. After the audio delay time,
the audio one-shot is triggered. The audio
switching MOSFETs (leads 9-10 and
leads11-12) are controlled by the audio one-
shot whose gate time is determined by the
values of R6 and C8:
Audio Gate Time (µs) = 1.9 x R6 x C8
The MOSFET audio gates also include
charge-balancing circuits to eliminate switch-
ing transients.
TYPICAL APPLICATION
A typical application uses the A3845xLW in a C-QUAM® AM
stereo car-radio tuner with its input from between the RF tuned circuits
and the mixer input. Although there is a 1.5 µs delay from the begin-
ning of the noise pulse to the start of blanking, this is small compared
with the impulse response time of the receiver. It takes almost 10 µs
for the RF noise burst to reach 70% amplitude at the mixer input. The
blanker RF input could be connected to the collector of the discrete RF
amplifier, but the bandwidth is much wider there and false triggering
from strong adjacent channel signals could occur.
The A3845xLW noise blanker can also be used in dual-conversion
AM tuners. The blanker RF input would then be connected at the first
IF amplifier input and the blanker RF gate connected at the second
mixer output. Because the first IF band-width is usually relatively wide,
the noise pulses are narrower, and the RF blanking time will be
correspondingly less. In this case, it may be necessary to reduce the
value of capacitor C13 so that the noise separator does not extend the
RF blanking time.
COIL INFORMATION FOR HIGH-PERFORMANCE ETR
AM STEREO RECEIVER WITH
NOISE BLANKING
TYPICAL RF FREQUENCY
RESPONSE
0
-5
Antenna
RF
Symbol Q
T1
T2, T3 120
Local Osc.
T4
120
Mixer
T5
Detector
L2
100
N1:N2
1:1.6
N1:N3
10:1
5:1
2:1
8.9:1
Toko Part Number
7HN-60064CY
RWOS-6A7894AO,
L = 178 µH
7TRS-A5609AO
7LC-502112N4,
CT = 180 pF
A7BRS-T1041Z,
CT = 1000 pF
® Registered trademark of MOTOROLA, INC.
-10
-15
0.1
1.0
10
RF INPUT FREQUENCY IN MHz
Dwg. GS-006
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
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