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3845
AM NOISE BLANKER
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
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
μ
s delay from the beginning of the noise
pulse to the start of blanking, this is small compared with the impulse re-
sponse time of the receiver. It takes almost 10
μ
s 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 nar-
rower, and the RF blanking time will be correspondingly less. In this case, it
may be necessary to reduce the value of capacitor C
13
so that the noise
separator does not extend the RF blanking time.
COIL INFORMATION FOR HIGH-PERFORMANCE ETR
AM STEREO RECEIVER WITH
NOISE BLANKING
Symbol
Q
N1:N2
N1:N3
Toko Part Number
Antenna
T
1
1:1.6
7HN-60064CY
RF
T
2
, T
3
120
10:1
RWOS-6A7894AO,
L = 178
μ
H
Local Osc.
T
4
120
5:1
7TRS-A5609AO
Mixer
T
5
2:1
8.9:1
7LC-502112N4,
C
T
= 180 pF
Detector
L
2
100
A7BRS-T1041Z,
C
T
= 1000 pF
0
-5
-10
-15
0.1
1.0
10
RF INPUT FREQUENCY IN MHz
R
Dwg. GS-006
TYPICAL RF FREQUENCY
RESPONSE
The audio delay is determined by the value of
R
5
:
Audio Gate Delay (
μ
s) = 157 x 10
-12
x R
5
where R
5
should be greater than 33 k
. 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 R
6
and C
8
:
Audio Gate Time (
μ
s) = 1.9 x R
6
x C
8
The MOSFET audio gates also include
charge-balancing circuits to eliminate switching
transients.
Registered trademark of MOTOROLA, INC.