Elecraft KNB2 Manual - Manual (Page 8)

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Circuit Details
U1 amplifies the I.F. signal, which is then detected by D2 (1N34A). Q1 reduces the gain of U1 in proportion to overall
signal strength, keeping the signal at D2 relatively constant. Negative-going pulses that are above the threshold
established by D3 and R3 turn Q2 on, triggering the one-shot (Q3/Q4). Q6 can be used to switch in a larger-value
capacitor, increasing the blanking time. The pulse output of the one-shot (GATE) saturates Q5, attenuating the signal at
the output of the band-pass filter. The band-pass filter creates a small delay between input and output, so that the GATE
signal arrives just ahead of the noise pulses. The controller, U3, decodes auxBus commands from the main processor. It
then controls power to the blanker (via Q7), pulse width (via Q6), and blanking threshold (via R3).
= On bottom of PC board.
KNB2 Noise Blanker
Band-Pass Filter
To RF Board, J12
0V = NARROW
PULSE WIDTH
W. Burdick
Noise Gate
E. Swartz
Pulse Amp
0V = HIGH
6V = WIDE
6V Switch
THRESHOLD
Elecraft
One-shot
6V = LOW
6V = OFF
PN2222A
0V = ON
2 N 3 9 0 6 ,
PN2222A
PN2222A
MC1350
1 of 1
AUXBUS
RF Amp
RF OUT 2 N 3 9 0 6
2 N 3 9 0 6
2 N 3 9 0 6
2 N 3 9 0 6
2 N 7 0 0 0
12C509A
40 µH
2 N 7 0 0 0
7 8 L 0 6
Rev. Sht.
1N34A
1N34A
7 8 L 0 6
1 0 0
Date
1.2 µH
. 0 0 1
3 9
. 0 0 6 8
. 0 0 1
2 2 µ F
3.9K
1 0 0 K 3.9K
1.8K
3.9K
3.9K
3.9K
3.9K
3.9K
2.7K
1.2 µH
3.9K
GATE
GATE
C10
AGC
1 8 0 0
0 . 1
OUT
GP5
GP4
VDD
GP3 GP2
GP1
GP0
VSS
RP1
RP1
RP2
RP2
RP1
C15
12V
. 0 0 1
C13
. 0 1
C14
6NB
6NB
6NB
6NB
RP2
C16
RP2
C12
ALC
C11 2 2 0
2 2 0
RP1
GND OUT
2 2 0
. 0 1
1 5 0 0
IN
U1
C7
C4 C6
U2
B y
U3
Q6
Q1
Q2
Q4
Q3
L 3
R1
R4
R6
P1
L 1 L 2 Q7
C9
R7
R3
D2
6V
6V
D3
IN
R5
Q5
C8
1 2 3 4
5 8 6 7
5
6
7
8 1
2
3
4
1 2 3 4 5 6 7 8
+
2
1
S D
G
3 4 6
5
4
3
2
1
5 6
7
8
7
8
S
G D E B C
MC1350, 12C509A
1
2
3
4 5
6
7
8
Controller
1 M
R10
3 0 0
R9
1 2 0
R8
8 2 0
C3
. 0 1
C17
(-6 dB)
D 8 - 1 9 - 0 2