Noise Blanker Circuit - Kenwood TK-80 Service Manual

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The first IF signal of 73.045MHz passes through the MCF
(XF1), is amplified by the IF amplifier Q10, and mixed with the
62.35MHz LO2 signal by the second mixer, Q11 and Q12, to
produce a second IF signal of 10.695MHz.
The second IF signal of 10.695MHz is split into two. One
signal goes to the NB amplifier and the other passes through
the NB gate FET Q19. The signal then goes to the IF filter.
There are four types of IF filters: 6kHz, 2.7kHz, 2.2kHz, and
500Hz (2.7kHz and 500Hz are optional). The signal passing
through the IF filter is amplified Q21 and Q30, Q31 SSB, CW,
FSK modes are product-detected in D52, D53 and AM mode
envelope-detected in D54 and condenser.
After detection, the AF signal for each mode passes
through analog switch IC3 and goes to AF preamplifier Q48.
After the preamplifier, the signal passes through the mute
circuit Q49 has the volume controlled at IC6 and is amplified
to the necessary electric power level at AF power amplifier
IC7.
2nd IF
W1
Q801
TP801
Q800
NBS
CIRCUIT DESCRIPTION
Q19
Q18
TH1
L802
L803
Q802
Q804
Noise
AMP
Q803
+
+
Fig. 5 Noise blanker circuit
3-1. Receiver front-end
The signal input to the TX-RX unit passes through the 30
MHz low-pass filter, and signal above 1.605MHz goes to
seven band-pass filters. When AIP is off, the signal passes
through each band-pass filter, D25 and D26 turn on and D23
and D24 turn off, and the signal is amplified by about 13dB by
Q8, Q9 and output to the first mixer. If AIP is on, D25 and
D26 turn off and D23 and D24 turn on, and the signal is out-
put directly to the first mixer without passing through Q8 and
Q9. The first mixer is a quad balanced mixer, Q4 to Q7 (Fig.
4).

3-2. Noise blanker circuit

The 10.695MHz IF signal generated from the first IF of
73.045MHz by the second mixer is input to the IF amplifier
Q19, sent through Q18 amplified by noise amplifier Q801,
Q802 and Q804, sent through buffer Q806 and noise-de-
tected by D800. This signal switches Q805 and Q807, Q808
and controls Q19 in the TX-RX unit. Q19 controls Q20 and
blanks the noise (Fig. 5).
L51
Q20
SW
Q808
+
Q806
D800
Q807
Q805
TK-80
RXB
AGC
to IF filter
8F
RBK
8A
13

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