Noise Blanker Circuit (Nb Circuit); Transmitter Circuit; Blanking Circuit; From Transmission Signal (Tif) To Drive Output - Kenwood TS-590S Service Manual

Noise Blanker Circuit (NB Circuit)

■ Noise detection circuit
The Noise detecting circuit is the circuit that removes
short frequency pulse noise.
The 10.695MHz or 11.374MHz IF signal passes through
the buffer (Q641) in the TX-RX unit (X57-785 A/2), and is
sent from CN674 to the NB unit (X57-785 B/2) CN667 (NB
IF) through a co-axial cable.
The signal input into the NB unit (X57-785 B/2) is ampli-
fied by the 4 stage amplifier (Q651, Q652, Q653, Q663,
Q664, Q666), passes through the buffer (Q670), and then
noise-detected by D662. This noise-detected voltage
switches Q669, turns Q671 ON, attenuates the IF signal in
the blanking circuit (D461 to D464) depending on the noise,
and removes the pulse noise.
When long-frequency signal is input, the AGC functions
X57-785 (A/2) X57-785 (B/2)
Q641
Buffer
IF input
10.695MHz/
11.374MHz
Q651
NB Amp1

■ Blanking circuit

The Noise blanking circuit is a circuit which attenuates
the IF signal by the signal detected in the noise detection
circuit and removes pulse noise.
If pulse noise is not detected in the NB unit, BLNK turns
to "H", Q456 turns to ON and Q455 turns to OFF. Also, di-
odes D461 to D464 turn OFF and the IF signal is attenuated.
+8V
Blanking ON: 4V
Blanking OFF: 1.6V
D461
L461
IN
D462
Q455,Q456
Fig. 11 Blanking circuit
CIRCUIT DESCRIPTION
Q652/Q653 Q663/Q664 Q666
NB Amp2 NB Amp3 NB Amp4
AGC
Fig. 10 Noise detection circuit
D463
L462
OUT
Blanking ON: 8V
D464
Blanking OFF: 0V
BLNK
H: Blanking ON
L : Blanking OFF
and the gains of the NB amplifi er 2 (Q652, Q653), NB ampli-
fi er 3 (Q663, Q664), and NB amplifi er 4 (Q666) are lowered,
which results in low detecting voltage, and Q671 is not
turned ON. Thus, the IF signal is not attenuated. Because
the AGC time constant is large, a pulse with short frequency
does not let the AGC function and lower the detecting volt-
age, Q671 becomes ON and the blanking circuit can func-
tion.
When the NB function is turned ON, the threshold vari-
able direct current "NBL" is added to the Q669 emitter from
the D/A converter (IC13). By changing the emitter's voltage,
the NB effect can be adjusted. Also, when the IF signal is
10.695MHz, Q654, Q665, and Q668 becomes ON and the
NB amplifi er 2, NB amplifi er 3, and NB amplifi er 4's tuning
point changes.
Q670 D662
Buffer Detection
Q669
Q667
NB AGC Amp Switching
NBL
(Noise Blanker threshold level)

Transmitter Circuit

■ From transmission signal (TIF) to drive output

The path of the transmission signal (TIF) is divided by the
analog switch (IC68) in the Control unit (X53-446 A/3) in ac-
cordance with whether it's FM mode or a mode other than
FM.
In FM mode, the transmission signal is switched by the
analog switch (IC68), and the frequency is modulated in
audio signals into the LO1 (first local oscillator signal). In
modes other than FM, the transmission signal is switched
by the analog switch (IC68), and then input into the balanced
mixer (IC701). The 24kHz IF signal and LO3 (third local oscil-
lator signal) are input into this balanced mixer to generate
the 10.695MHz IF signal.
The 10.695MHz transmission IF signal passes through
the IF fi lter (XF471) with the 6kHz bandwidth, and controls
the transmission IF amplifi er's (Q721) second gate by the
ALC voltage and adjusts the gain. The pin diode (D721) oper-
ates the variable attenuator by current control, and changes
the transmission IF signal gain. This variable gain provides
the TGC function which controls each band's transmission
gain.
TS-590S
Q671
Switching
BLNK signal
(NB blank control)
IBK
15