Kenwood TS-590SG, Руководство по обслуживанию

Страница: 15 / 155

TS-590SG
15
Noise Blanker Circuit (NB Circuit)
n
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 (XC1-024 A/2), and is
sent from CN674 to the NB unit (XC1-024 B/2) CN667 (NB
IF) through a co-axial cable.
The signal input into the NB unit (XC1-024 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 func-
tions and the gains of the NB amplifi er 2 (Q652, Q653), NB
amplifier 3 (Q663, Q664), and NB amplifier 4 (Q666) are
lowered, which results in low detecting voltage, and Q671
is not turned ON. Thus, the IF signal is not attenuated. Be-
cause the AGC time constant is large, a pulse with short
frequency does not let the AGC function and the detecting
voltage increases, Q671 becomes ON and the blanking cir-
cuit can function.
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 to 10.695MHz.
Fig. 9 Noise detection circuit
Q641
Buffer
IF input
10.695MHz/
11.374MHz
Q651
NB Amp1
Q652/Q653
NB Amp2
AGC
XC1-024 (A/2) XC1-024 (B/2)
Q663/Q664
NB Amp3
Q666
NB Amp4
Q670
Buffer
D662
Detection
Q669
Switching
NBL
(Noise Blanker threshold level)
Q671
Switching
IBK
BLNK signal
(NB blank control)
Q667
NB AGC Amp
n
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,
diodes D461 to D464 turn OFF and the IF signal is attenu-
ated.
Fig. 10 Blanking circuit
Transmitter Circuit
n
From transmission signal (TIF) to drive output
The path of the transmission signal (TIF) through the
analog switch (IC68) in the Control unit (XC1-021 A/3) is dif-
ferent during FM mode and mode other 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 (fi rst local oscillator signal). In
modes other than FM, the transmission signal is switched
by the analog switch (IC68), and then input into the bal-
anced mixer (IC701). The 24kHz IF signal and LO3 (third
local oscillator 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 6kHz bandwidth, and the signal is
amplifi ed at the 10M transmission IF amplifi er (Q721). The
pin diode (D721) operates the variable attenuator by cur-
rent control, and changes the transmission IF signal gain.
This variable gain provides the TGC function which controls
each band’s transmission gain.
CIRCUIT DESCRIPTION
D461
D462
D463
L462 L461
+8V
OUT IN
D464
Q455,Q456
BLNK
H: Blanking ON
L : Blanking OFF
Blanking ON: 8V
Blanking OFF: 0V
Blanking ON: 4V
Blanking OFF: 1.6V