Noise Detector Circuit - Kenwood TS-990S Service Manual

TS-990S
SSB/FM SSB/PSK*2
at Slope Tune [Hz] at VBT [Hz]
LOW CUT HI CUT WIDTH SHIFT
0 1 0 0 0 5 0 1
5 0 1 2 0 0 8 0 1
100 1400 100 1200
200 1600 150 1300
300 1800 200 1400
400 2000 250 1500
500 2200 300 1600
600 2400 400 1700
700 2600 500*3 1800
800 2800 600 1900
900 3000 1000 2000
1000 3400 1500 2100
4000 2000 2210
5000 2200
2400
2600*4
2800
3000
*1 In CW mode, the DSP fi lter width is displayed as 500Hz; however, the DSP fi lter width is actually 600Hz.
*2 LOW CUT: Low-cut frequency; HI CUT: High-cut frequency; WIDTH: Bandwidth; SHIFT: Center frequency (SSB);
SHIFT (L/H): Amount of change from the center frequency (CW); IFBW: Bandwidth with the roofi ng fi lter set to AUTO (default value).
*3 The initial value of PSK mode is 500 Hz.
*4 The initial value of SSB (WIDTH/SHIFT) mode is 2600 Hz.
4-8. Noise Blanker (NB) Circuit

Noise Detector Circuit

The Noise detector circuit generates the blanking signal,
depending on the short cycle pulse noise.
The 10.695MHz or 11.374MHz IF signal passes through
the buffer (Q881) in the TX-RX unit (X57-827 A/2), and is
sent from CN900 to the NB unit (X57-827 B/2) CN950 (SN-
BIF) through a coaxial cable.
The signal input into the NB unit is amplified by the 4
stage amplifi er (Q882, Q883/Q884, Q886/Q887 and Q890),
passes through the buffer (Q893), and is then noise de-
tected by D881. This noise-detected voltage switches Q892,
turns Q894 ON, attenuates the IF signal in the blanking cir-
cuit (D481 to D484) depending on the noise, and removes
the pulse noise.
IF input
(10.695M/11.374M)
TX-RX unit (X57-827 A/2)
24
CIRCUIT DESCRIPTION
AM [Hz]
IFBW LOW CUT HI CUT IFBW
0 0 0 5 0 0 0 2 5 . k 6
1 0 0 5 0 0 1 0 0 3 0 . k 6
500 200 4.0k 15k
500 300 5.0k 15k
500
500
500
500
500
2.7k
2.7k
2.7k
2.7k
2.7k
2.7k
2.7k
6k
6k
Table 16 RX IF fi lter settings
Q881 Q883/Q884 Q886/Q887
Buffer NB AMP2 NB AMP3
Q882
NB AMP1
AGC
NB unit (X57-827 B/2)
Fig. 24 Noise detector circuit
CW [Hz]
WIDTH IFBW SHIFT(L) SHIFT(H) WIDTH
k 5 0 5 0 0 8 - 0 0 0
k 8 0 5 0 0 7 - 5 0 5 0
100 500 -700 100
150 500 -650 150
200 500 -600 200
250 500 -550 250
300 500 -500 300
400 500 -450 350
500*1 500 -400 400
600 2.7k -350 450
1000 2.7k -300 500
1500 2.7k -250 550
2000 2.7k -200 600
2500 2.7k -150 650
-100 700
-50 750
800
When a long cycle signal is applied, the AGC works and
the gains of the NB amplifi er 2 (Q883/Q884), NB amplifi er 3
(Q886/Q887), and NB amplifi er 4 (Q890) fall, which results
in a low detecting voltage, and Q894 is not turned ON. Thus,
the IF signal is not attenuated. The AGC time constant is
long and as a result, the AGC does not work for short pulses
to drop the detecting voltage. Therefore, Q894 is turned ON
and the noise blanker operates.
When the NB function is turned ON, the threshold vari-
able direct current "SNBL" is added to the Q892 emitter from
the D/A converter (IC55). By changing the emitter's voltage,
the NB effect can be adjusted. Also, when the IF signal is
10.695MHz, Q885, Q888 and Q891 become ON and the NB
amplifi er 2, NB amplifi er 3, and NB amplifi er 4's tuning points
change.
Q890 Q893
NB AMP4 Buffer
D881
Detection
Switching
Q889
NB AGC AMP
(Sub noise blanker
threshold level)
FSK
FSK [Hz]
tone
IFBW
2 5 0 5 0 0
1275Hz
3 0 0 5 0 0
400 500
500 500
1000 2.7k
2125Hz
1500 2.7k
Initial value
Q892 Q894
Switching
SBLNK signal
(Sub NB blank control)
SNBL
SIBK
FSK Center
shift frequency
170Hz 1360Hz
2 0 0 H z 1 3 7 5 H z
4 2 5 H z 1487.5Hz
850Hz 1700Hz
170Hz 2210Hz
200Hz 2225Hz
425Hz 2337.5Hz
850Hz 2550Hz