Squelch Circuit; Rx Bpf (L110~112); Vco/Pll Circuit; Squelch And Rssi Voltage Vs Ant Input Level - Kenwood TK-790 Service Manual

TK-790/ ( B

3. Squelch Circuit

The output from IC101 enters FM IC again, then passed
through a band-pass filter. The noise component output
from IC101 is amplified by Q109 and rectified by D103 to
produce a DC voltage corresponding to the nose level.
The DC voltage is sent to the analog port of the CPU
(IC516/pin 27). And IC101 output a DC voltage (RSSI) corre-
sponding to the input of the IF amplifier. The CPU reads the
RSSI signal via pin 26. IC101 determines whether to output
sounds from the speaker by comparing the input voltage of
pin 27 with the present value.
Only during scan, the RSSI DC voltage is used together
with the noise wave detection pin's DC voltage (pin 27).
SQ close
SQ open
ANT input level
Fig. 7 Squelch and RSSI voltage vs ANT input level

4. RX BPF (L110~112)

The signals are then fed into band-pass filter that uses
varactor diode tuning to reject unwanted signal compo-
nents, and is fed to the 1st mixer.
28
) /H ( B )
CIRCUIT DESCRIPTION
Preset
value
Preset value
ANT input level
Fig. 8

5. VCO/PLL Circuit

The VCO of TK-790 consists of two VCO circuits which
one oscillates the transmit signal with Q306 and the other
does the first local receive signal with Q305.
Each oscillators are switched by turning the source line
for Q307, Q308 and Q309 on and off. The signal output of
the VCO is amplified by the buffer amplifier Q310 and di-
vided into two signals which one is amplified by Q312 and
the other is done by Q311. The signal from Q312 passes
through D306 which is transmit/receive switch, is amplified
by Q201 and Q204, and is output from the CN203.
The signal from Q311 passes through the low-pass filter
and is applied to IC301 which is PLL frequency synthesizer
with VCXO ; X301. The VCXO of which the frequency stabil-
ity is within 2.0ppm (temperature range of –30 to +60°C)
generates 16.8MHz. The PLL-IC consists of three modulus
prescaler, fractional divider, reference divider, digital phase
comparator with charge pump output.
This PLL-IC is Fractional-N type synthesizer and performs
is 40 or 50 or 60kHz reference signal which is eighth of the
channel step (5 or 6.25 or 7.5kHz). The input signal from the
pins 5 and 8 of the PLL-IC is divided down to the 40 or 50 or
60kHz and compared at digital phase comparator.
The pulsed output signal of the digital phase comparator
is applied to the charge pump and transformed into DC sig-
nal. The DC signal from the pin 14 of the PLL-IC passes
through the active low-pass filter (loop filter), is applied to
the VCO and controls to keep the frequency of the VCO.
The serial data (DT, CP, EP) from the microprocessor
IC516 is input to the PLL-IC. And PLL lock condition is al-
ways monitored by the pin 28 (UL) of IC516.
TX-RX UNIT
8CL
MO
Q306
TX VCO
Q307
STR
SW
Q309 SW Q308
SW
8CL
Q305
RX VCO
9CL 5C
IC301
LPF PLL-IC
Q303,304
IC516
µ-com
Fig. 9 VCO/PLL circuit
8CL 8C 8T
Q310 Q312 D306
TX
BUFF
AMP SW
AMP
RX
8R
X301
Q311
AMP VCXO
5C 5C
MB MO
CONTROL UNIT