28
TK-790/
(
B
)
/H
(
B
)
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.
CIRCUIT DESCRIPTION
Fig. 8
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 voltage
ANT input level
SQ close
SQ open
Preset
value
RSSI voltage
ANT input level
Preset value
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.
8CL
TX VCO
PLL-IC
IC516
µ
-com
RX VCO
BUFF
AMP
AMP
AMP
SW
SW
SW
LPF
MO
8CL
STR
Q306
Q307
SW Q308
Q309
Q305
IC301
DT
CP
UL
Q311
5C
X301
MB
MO
8R
Q310
Q312
D306
TX
RX
EP
TX-RX UNIT
CONTROL UNIT
8C
8T
8CL
VCXO
5C
5C
9CL
Q303,304
Fig. 9
VCO/PLL circuit
