Kenwood NX-411 Service Manual page 16

NX-411
5-3. PLL IC (IC503)
The PLL IC compares the differences in phases of the
VCO oscillation frequency and the TCXO reference fre-
quency, returns the difference to the VCO CV terminal and
realizes the "Phase Locked Loop" for the return control. This
allows the VCO oscillation frequency to accurately match
(lock) the desired frequency.
When the frequency is controlled by the PLL, the fre-
quency convergence time increases as the frequency differ-
ence increases when the set frequency is changed. To sup-
plement this, the ASIC is used before control by the PLL IC
to bring the VCO oscillation frequency close to the desired
frequency. As a result, the VCO CV voltage does not change
and is always stable at approximately 2.8V at the time of re-
ceive mode. At the time of transmitit mode, it is always stable
at approximately 3.0V.
The desired frequency is set for the PLL IC by the ASIC
(IC309) through the 3-line "SDO1", "SCK1", "/PCS_RF" se-
rial bus. Whether the PLL IC is locked or not is monitored by
the ASIC through the "PLD" signal line. If the VCO is not the
desired frequency (unlock), the "PLD" logic is low.
"PLLMOD" receives the modulation data from the DSP
(IC304).
5C
IC309
ASIC
DSP
16
CIRCUIT DESCRIPTION
Q505
Q503 Q506
Ripple
filter
/T_R
Q501 IC504
Ripple
BUF
filter
Q502
Q504, Q507
CV
BUF
IC503
PLL IC
SD1, SCK1, PLD
IC304
TCXO
PLL MOD
Fig. 7 PLL block diagram
5-4. Doubler (Q509)
The doubler (Q509) extracts the twice harmonic com-
ponent from the signal from the VCO. This twice harmonic
components is fed into the PLL (IC503) and buffer (Q510)
through a band-pass fi lter.
5-5. Local Switch (D513, D601)
The destination of the signal from the buffer amplifier
(Q510) is changed with the diode switch (D601) that is con-
trolled by the transmission power supply, 50T, and the diode
switch (D513) that is controlled by the receive power supply,
50R. If the 50T logic is high, it is connected to a send-side
pre-drive (Q610). If the 50T logic is low, it is connected to a
receive-side mixer (IC804).
Q508 Q509
RX VCO BUF x2
TX VCO
VCO_MOD
IC505 IC3
BUF DAC
X501
Q510 D513, D601
TX/RX
BUF
SW