14
Circuit Description
PLL
The first local signal for the receiver, and the carrier
for the transmitter (at the transmitter frequency) are gen-
erated by the PLL. This circuit consists two voltage con-
trolled oscillator (VCOs), prescalar, programmable divid-
er, reference oscillator, phase detector, charge pump and
low pass filter.
The VCO (on the VCO Unit) consists of Q2401, Q2404
and varactor diodes D2401, D2402 and D2403 (mentioned
above). The oscillating frequency is controlled primarily
by the level of DC voltage fed from the loop filter (low-
pass filter) to the varactor diodes. The VCO output is buff-
ered by Q2406 (
2SC4226
), and then to prescalar within
Q2105 (
MB1505
) on the PLL Unit, which divides the VCO
frequency by 64 or 65, according to a control signal from
the prescalar control logic section of PLL IC Q2105.
The divided signal from the prescalar is fed to the pro-
grammable divider section of Q2105, where it is further
divided down to 10/12.5 kHz according to data from mi-
croprocessor Q1017 on the Control Unit. Meanwhile, the
reference oscillator section of Q2105 generates the refer-
ence frequency with crystal X2101, which signal is divid-
ed by Q2105.
The reference and the divided VCO signal are applied
together to the phase detector section of Q2105, from
which any phase difference between the two signals re-
sults in a pulse train from the phase detector. The pulses
are applied to the charge pump Q2102 (
IMD3
) and then
through low-pass filter R2101, R2103 ~ R2106, R2122 and
C2102, C2104 ~ C2107, to produce a DC voltage at a level
corresponding to the difference in phase between the ref-
erence and the divided VCO signal. This DC voltage is
returned to the varactor diodes on the VCO Unit, locking
the frequency of the VCO to the crystal reference oscilla-
tor.
In the Tx mode, Tx 5 volts applied to inverter Q2106
pulls analog switch Q2101-1/2 off, removing R2106 and
R2122 from the PLL loop.
Also, the transmitter VCO is modulated by the filtered
speech audio applied to modulating varactor diode D2403,
as previously described. If Digital Coded Squelch is in
operation, the DCS signal modulation is applied to the
PLL reference, via varactor D2102 & D2103 (
HVU350
).
Control Unit & Supply Buses
Microprocessor Q1017 (
M38067MC
) on the control unit
contains programming in masked ROM to generate seri-
al data to control the Liquid Crystal Display driver IC
Q5001 (
LC75821E
) on the LCD Unit, and the program-
mable divider in the PLL according to channel frequency
data stored in externally programmable EEPROM. Q1017
also includes programming for channel frequency scan-
ning. DCS encode/decode, CTCSS IC Control, option unit
control, selectable channel steps and frequency range.
The microprocessor receives an indication of the con-
dition of the noise squelch from the FM receiver subsystem
IC on the IF Unit, by which scanning is activated or deac-
tivated.
Q1017 also controls the power saver function and trans-
mit/receive switching by selecting the supply buses on
the regulator unit:Q2209 (
D T B 1 2 3 E K
), Q2205
(
DTA143XK
) and Q2210 (
DTC144EU
) disables the RX 5V
bus when the power saver is active.
When the PTT switch is pressed, the impedance change
on the microphone line is detected by Q1015 (
2SA1586Y
)
on the control unit, which signals the microprocessor that
the transmitter is active. The microprocessor then activates
LED indicator D5001 to glow red (TX).
Voltage comparator Q1012 (
RH5VL45AA
) controls
power-up resetting of the microprocessor.
Содержание VX-520U
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