Transmit Signal Path; Apc (Automatic Power Control); (Cntl) Unit; Base Operation (Tx, Line-Input Audio) - Vertex Standard VXR-7000 Service Manual

Circuit Description
ceiving frequency. The RX VCO output passes through buffer
amplifier Q3009 and first local amplifier Q3011 to the first mixer
D3014, as described previously. A portion of the RX VCO out-
put is applied to the prescaler/swallow counter section in the
PLL IC, Q3001. There the RX VCO signal is divided by 64 or
65, according to a control signal from the Main CPU Q4012 on
the CNTL Unit, before being applied to the programmable di-
vider section of the PLL IC Q3001.
The data latch section of the PLL IC Q3001 also receives
serial dividing data from the Main CPU Q4012, which causes
the pre-divided RX VCO signal to be further divided by 22,920
~ 30,520 in the programmable divider section in the PLL IC
Q3001, depending upon the desired receive frequency, so as to
produce a 5 kHz or 6.25 kHz derivative of the current RX VCO
frequency. Meanwhile, the reference divider section of the PLL
IC Q3001 divides the 14.4 MHz crystal reference from the ref-
erence oscillator X3001 and Q3002 (2SC4116GR) by 2880
(or 2304) to produce the 5 kHz (or 6.25 kHz) loop reference.
The 5 kHz or 6.25 kHz signal from the programmable di-
vider (derived from the RX VCO) and that derived from the
crystal are applied to the phase detector section of the PLL IC
Q3001, which produces a pulsed output with pulse duration
depending on the phase difference between the input signals.
This pulse train is then converted to DC, low pass filtered, then
fed back to the RX VCO varactor diodes D3005, D3006, D3009,
and D3010.
Changes in the DC voltage applied to the varactor diodes
D3005, D3006, D3009, and D3010 affect the reactance in the
tank circuit RX VCO Q3007, changing the oscillating frequen-
cy according to the phase difference between the signals de-
rived from the RX VCO and the crystal reference oscillator.
The RX VCO is thus phase-locked to the reference frequency
standard.

Transmit Signal Path

The TX VCO, consisting of FET Q2005 (2SK508) and var-
actor diodes D2004 and D2005, oscillates between 136 MHz
and 174 MHz according to the programmed transmit frequen-
cy. The theory of operation of the remainder of the PLL circuit-
ry is similar to that of the RX PLL circuit; however, dividing
data from the Main CPU Q4012 on the CNTL Unit is such that
the VCO frequency is the actual transmit frequency.
The speech audio from the CNTL Unit is applied to varac-
tor diode D2005 (HVU350), which frequency modulates the
TX VCO up to 5 kHz (wide band) or 2.5 kHz (narrow band)
from the unmodulated carrier at the transmit frequency. The
modulated transmit signal is buffered by Q2008 (2SC5226),
then passes through the RF amplifier Q2010 (2SC3357) and
RF diode switch D2008 (RN739F) to the PA Unit.
The transmit signal is applied to the RF amplifier Q1501
(2SC3357) and RF power module IC Q1502 (PF0310A), then
22
finally amplified by power amplifier Q1507 (2SC5125) up to
50 Watts. Harmonic and spurious radiation in the final output
is suppressed by a low pass filter consisting of coils L1511 ~
L1515, plus capacitors C1546, C1547, C1554, C1556, C1560,
and C1566 on the PA Unit, before delivery to the TX antenna
jack.

APC (Automatic Power Control)

RF power output from the final amplifier Q1507 is sampled
by C1548/C1558 and is then rectified by D1503/D1504 (both
1SS319). The resulting DC voltage is applied to the compara-
tor Q1509 (TA75S01F), where the voltage is compared with a
reference voltage from the Main CPU Q4012 on the CNTL
Unit, to produce a control voltage for the Automatic Power Con-
troller Q1503 (2SB1122S) and Q1504 (2SC4116GR), which
regulates supply voltage to the RF power module IC Q1502, so
as to maintain stable high (or low) output power under varying
antenna loading conditions.
C

(CNTL) Unit

The CNTL Unit consists of 8-bit CPU Q4012
(HD64F3337YF16), EEPROM Q4008 (NM93C86A), RX and
TX speech audio circuits, and various analog switches for the
CPU and repeater interconnections.
Microprocessor operational code is stored in Q4008, while
channel data and repeater configuration information is pro-
grammed from an external PC connected to the front panel's
MIC jack via a VPL-1 programming cable.
The output from the Main CPU, Q4012, contains serial con-
trol data used for REPEATER/BASE mode control, as well as
TX and RX PLL data. Crystal X4001 oscillates at 16 MHz, and
provides stable clock timing for the Main CPU. When the re-
peater is powered on, the voltage at pin 8 of Q4012 becomes
stable, and the output of voltage detector IC Q4005
(RH5VL45AA), which is tied to Q4012 (pin 1-RST) becomes
high, resetting the Main CPU.

Base Operation (Tx, Line-Input Audio)

Line input from J4009 (pins 3 and 4) is impedance matched
by transformer T4001, then passes through the audio amplifier
Q4018 (NJM2902M) and audio switch Q4030 (NJU4066BM)
to the pre-emphasis network at Q4018-1, where the signal is
processed in the same manner as previously described. The line
level can be attenuated by switch S4002, and line sensitivity
can be adjusted to 10 dBm ~ +10 dBm by potentiometer
VR4003 to compensate for audio line level variations.
Base Operation (Tx, Mic-Input Audio)
Microphone input is delivered past the MIC MUTE switch
Q4026 (DTC323TK), then passes through the audio amplifier
and active low pass filter at Q4021 (NJM2902M) to the pre-
emphasis network at Q4018-1, where the signal is processed in
the same manner as previously described.