Circuit Description - Kenwood TKR-740 Service Manual

TKR-740
1.Outline
The TKR-740 is a VHF repeater operating in the158-174MHz
(RX), 146-174M Hz (TX) frequency range. It has the follow ing
features:
High-performance model w ith enhanced features.
●
32 channel radio w ith 16 QT / DQT.
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Remote functions for use by base stations.
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Fine frequency steps using Direct Digital Synthesizer (DDS).
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Signalling decoding and AF processing using Digital Signal
●
Processor (DSP).
2.2 Transmitter reference PLL circuit
The t ransm it t er ref erence PLL circuit generat es t he
reference frequency signal (19.2M Hz) for the transmitter DDS
and modulates the low-frequency components of QT and DQT.
This circuit consists of IC201, X201, Q201, and Q202. The
VCO consists of Q201, X201, D203, D201, and D202.
The signal generated by the VCO is fed to buffer amplifier
Q202 and unwanted harmonics are removed with an LPF. The
resulting signal goes to the PLL IC (IC201), and its phase is
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CIRCUIT DESCRIPTION

2.Transmitter Unit
circuits: (1) internal/external reference circuit, (2) transmitter
reference PLL circuit, (3) transmitter DDS circuit, (4) transmitter
m ain PLL circuit, (5) driver circuit, (6) m odulation level
adjustment circuit, and (7) other circuits.
2.1 Internal/ external reference circuit
the internal +/-1.0ppm/20M Hz TCXO (X101) and the 10M Hz
external reference automatically. If there is no external
reference signal applied to the unit, the internal TCXO is used
as the reference frequency. When an external reference
(10M Hz/-10dBm or higher) is applied, the external reference
is automatically used as the reference frequency. The circuit
consists of Q102, Q106, XF210, Q109, D101, D103, Q15,
X101, Q205, D205, Q206, IC204, Q110, Q114, Q112, Q113,
Q108, XF211, and Q115.
Fig.1 Internal/ external reference circuit
compared w ith that of the reference frequency using the
comparison frequency of 200 kHz. The phase difference signal
is converted to a DC voltage by a lag-lead type loop filter. The
capacitance of D201 and D204 are varied by the DC voltage to
keep the VCO oscillator frequency at 19.2M Hz. The 19.2M Hz
oscillator signal is fed to Q241 and used as the reference
frequency signal for the transmitter DDS.
The transmitter unit (X56-307 A/3) consists of the follow ing
The internal/external reference circuit sw itches betw een