Circuit Description - HYT TC-518 Service Manual

TC-518 Service Manual

Circuit Description

1.1 General Principle
Tx section: The modulated signal from MIC is directly sent to MCU chip (U604) for A-D conversion to digital signal,
processing such as filtering, signal compression, encryption, pre-emphasis, Tx gain control and amplitude limiting, and
restoration to modulated signal via D-A conversion. The modulated signal from MCU passes through RC low-pass filter
circuit, and enters VCO together with CTCSS/CDCSS signal generated by MCU for direct frequency modulation (The
PLL system is composed of U101, Q104 and Q105.). The modulated audio signal and CTCSS/CDCSS signaling are
transferred to RF carrier signal. The modulated RF carrier signal is converted to RF signal by the buffer amplifier Q106
and Q109, and then enters the preamplifier Q402, where the signal power is amplified. The RF signal is further amplified
by the pre-driver Q403 to drive the final power amplifier Q404 to get signals of the desired power. The amplified signal
enters the low-pass filter circuit through the diode D402 and D404, and then it is transmitted from the antenna after
high-order harmonics are removed. D402/D404 and D501/D502 compose the Rx-Tx switch circuit.
Rx section: Desired/Undesired RF signal is received via the antenna, and goes through the bandpass filter (composed of
D503, D504 and D505) to filter the undesired out-of-band interference signal. Then the desired RF signal enters the
high-frequency low-noise amplifier Q618, where the desired signal is amplified. The amplified signal goes through the
bandpass filter (composed of D506 and D507) again to filter the undesired out-of-band interference signal. The desired
RF signal is mixed with the first local oscillator signal (output from VCO) at Q502. The newly generated signal is
processed by the crystal filter (XF503), which brings the first IF signal (desired). After amplified by Q505, this IF signal
enters the IF demodulation IC (U301), and it is mixed with the second local oscillator signal to generate the second IF
signal. Then the second IF signal is demodulated by U301 and outputs in the form of audio signal, which is sent to MCU
for A-D conversion and filtering. Afterwards, the audio signal is divided into two flows for further processing: one flow is
processed by CTCSS/CDCSS decoder; the other is restored to audio signal through filtering, de-emphasis, decoding and
D-A conversion. Then the restored audio signal is processed by the audio power amplifier (U302) to drive the speaker
directly.
Power Supply Section: The 7.4V battery voltage goes through LDO IC U601, and generates 5V DC steady voltage,
which is divided into two flows. One flow is converted to 4.5V voltage output (one flow supplies the Tx circuit with 4.5T and
the other flow supplies VCO as V_SAVE under MCU control; V_SAVE flow is converted to 4.5R through Q604 and
supplies the Rx circuit.) through double-channel LDO U602. The other flow is converted to 3.0V voltage output through
double-channel LDO U603, and supplies MCU and PLL.
Signaling Code: CTCSS/CDCSS generated by MCU is divided into two flows. One flow. is output through MOD port, and
sent to VCO together with audio signal for modulation through RC filter network. The other flow is output through D/A port,
and goes through RC network to modulate the PLL reference frequency oscillator.
8