Circuit Description - HYT TC-320 Service Manual

TC-320 Service Manual

Circuit Description

1. General Diagram
Tx Section: Audio signal from MIC is sent to the RF chip (U501) directly. In U501, the signal is amplitude limited and
amplified, and then sent to the MCU chip (U601). The baseband signal processing circuit in U601 will convert the
amplified audio signal to digital signal via A/D conversion, and then convert the digital signal to modulation signal via D/A
conversion after processing such as filtering, signal compression, encryption, pre-emphasis, Tx gain control and
amplitude limiting are completed. The modulation signal from MCU will pass through the low-pass filter circuit and then
will go to VCO for direct frequency modulation. The modulated carrier signal is converted to RF signal by the buffer
amplifier (U501), and then enters front-stage power amplifier (Q401) for amplification. The amplified RF signal will be
further amplified by the pre-driver (Q402) and final-stage power amplifier (Q403), and then will enter low-pass filter circuit
through the switching diode (D401), and will be finally transmitted via antenna after ultroharmonics are removed. D402
and D501 compose the Rx-Tx switch circuit.
Rx section: Desired and undesired RF signals received via the antenna are fed to the bandpass filter (composed of
SAWF501 and SAWF502) to filter out undesired out-of-band interference signal, and then amplified by the RF amplifier
(Q506). The amplified signal is then fed to the bandpass filter (SAWF503) to filter out undesired out-of-band interference
signal. The desired RF signal is mixed with the first local oscillator signal (output from VCO) by Q505. The newly
generated signal is processed by the crystal filter (XF501) to get the first IF signal (desired signal). After being amplified
by Q504, this IF signal enters the RF chip (U501), and is mixed with the second local oscillator signal to generate the
second IF signal. The second IF signal is demodulated by U501 to output audio signal, which is sent to MCU for A/D
conversion and filtering. Afterwards, the audio signal is divided into two flows. One flow is processed by CTCSS/CDCSS
decoder; the other is restored to audio signal through filtering, de-emphasis, decoding, signal expansion and D/A
conversion. Then the restored audio signal is amplified by the audio power amplifier (U301) to drive the speaker directly.
Power Supply Section: The 3.7V battery voltage is converted to two flows of 3.0V DC voltage by the LDO module (U602).
One flow supplies 3.0V VCC for the CPU; the other outputs 3.0V V-SAVE voltage, which goes through Q604 and Q601
respectively to supply, under the control TX-CTRL, 3.0V VT for Tx circuit. The V-SAVE voltage also goes through Q602 to
supply 3.0V VR for Rx circuit.
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