1. transmitting part: the transmitted RF carrier signal is generated by a phase-locked loop and a voltage-controlled oscillator (VCO), and the rated RF power is generated by buffer amplification, excitation amplification and power amplification, and then the harmonic components are suppressed by an antenna switch and a low-pass filter, and then transmitted through an antenna.

2. Receiving part: The receiving part generally adopts the superheterodyne mode of secondary frequency conversion. The signal input from the antenna passes through the transceiver conversion circuit and the band-pass filter, and then enters the first mixer through the band-pass filter. In a first mixer, an amplified signal from a radio frequency is mixed with a first local oscillator signal from a PLL frequency synthesizer circuit to generate a first intermediate frequency signal. The first intermediate frequency signal is further eliminated by the crystal filter. The filtered first intermediate frequency signal enters the intermediate frequency processing chip and is mixed with the second local oscillator signal again to generate a second intermediate frequency signal. The second intermediate frequency signal is filtered by two ceramic filters, amplified and frequency discriminated to generate an audio signal. After amplification, band-pass filtering, de-emphasis and other circuits, the audio signal enters the volume control circuit and audio power amplifier for amplification, and drives the speaker to get the information people need.

3. Modulation signal and modulation circuit: The human voice is converted into an audio signal through a microphone, and the audio signal enters a voltage controlled oscillator through an amplification circuit, a pre-emphasis circuit and a bandpass filter for direct modulation.

4. Signaling processing: CTCSS/DTCSS signals generated by CPU are amplified and adjusted, and then enter the voltage-controlled oscillator for modulation. A part of low-frequency signals obtained after receiving frequency discrimination are amplified, filtered and shaped by a sub-audio band-pass filter, and then enter the CPU, where they are compared with preset values, and the results control the output of audio power amplifiers and speakers. That is, if it is the same as the preset value, turn on the speaker; If not, please turn off the speakers.

5. Power supply control: When the CPU is controlled in different states, it sends out different power supplies.

Receiving power supply: usually in intermittent working mode to ensure power saving.

Transmitting power supply: there is only electricity when transmitting.

CPU power supply: stable power supply

Circuit description

1. Circuit composition

The receiver adopts secondary frequency conversion superheterodyne mode. 1 IF is 2 1.7MHz, and the second IF is 455kHz. 1 LO frequency is generated by a phase-locked loop (PLL) circuit. The transmitter directly generates the required frequency through the PLL circuit.

2. Receiving department

2- 1 Pre-stage (RF amplifier)

A received signal input from an antenna is amplified in an RF amplifier through a transceiver circuit composed of diodes. Then, it passes through a band-pass filter (BPF) and enters the mixer.

No.2-2 1 mixer

The signal from the previous stage is mixed with the 1 local oscillator signal from the phase locked loop (PLL) circuit in the mixer to generate the 1 intermediate frequency signal (2 1.7MHz). The crystal filter is used to filter the signal and remove the adjacent clutter signal, thus ensuring the necessary technical indicators such as adjacent channel selectivity.

2-3 IF amplifier (IF amplifier)

The signal passing through the crystal filter is amplified by 1 IF amplifier and then enters the IF integrated circuit (MC336 1). The IC is an integrated circuit chip that integrates a second local oscillator, a second mixer, a second intermediate frequency amplifier, a frequency discriminator, a noise amplifier and a noise rectification circuit.

The signal entering the integrated circuit is mixed with the second local oscillator signal to generate a second intermediate frequency signal of 455kHz, which is amplified by an intermediate frequency amplifier and then filtered by a ceramic filter of 455KHz to ensure the necessary selectivity.

Finally, the IF signal of the filter is frequency-discriminated in the integrated circuit to produce audio signal output.

2-4 Audio Amplifier (AF Amplifier)

The audio signal output from the IF integrated circuit passes through the de-emphasis circuit to restore the original frequency characteristics of the audio signal. Then, the audio signal passes through the volume control circuit (AF VOL) and is amplified by the audio power amplifier (MC34 1 19) to drive the speaker.

2-5 Silence

A part of the audio signal output from the IF integrated circuit enters the FM integrated circuit again, and its noise component is rectified by a filter and an amplifier to generate a DC voltage corresponding to the noise component. Analog port of microprocessor (MCU). Compare the input DC voltage with the preset voltage value, and IC 1 controls to turn on or off the speaker output according to the comparison result.

When the speaker sounds, the AFCO line is set to (HI) high level, and the amplifier is turned on through the reflection of the triode, and the speaker sounds.

2-6 receiving CTCSS signaling

(T-260CT only)

Part of the signals output by the IF integrated circuit enter the special accessories for CTCSS coding and decoding through special plugs, and various processing judgments are made in the accessories to analyze whether the received consonants are consistent with the preset values. The judgment result and the judgment result of static noise jointly control AFCO to determine whether the speaker sounds.

3. Phase locked loop (PLL) circuit

The PLL circuit generates the 1 local oscillator signal of the receiver and the RF carrier signal of the transmitter.

3- 1 PLL

Receiving and transmitting use the same voltage controlled oscillator (VCO). The oscillating signal enters the PLL integrated circuit (LM3 1202) after passing through the buffer. The integrated circuit includes a reference oscillator frequency divider and a phase comparator. An input oscillator signal is divided by a predetermined frequency to become a 5kHz or 6.25kHz signal, and then it is added to a phase comparator for phase comparison together with a 5kHz or 6.25kHz signal generated by frequency division of a reference oscillator, thereby generating a phase difference signal, which is generated by a charge pump to generate a frequency control signal. The control signal passes through a passive low-pass filter (LPF) and is applied to the varactor diode of VCO to control its output frequency.

3-2 reference oscillator

The reference signal of PLL is the 14.4MHz oscillation signal generated by the internal oscillation circuit of PLL integrated circuit. In order to ensure the frequency stability, the imported 14.4MHz crystal with temperature compensation is adopted.

4. Launching department

4- 1 transmit audio

The voice signal input by the microphone is pre-emphasized, and then amplified, limited and frequency offset controlled in the voice amplifier circuit. Complete the instantaneous frequency offset control (IDC) of the input signal. Then the part of the signal with frequency above 3kHz is filtered by a low-pass filter, and then enters the VCO from the modulation end for direct frequency modulation (FM).

4-2 CTCSS signaling encoder

(T-260CT only)

The CTCSS code is generated by a special external accessory. This signal is mixed with the voice signal sent by the voice amplifier and sent to the VCO where it is frequency modulated.

4-3 VCO and RF amplifier

The modulation signal modulates the VCO at T 1 The RF signal output by PLL is amplified by R25 to reach the excitation level required by the final amplifier.

4-4 Final Power Amplifier

MOS FET final power amplifier (BLT50) is used for power amplification.

4-5 antenna (ant) conversion and low-pass filter (LPF)

The signal output by the final power amplifier passes through a PIN diode and a low-pass filter, and then is emitted from the antenna terminal.

5. Power supply

The 5V power supply system includes 5V, 5M, 5C, 5R and 5T * * * power supplies for microprocessors. The 5V voltage used by the microprocessor produces an output as soon as the power supply is turned on. 5M is the normal output, but once the power switch is turned off, this output will be turned off at the same time to prevent the microprocessor system from misoperation.

5C is * * * 5V power supply. In the power saving function, it outputs everything except the "sleep" state.

5R is a 5V power supply for receiving, which outputs when receiving.

5T is a 5V power supply for transmission, which is output during transmission.

6. The control system IC 1 Central Processing Unit (MCU) works with a clock of 4.00MHz.