Introduction to the working principle and function of the walkie-talkie

Introduction to the working principle and function of the walkie-talkie

 

Frequency and spectrum:

The electromagnetic spectrum is the sum of all frequencies of electromagnetic radiation, ranging from the longest radio waves to the shortest cosmic rays. The spectrum can be divided into several parts, which are called frequency bands. The electromagnetic spectrum is divided into the following bands:

100Hz 300KHz 3MHz 30MHz 300MHz 3GHz 30GHz 300GHz

AM AM broadcast MW medium wave SW short wave HF high frequency

VHF VHF UHF UHF SHF UHF EHF Extremely High Frequency

The frequency band used by the walkie-talkie:

VHF 136-174MHz, divided into two segments: 136-150MHz, 150-174MHz UHF 350-370MHz 370-390MHz 400-420MHz 450-470MHz

 

Radio communication band characteristics:

Propagation; the atmosphere surrounding the Earth attenuates and refracts radio signals as it attenuates and refracts light. The usual rule is: The lower the frequency, the smaller the attenuation loss. The way of propagation is: Ground wave propagation (radio below 300KHz) Wave); sky wave propagation (radio waves in the range of 300KHz-30MHz, propagating through the reflection and refraction of the ionosphere); line-of-sight propagation (30MHz-900MHz best for wireless walkie-talkie communication) noise: electromagnetic noise interference comes from machines and engines because of noise signals The frequency is low, therefore, the low frequency signal is more sensitive to noise. Transmission distance: the transmission distance is the largest in rural and suburban VHF; the transmission distance in urban UHF is the largest.

 

Principle overview:

The transmitting part: the phase-locked loop and the voltage-controlled oscillator (VCO) generate the transmitted RF carrier signal, and after buffer amplification, excitation amplification, and power amplifier, generate the rated RF power, pass the antenna low-pass filter, suppress the harmonic components, and then pass The antenna is launched.

Receiving part: divided into two-frequency super-heterodyne mode, the signal input from the antenna is amplified by the transceiving conversion circuit and the band-pass filter, and then amplified by a band-pass filter to enter a mixing frequency to amplify the signal from the radio frequency. A first local oscillator signal from the phase locked loop frequency synthesizer circuit is mixed at the first mixer and a first intermediate frequency signal is generated. The first intermediate frequency signal further cancels the adjacent channel clutter signal through the crystal filter. The filtered first intermediate frequency signal enters the intermediate frequency processing chip, and the second local oscillation signal is mixed again to generate a second intermediate frequency signal, and the second intermediate frequency signal is filtered and removed by a ceramic filter to filter out unwanted spurious signals. Generate an audio signal. The audio signal is amplified by amplifying, bandpass filter, de-emphasis, etc., into the volume control circuit and the power amplifier to amplify, drive the speaker, and get the information people need.

Modulation signal and modulation circuit: The human voice is converted into an audio electrical signal through a microphone, and the audio signal is directly modulated by the amplification circuit, the pre-emphasis circuit and the band pass filter into the voltage controlled oscillator.

Signaling processing: The CPU generates the CTCSS/DTCSS signal and adjusts it to enter the voltage controlled oscillator for modulation. The low frequency signal obtained after receiving the frequency discrimination is filtered and shaped by the bandpass filter of the amplification and sub-audio, enters the CPU, compares with the preset value, and controls the output of the audio power amplifier and the speaker. That is, if it is the same as the preset value, the speaker is turned on, and if it is different, the speaker is turned off.

 

Common terms are explained:

Carrier: A carrier of useful signals such as voice, digital signals, and signaling, and high-frequency electromagnetic waves that are easy to transmit.

Frequency: The number of complete cycles per second, such as alternating current or radio waves, usually measured in Hertz.

Channel: Channel refers to the frequency value occupied when transmitting and receiving.

Channel spacing: refers to the frequency difference between adjacent channels called the channel spacing. There are 25KHz (wideband), 20KHz, 12.5KHz (narrowband), etc. between the specified channels.

Simplex: At the same time, information can only be transmitted in one direction, which can be divided into two modes: same-frequency simplex and different-frequency simplex.

PTT: Abbreviation for Push To Talk, press to talk, then the walkie-talkie enters the launch.

Audio: Audio, the frequency of the voice of a person speaking, usually refers to the frequency band of 300Hz - 3400Hz.

Modulation: The parameter of the carrier is controlled by one or several parameters of the modulated signal. According to different control parameters, the modulation can be divided into amplitude modulation, frequency modulation and phase modulation.

Coverage: Refers to our confidence in the estimated communication conditions in a certain area. At a coverage of 90% of the time and 90% of the locations, a walkie-talkie can talk.

Squelch: The squelch circuit is used to reduce or eliminate unwanted signals and noise before reaching the speaker. There are two types of squelch: carrier squelch and code squelch.

Carrier squelch: Often referred to as noise compensated squelch, it is used to prevent noise from being heard from the speaker when no information is transmitted.

Code squelch: allows the listener to receive only the information sent to them. The speaker will only turn on when the received message has a predetermined code. There are two common types of code squelch: tone code squelch (CTCSS) , PL 38/42 group), digital coded squelch (DPL, DTCSS 84 group)

Antenna: A device designed to transmit radio signals and transmit such signals into the atmosphere. How far this signal can be transmitted is affected by many factors, of which the antenna plays a major role.

MONITOR: A listening method used to hear weak signals. By pressing the dedicated button to forcibly turn on the receiving signal channel, the operator uses the ear to discern the weak sound in the SPK for the purpose of listening.

Scan (SCAN): A form of listening used to hear calls on all channels. By pressing the dedicated button, the receiving circuit receives a channel for a period of time in a certain order to listen to the signal in the channel. If the reception time per channel is 100ms, then ten channels can be scanned per second, that is, the scanning speed is 10ch/s.

Voice Control (VOX): The sound control function that initiates the launch operation by speaking and transmits the sound. It is not necessary to press the PTT button. Stop talking and the launch stops.

Low voltage alarm: When the voltage of the battery reaches the preset low voltage value, the intercom will flash and an alarm tone will be sent to inform the user that the battery should be replaced or charged.

Power saving function: In order to save electricity and extend standby time, the walkie-talkie will not operate for a period of time and will operate with a period of time and a period of time. This mode is called power saving mode. The switch length ratio is approximately 1:4. When receiving a signal or having a button operation, the walkie-talkie immediately exits the power-saving state and enters the normal state.

Squelch level: The strength of the noise in the received signal is corresponding to the strength of the signal. The stronger the signal, the weaker the noise. The maximum noise and the minimum noise are divided into several files, and each file is called a level. The number of files divided is called the squelch level.

Tail cancellation: During a call, the transmitter releases the PTT button to stop transmitting, and the receiver will hear a harsh “ca...” sound called the tail. To eliminate this sound, some special techniques can be used for processing.

Others: lighting, lock keys, noise canceling channels, etc.