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 parts, called frequency bands. The electromagnetic spectrum is divided into the following frequency bands:
100Hz 300KHz 3MHz 30MHz 300MHz 3GHz 30GHz 300GHz
AM Amplitude Broadcasting MW medium wave   SW Shortwave   HF high frequency
VHF Very High Frequency UHF Ultra High Frequency SHF Super High Frequency         EHF Extremely High Frequency
Interphone use of the band:
VHF 136-174MHz, is divided into two sections: 136-150MHz, 150-174MHzUHF 350-370MHz 370-390MHz 400-420MHz 450-470MHz
Radio frequency band characteristics:
The atmosphere around the earth will attenuate and refract radio signals, as it attenuates and refracts light. The usual rule is that the lower the frequency, the smaller the attenuation loss is: Ground wave propagation (radio below 300KHz (30MHz-900MHz most suitable for wireless walkie-talkie communication) Noise: electromagnetic noise from the machine and the engine, because the noise signal (the frequency range of the radio frequency of the radio frequency) The frequency of low-frequency signals, therefore, the low-frequency signal is more sensitive to noise.Transfer distance: in rural and suburban VHF transmission distance of the largest urban UHF transmission distance in the largest.
Principle Overview:
1、Transmitter：
Phase-locked loop and voltage-controlled oscillator (VCO) generate the transmitted RF carrier signal, after buffer amplification, excitation amplification and power amplifier will generate the rated radio-frequency power, which using the antenna low-pass filter to suppress harmonic components, then transit through the antenna.
2、Receiver：
Receiving part is the dual frequency conversion super-heterodyne mode. The signal inputted from antenna will magnify radio frequency through the transceiver conversion circuit and the band-pass filter, then through a band-pass filter into a mixing frequency, which mix the amplified signal from the radio frequency with the first local oscillator signal from the phase-locked loop frequency synthesizer circuit at the first mixer and generate a first intermediate frequency signal. The first intermediate frequency signal further eliminates the clutter signal of the adjacent channel through the crystal filter. The filtered first intermediate frequency signal enter into the intermediate frequency processing chip, then mix again with the second local oscillator signal to generate the second intermediate frequency signal, which filter unwanted spurious signals through a ceramic filter, then it was amplified and frequency discrimination to generate audio signal. Audio signal go through the amplification, band-pass filter, de-emphasis circuit, enter into the volume control circuit and magnify by power amplifier and drive speakers, then get the information people need.
3、Modulating Signal and Circuit:
The human voice convert to the audio signal by microphone, which enter into the voltage-controlled oscillator and modulate directly through amplifying circuit, pre-emphasis circuit and band-pass filter
4、Signaling Processing:
CPU generates CTCSS / DTCSS signal that was magnified and adjusted then, and enter into the voltage-controlled oscillator for modulation. After receiving the low frequency signal obtained by frequency discrimination, part of them process filter shaping through the amplification and band-pass filter of sub-band, then enter into the CPU, compared with the preset value, and the results will control the audio power amplifier and speaker output. That is, if same as the preset value, the speaker will turn on, and if it is different, the speaker is will turn off.
Commonly used to explain:
Carrier: voice, digital signals, signaling and other useful signal carrier, easy to transmit high-frequency electromagnetic waves.
Frequency: The total number of 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 is called channel spacing. There are 25KHz (wide band), 20KHz, 12.5KHz (narrowband) among the prescribed channels.
Simplex: At the same time, only in one direction to send information, can be divided into the same frequency and simplex simplex two ways.
PTT: Push To Talk abbreviation, press the call, then enter the radio-like radio.
Audio: Audio, refers to the human voice frequency, usually refers to 300Hz-3400Hz frequency band.
Modulation: Modulate the signal of one or several parameters to control the carrier parameters of the change, in accordance with the different control parameters, modulation can be divided into amplitude modulation, frequency modulation and phase modulation.
Coverage: refers to the degree of confidence we have in estimating the state of communication in a given area. A certain radio can talk within 90% of the time and 90% of the coverage within the coverage area.
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 loudspeaker when no information is being transmitted.
Coding Squelch: allows the listener to receive only the information sent to them, only when the received information with a pre-determined code will open the speaker, there are two common squelch code: Tone encoding squelch (CTCSS , PL 38/42 group), digital coding squelch (DPL, DTCSS 84 group)
Antenna: A device specially designed to transmit radio signals and transmit such signals to the atmosphere. This signal can be transmitted much affected by many factors, including antenna plays a major role.
MONITOR: A listening mode used to hear weak signals. By pressing the dedicated key to force the receive signal channel, the operator recognizes the weak sound in the SPK with the ear to achieve the purpose of listening.
Scan (SCAN): In order to hear all the channels of the call, and used a listening mode. By pressing the dedicated key, the receiving circuit in a certain order by a channel to receive a period of time to listen to the signal in the channel. If the time for each channel to receive 100ms, you can scan ten channels per second, the scanning speed of 10ch / s.
Voice Control (VOX): voice control function, through the launch operation can start the voice, the sound transmission out. You do not have to press the PTT key. Stop talking, the launch also stops.
Low voltage alarm: When the battery voltage reaches the preset low voltage value, the intercom will have indicator light flashing and the alarm beep sounded to inform the user should replace the battery or charge the battery.
Power saving function: to save electricity, extend the standby time, the radio in a period of time without launch and key operation, will be shut down for some time, a period of time to boot the work, this approach is called power saving. The switch-on time is approximately 1: 4. When receiving a signal or a key operation, the radio immediately exit the power-saving state, into the normal state.
Squelch Series: the strength of the received signal noise and the strength of the signal was the corresponding relationship, the stronger the signal the stronger the noise. The maximum noise and minimum noise is divided into several files, each file is called a level. Divided into the number of files called squelch series.
Tone elimination: the call, the PTT button to launch the release of the launch, the receiving side will hear the harsh "Cha ..." sound, called the tail. To eliminate this sound, can use some special techniques for processing.
Other: lighting, lock keys, eliminate noise channels, and so on.