What Is High Frequency Ventilation?

High-frequency signals, as the name implies, are higher frequency signals. There are different judgment standards in electronics and high-speed digital design.

Basic Information

Chinese name

High frequency signal

Foreign name

High frequency signal

Definition

Higher frequency signals

Frequency band

Related to the division

High frequency is a signal frequency between 3 and 30MHz, which is only a narrow understanding of high frequency. The high frequency includes the frequency range from 3MHz to X00GHz and can be called high frequency. After receiving a high-frequency signal from a certain channel, the television must demodulate the entire television signal from the high-frequency signal in order to reproduce the video image on the screen.

In order to be able to propagate TV signals in the air, the entire video signal of the video must be modulated into high-frequency or radio-frequency (RF-Radio Frequency) signals. Each signal occupies a channel. . Adopted by our country

1. Very high frequency communication system (VHF: Very High Frequency) uses very high frequency radio waves.

Its effective range is short, only within the visual range, the range of action varies with altitude, and the distance is 74 kilometers at a height of 300 meters. It is the main communication tool for civil aviation aircraft at present. It is used for two-way voice communication between the crew and the ground control personnel when the aircraft is taking off, landing, or through controlling airspace. The take-off and landing periods are the busiest times for pilots to deal with problems and the most prone to accidents during flight. Therefore, VHF communication must be highly reliable. Civil aviation aircraft are generally equipped with more than one backup system.

VHF communication system is composed of three parts: transceiver unit, control box and antenna. The transceiver unit uses a frequency synthesizer to provide a stable reference frequency, which is then transmitted through the antenna with the signal. The receiving part receives the signal from the antenna, after amplification, detection, and noise suppression, it becomes an audio signal, which is input to the driver's headset. The antenna is knife-shaped and is generally installed on the belly and back of the machine.

The frequency range used by VHF is 118.000 ~ 135.975MHZ in accordance with the unified regulations of ICAO. Every 25KHZ is a channel, and 720 channels can be set for selection by aircraft and ground console.

The specific frequency allocations are: 118.000 ~ 121.400MHZ, 123.675 ~ 128.800MHZ, and 132.025 ~ 135.975MHZ. The three frequency bands are mainly used for communication between air traffic controllers and aircraft pilots, which are mainly concentrated in 118.000 ~ 121.400MHZ; 121.100MHZ ~ 121.200 MHZ is used for air flight information service; 121.500MHZ is designated as the world s unified channel for distress calls. 121.600 ~ 121.925MHZ is mainly used for ground control; it is worth noting that the communication signal is amplitude-modulated. The two parties on the call use the same frequency.

2. High-frequency signal system (HF: High Frequency) is a long-distance communication system. It uses electromagnetic waves with the same frequency range as short-wave broadcasting. It uses the reflection of the ionosphere, so the communication distance can reach thousands of kilometers, and it is used to maintain communication with bases and distant terminals in flight. The frequency range used is 2 ~ 30MHZ, one channel per 1KHZ. Large aircraft are generally equipped with two sets of high-frequency communication systems, using single-sideband communication, which can greatly compress the occupied frequency band and save transmission power. The high-frequency communication system is composed of a transceiver unit, an antenna coupler, a control box, and an antenna. Its output power is large, and it needs a ventilation and heat dissipation device. High-frequency communication antennas used in modern civil aircraft are generally buried in the skin of the aircraft and installed at the rear of the aircraft, but this system is rarely used at present.

High and low frequency division:

ELF below 3KHZ

VLF 3 ~ 30KHZ

Low frequency LF 30 ~ 300KHZ

MF 0.3 ~ 3MHZ

HF HF 3 ~ 30MHZ

VHF VHF 30 ~ 300MHZ (TV channels 1--12)

UHF UHF 0.3 ~ 3GHZ (TV channel 13 or above)

UHF SHF 3 ~ 30GHZ

There are also divisions like this:

The frequency is divided according to regulations so as to have a professional communication language:

Ultra-low frequency: 0.03 ~ 300Hz

Extremely low frequency: 300 ~ 3000Hz (audio)

VLF: 3 ~ 300KHz

But for high-speed electronic design engineers, the understanding should be deeper. In addition to the natural frequency of the signal, we should also consider the impact of the higher-order harmonics that accompany the signal transmission. Generally, we use the following formula to define the signal Transmission bandwidth, sometimes called EMI emission bandwidth:

F = 1 / (Tr * )

F is the frequency (in GHz), and Tr is the rise or fall time (in ns) of the signal.

Usually when F> 100MHz, it can be called a high frequency circuit. Therefore, in a digital circuit, whether it is a high-frequency circuit does not depend on the level of the signal frequency, but mainly depends on the width of the rising and falling edges. According to this formula, when the rise or fall time is less than 10 / ns, we consider it to be a high-frequency circuit.