High frequency signal, as its name implies, is a signal with higher frequency.In electronics and high-speed digital design, there are differentJudgment criteria。
High frequency is the signal frequency whose frequency is between 3~30MHz, which is only a narrow understanding of high frequency.High frequency includes the frequency range from 3MHz to X00GHz, which can be called high frequency.After receiving the high-frequency signal of a certain channel, the TV shouldFull TV signalThe video image can be reproduced on the screen only after it is demodulated from the high-frequency signal.
In order to spread in the airTV signal, the video full TV signal must be modulated into high frequency or radio frequency（RF- Radio Frequency) signal, each signal occupies one channel, so that multiple TV programs can be transmitted simultaneously in the air without causing confusion.Adopted in ChinaPALSystem, each channel occupies 8MHzBandwidth;The NTSC system is adopted in the United States. TV channels range from 2 to 69. The bandwidth of each channel is 4 MHz. The TV signal band occupies a total of 54 MHz to 806 MHz channels.Cable TVCATV (Cable Television) works in a similar way, except that it transmits TV signals through cables rather than over the air.
Its effectivenessScope of actionShort, only within the visual range,Operating distanceThe distance is 74 km at the altitude of 300 m, depending on the altitude.It is the main communication tool of civil aviation aircraft, which is used for two-way communication between aircrew and ground controllers when the aircraft is taking off, landing or passing through the control airspace.The takeoff and landing period is the busiest time for pilots to deal with problems, and also the time when accidents are most likely to occur in flight, so it must be ensured thatVHF communicationHigh reliability, civil aviation aircraft are generally equipped with more than one set of standby system.
veryHigh frequency communication systemBy transceiverControl boxAnd antenna.For transceiverfrequency synthesizer Provide a stable reference frequency, and then transmit it with the signal through the antenna.The receiving part receives the signal from the antenna and becomesTransmission signal, enter the driver's device.The antenna is knife shaped and is generally installed on the belly and back of the machine.
Specific frequency allocation: 118.000~121.400MHZ, 123.675~128.800MHZ and 132.025~135.975MHZair traffic controlPeople andpilotCall between, mainly 118.000~121.400MHZ;121.100MHZ~121.200MHZ for air flightinformation service； 121.500MHZ is set as the world's unified channel for distress calls.121.600~121.925MHZ is mainly used for ground control;It is worth noting thatcommunication signals It is amplitude modulation. Both sides of the call use the same frequency. When one party finishes transmitting, stop transmitting and wait for the other party's signal.
2. High frequencysignalling system(HF: High Frequency) is a long distancecommunication system 。It uses andShortwave broadcastingWith the same frequency rangeelectromagnetic wave, which usesthe ionosphereAs a result, the communication distance can reach thousands of kilometers, which is used to keep contact with the base and remote terminal during flight.The frequency range used is 2~30MHZ, and every 1KHZ is a channel.Large aircraft are generally equipped with two setsHigh frequency communication system, usingSingle sideband communication, which can greatly reduce the occupiedfrequency bandTo save transmission power.High frequency communicationThe system consists of transceiver unit and antennacoupler, control box and antenna, itsoutput powerLarge, ventilation and heat dissipation device is required.High frequency communication antenna for modern civil aviation aircraft is generally embeddedAircraft skinIt is installed in the tail of the aircraft, but this system is rarely used.
But for high-speedElectronic Design EngineerFor example, the understanding should be more profound. We should not only care about the signalnatural frequency, signals should also be consideredAt launchHigher order of simultaneous concomitant productionharmonicGenerally, we use the following formula to define the signal transmission bandwidth, sometimes called EMI transmission bandwidth:
Generally, when F ＞ 100MHz, it can be calledHigh frequency circuit。So, indigital circuit Medium, whether it is a high-frequency circuit is not determined bysignal frequency And mainly depends onRising edgeandFalling edgeThe width of.According to this formula, when the rising or falling time is less than 10/π ns, we consider it as a high-frequency circuit.
