What is spread spectrum?
Spread spectrum is an information processing and transmission technology. Spread-spectrum technology is to spread the spectrum of the transmitted signal using codes that are not related to the data (information) to be transmitted, so that it occupies far more than the minimum bandwidth necessary for the transmitted information.
Principle of spread spectrum technology
The information input at the sending end is first modulated by the information to form a digital signal, and then the spreading code sequence generated by the spreading code generator modulates the digital signal to broaden the frequency spectrum of the signal. The stretched signal is then modulated to the radio frequency and sent out. The broadband radio frequency signal received at the receiving end is converted to an intermediate frequency, and then the locally generated spreading code sequence that is the same as the transmitting end is de-correlated and de-spread. The information is demodulated and restored to the original information output.
It can be seen from this that the general spread spectrum communication system has to perform three times of modulation and corresponding demodulation. Primary modulation is information modulation, secondary modulation is spread spectrum modulation, tertiary modulation is radio frequency modulation, and corresponding information demodulation, despreading and radio frequency demodulation. Compared with general communication systems, spread spectrum communication is more spread spectrum modulation and despreading.
Spread spectrum working method
According to different ways of spreading spectrum, existing spread spectrum communication systems can be divided into:
Direct Spread
Direct sequence spread spectrum (DirectSequenceSpreadSpectrum) working mode, referred to as direct spread (DS) mode. The so-called direct sequence (DS-DirectSequency) spreading is to directly use a spreading code sequence with a high code rate to spread the signal spectrum at the sending end. At the receiving end, the same spreading code sequence is used for despreading, and the spread spreading signal is restored to the original information. The principle of direct sequence spread spectrum is shown in the figure.
Carry out two-phase phase shift keying modulation on a certain carrier wave with a narrow pulse sequence. If a balanced modulator is used, the modulated output is a two-phase phase shift keying signal, which is equivalent to an amplitude-modulated double-sideband signal with carrier suppression. In the figure, the frequency of the input carrier signal is fc, and the spectral function of the narrow pulse sequence is G (C), which has a very wide frequency band. The output of the balanced modulator is twice the width of the pulse spectrum, and the spread spectrum signal of the double-sided band of fc being suppressed has a spectrum function of fc + G (C). Apply the same balanced modulator as the despreader at the receiving end. The spread spectrum signal with frequency spectrum fc + G (C) can be re-modulated with the same code sequence to restore it to the original carrier signal fc.
Frequency hopping
Frequency hopping (FrequencyHopping) working mode, referred to as frequency hopping (FH) mode. Another way to spread the signal spectrum is called frequency hopping (FH-Frequency Hopping). The so-called frequency hopping means more precisely: multi-frequency frequency shift keying with a certain code sequence for selection. That is to say, the spreading code sequence is used to perform frequency shift keying modulation, and the carrier frequency is continuously hopped, so it is called frequency hopping.
Simple frequency shift keying, such as 2FSK, has only two frequencies, representing the mark and the space, respectively. The frequency hopping system has several, dozens, or even a frequency, which is selected and controlled by the combination of the transmitted information and the spreading code, and continuously hops.
Time-hopping
TIme Hopping working mode, referred to as time-hopping (TH) mode. Similar to frequency hopping, time hopping (TH-TIme Hopping) makes the transmitted signal hop on the time axis. First divide the time axis into many time slices. In a frame, which chip transmission signal is controlled by the spreading code sequence. Time hopping can be understood as: time-shift keying of multiple time slices selected with a certain code sequence.
Since a much narrower time slice is used to transmit the signal, the spectrum of the signal is relatively broadened. The figure on the right is the principle block diagram of the time-hopping system. At the sending end, the input data is stored first, and the on-off switch is controlled by the spreading code sequence of the spreading code generator. After two-phase or four-phase modulation, it is transmitted after being modulated by radio frequency. At the receiving end, the intermediate frequency signal output by the radio frequency receiver is controlled by the same spreading code sequence generated locally as the sending end to control the on-off switch, and then sent to the data storage and retiming through the two-phase or four-phase demodulator. data. As long as the sending and receiving ends are strictly synchronized in time, the original data can be restored correctly.
The tubular oil level gauge is used to judge whether the transformer is short of oil through the color in the observation window by floating the float up and down. Generally, the observation is full of red marks indicating that the transformer tank is short of oil and needs to be replenished; Patrol inspectors of glass tube type oil level gauge can directly see the amount of transformer oil through the height of the oil level line in the glass tube. When the oil level is below - 30 °, oil replenishment is required
This series of products are mainly applicable to oil immersed fully sealed transformers, which are installed on the upper part of the transformer oil tank cover to display the oil level in the oil tank. The oil level is normal when the blue mark appears in the upper window of the oil level gauge, and the oil level is above the tank cover; If the red mark appears, it means that the oil pointer is abnormal, and the oil level is below the tank cover, so it is necessary to add oil properly.
Tubular Oil Level Gauge,Special Oil Level Gauge,Screw Oil Level Indicator,Screw Type Oil Level Indicator
Henan New Electric Power Co.,Ltd. , https://www.newelectricpower.com