For the frequency source of vibration and the mechanism of deterioration of the phase noise of the crystal resonator and the corresponding reinforcement measures, this paper analyzes the mechanism and reinforcement measures of the phase noise introduced by the vibration in a coaxial resonator microwave equalizer, and gives the test results. The principle and structure of the coaxial resonator microwave equalizer and the high-power traveling wave tube have an important influence on the performance of radar and electronic countermeasure systems. Since the slow wave line of the traveling wave tube cannot be ideally uniform, the two ends of the attenuator in the tube cannot be completely matched, and the high frequency input of the traveling wave tube and the connection between the traveling wave tube and the pre-stage and post-stage cannot be achieved. For ideal matching and other reasons, when the traveling wave tube is in working state, its gain will fluctuate with frequency. Since the reflections in various places have certain fluctuations for different tubes, the situation of this gain fluctuation is irregular, and the fluctuations between the tubes are not completely consistent. Due to the whole machine fund project: the project supported by the National Natural Science Foundation of China ( 60071031) Telecommunications technology fund project papers can usually only provide equal power excitation to microwave tubes. Gain fluctuations lead to different working conditions of each frequency point during traveling wave tube and other excitations. The frequency points with high gain will be over-excited, and the frequency with low gain The point will work on under-excitation, which makes the output power of the traveling wave tube greatly fluctuate, and at the same time causes the adverse effects of microwave tube efficiency reduction and noise characteristics deterioration. With the continuous development of electronic technology, the requirements of the whole machine system on all aspects of traveling wave tubes continue to increase, and the problem of gain fluctuations has become more prominent. It has become one of the main factors restricting the performance of some tube types and transmitters.
In engineering, a microwave amplitude equalizer is generally used to solve the problem of gain fluctuation in the band of the traveling wave tube. A microwave amplitude equalizer is usually added to the input of the traveling wave tube. The equalizer attenuates the equal excitation power provided by the system to the appropriate excitation power required by the traveling wave tube, so that the output power of each frequency point of the system meets the requirements under equal excitation. The amplitude (attenuation) frequency characteristic of the microwave amplitude equalizer is mutually compensated for the gain frequency characteristic of the traveling wave tube. Because the gain fluctuations of high-power traveling wave tubes are usually irregular, and there will be some changes from tube to tube, the amplitude and frequency characteristics of microwave equalizers are often more complicated, and microwave equalizers need to have strong adjustment capabilities. . Therefore, microwave equalizers mostly use multiple resonators connected in parallel to the main transmission line. A typical structure of a microwave equalizer based on a coaxial resonant cavity is presented. The equalizer is coupled by multiple coaxial resonant cavities through a capacitor at the end of the probe and a coaxial transmission line to achieve quantitative attenuation of a specific frequency.
Due to the result of noise modulation, the spectral distribution of the output signal of the frequency source is no longer an ideal single spectral line, but extends to both sides of the actual signal spectral line of the output signal in the form of modulation sidebands. The phase characteristics of the signal when passing through three basic networks. For any microwave network, if its S is then when the system input is a pure carrier cosωt, the instantaneous value of the output signal can be expressed as if the network S changes with time, the signal Amplitude modulation and phase modulation components will appear. The following analyzes the phase characteristics of the signal through the series impedance, parallel admittance, and transmission line segments with different characteristic impedances.
For series impedance and parallel admittance, the phase factor of S has the same form, and when the reactance (admittance) is small, there are: The project paper of the Telecommunications Technology Fund can be seen from the above formula. The susceptance) component changes with time, and phase modulation is directly introduced. If only the resistance (conductance) component changes with time, when the resistance (conductance) is small, the expansion according to the power series: phase modulation will also be introduced. If both change with time, phase modulation will also be introduced.
For transmission line segments with different characteristic impedances: If the electrical length of the transmission line segment with characteristic impedance Z changes with time, a phase modulation component will be introduced. The single-chamber equivalent circuit of a microwave equalizer with phase noise introduced by random vibration in the equalizer. In the steady state, in the original design of Z, because the vibration characteristics of various components were not considered, and in order to reduce the insertion loss of the system, the main transmission line used air coaxial hard coaxial lines with fixed ends. After calculation, the first-order resonance frequency of the conductor in the coaxial line has fallen within the frequency range of the random vibration spectrum, and the first-order resonance frequency of the conductor in the coaxial resonance cavity has also fallen within the frequency range of the random vibration spectrum. This causes the conductor in the resonant cavity and the conductor in the coaxial main transmission line to deform in random vibration, so that the gap between the end of the conductor in the resonant cavity and the conductor in the main transmission line changes, resulting in a change in the coupling capacitance C. According to the analysis of the equivalent circuit, it can be simplified to consider the residual susceptance introduced by the random vibration at the resonance frequency, and this susceptance is a random process related to random vibration excitation. According to the analysis in the previous section, this random susceptance will cause the carrier to produce a random phase modulation, which introduces phase noise.
In the original design, the inner conductor of the coaxial main transmission line and the SMA flange connector at both ends are connected by a beryllium bronze chuck. Because the inner conductor of the main transmission line is deformed in random vibration, the impedance at the joint will change, which can be equivalent to a Parallel impedance. The contact resistance and related reactance components will change with time due to random vibration. According to the analysis in the previous section, this will also introduce phase noise.
From the above analysis, it can be seen that the phase noise introduced by random vibration is mainly caused by the natural resonance frequency of the parts in the structure falling into or near the frequency band of the random vibration test. Therefore, in the structural design, it is necessary to calculate the natural resonance frequency of the main parts, and to ensure that it does not resonate in the vibration test by adding support, adjusting the structure and dimensions. At the same time, the relevant joints need to be screwed or welded or conductive adhesive to avoid contact modulation caused by vibration and joint reactance changes to introduce noise modulation to generate additional phase noise components.
The modulation of the carrier wave by the random vibration signal generated by the resonance deformation of the part is the reason why the coaxial resonator microwave equalizer generates additional phase noise in the random vibration test. It can be seen from the mechanism that random vibration introduces additional phase noise, this problem may exist in similar microwave structures. In the structural design, it is necessary to calculate the natural resonance frequency of the part to avoid falling into or near the frequency range required by the random vibration test. At the same time, since the influence of amplitude modulation noise in the noise introduced by vibration may not be negligible, phase detector-like methods should be used to measure the phase noise introduced by vibration.
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