In the tuner of a satellite television receiver, the local oscillator operates at a high frequency and is in the microwave band. According to the requirements of down-conversion, the local oscillation frequency is different from the input signal frequency by an intermediate frequency. The frequency range of the first intermediate frequency is 950MHZ~1450MHZ. For the C-band, the high local oscillator frequency scheme is usually adopted, and the frequency range of the input signal is 3.70GHZ~4.20GHZ. The local oscillator frequency is 5.15 GHz. For the KU band, a low local oscillator scheme is usually used. The frequency range of the input signal is 11.70 GHz to 12.20 GHz, and the local oscillator frequency is 1.075 GHz.
Since the high frequency head is placed outside, the ambient temperature changes to form a large temperature difference, which often causes the vibration frequency of the present to drift, thereby causing a "running platform" phenomenon. Therefore, it is required that the maximum frequency deviation generated by the local oscillator frequency in different ambient temperatures (-330~+60°C) is less than ±2MHZ.
There are three kinds of frequency stabilization methods commonly used in oscillators: crystal frequency multiplication method, phase-locked loop frequency stabilization method and high-Q resonance frequency multiplication method, which are respectively introduced below.
1. Crystal frequency multiplication method: Because the crystal working frequency is generally not very high, a crystal oscillation circuit is first formed at a lower frequency, and then a high-order harmonic of the oscillation frequency is obtained by using a nonlinear circuit, and is selected by a frequency selection circuit. The higher harmonic component is output and amplified. Due to the large multiple of the multiplier, it is usually necessary to use a multi-stage multiplier cascade. Although this kind of frequency stabilization method can obtain higher stability, the structure is complicated, the volume is large, the efficiency is low, and the harmonic interference is large, which is only applied in the early microwave communication.
2. Phase-locked loop frequency stabilization method: The microwave oscillator is composed of a voltage oscillator. A part of the energy is taken out at the output end. After being divided by the microwave, the phase is compared with the reference frequency generated by a crystal oscillator, and the obtained error signal is passed through the loop. After smooth filtering, the path filter is used to control the control voltage of the microwave pressure oscillator so that the frequency of the oscillator is locked to the reference frequency. This frequency stabilization method can keep the frequency stability of the microwave oscillator and the frequency stability of the crystal oscillator in the same order of magnitude, the best effect, the phase noise of the local oscillator is the lowest, but the circuit is more complicated and the cost is higher, mainly used for satellite digital. TV, satellite digital communication and other requirements are high. Under the condition of 25 °C test, for the C-band and KU-band single-vibration single-polarization digital wide-band phase-locked loop high-frequency head, the PLL phase-locking technique is adopted, the local oscillator has good stability, and the general local oscillator frequency stability is ± 100KHZ; for C/KU dual-band dual local oscillator dual-polarized single-output (or dual-output) tuner, the general local oscillator frequency stability is ±500KHZ.
3. High-Q resonant cavity frequency stabilization method: The high-Q resonant cavity frequency stabilization method does not need to divide or octave the oscillation frequency, but directly stabilizes the oscillator at the working frequency, and the frequency stabilization effect is good. Bad is related to the Q value of the high Q resonator. The higher the Q value, the better the frequency stabilization effect. Generally, the no-load Q value of the high-Q resonator is as high as several thousand or even tens of thousands. At present, commonly used dielectric resonators have a square, a cylindrical shape and a circular shape, and the cylindrical shape is used most. The natural resonant frequency of a dielectric resonator refers to the resonant frequency of a dielectric resonator when it is in free space. In actual use, when it is placed on other dielectric materials or conductors, its resonant frequency changes because of a part of the electromagnetic field distribution. In its appearance, when it is close to other media or conductors, its external electromagnetic field distribution law will change. In general, when it is close to the metal conductor, the resonant frequency will increase, and the resonant frequency will be close to the dielectric material. Will fall. We are using this feature to slightly change the resonant frequency of the dielectric oscillator by slightly adjusting the distance between the metal conductor and the dielectric oscillator.
The coupling of the dielectric oscillator to other circuits (such as microstrip circuits) is also solved by the external electromagnetic field distribution characteristics of the dielectric oscillator. The energy coupling is realized by the magnetic field of the dielectric oscillator and the microstrip line on the microstrip circuit. The strength of the coupling can be adjusted by changing the distance between the two, but the loaded Q value of the dielectric oscillator will be reduced. It must be selected according to actual needs.
The high-Q resonator frequency-stabilized dielectric oscillator has high Q value and small volume, and the stability coefficient can be selected according to needs, and the cost is relatively low. The microwave oscillator with such a dielectric oscillator is stable in frequency and complexity. Both the degree and the cost are much better than the first two methods, and the performance is fully satisfied with the requirements of satellite TV reception. Therefore, most of the current high-frequency local oscillators of satellite television receivers use this frequency stabilization method.
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