遥遥领先MTRONPTI晶体滤波器词汇表

Linear Phase 

Family of filters, including Gaussian, Bessel and their derivatives, all roll off slowly at the band edge and consequently have reduced or minimal delay peaks. The rounded passband of these filters occurs because power is reflected to the source as frequency deviates from nominal center frequency. Consequently, the return loss of these filters is poor away from center frequency. Classical network theory shows that sensitivity to changes in element values accompanies poor return loss. Consequently this family demands tighter control of components and delay performance often departs from theoretical predictions. This sensitivity also results in increased manufacturing cost. In general, these designs work more predictably if the number of poles are restricted.

Intermodulation (IM) 

Occurs when a filter acts in a nonlinear manner causing incident signals to mix. The new frequencies that result from this mixing are called intermodulation products. They are normally third-order products, and for 1 dB increase in incident signal levels, the IM products increase by 3 dB. Out-of-band intermodulation occurs when two incident signals (typically -20 to -30 dBm) in the filter stopband produce an IM product in the filter passband. This IM is most prevalent in receiver application when an input signal is present simultaneously in the first and second channels adjacent to the passband of the filter. The IM performance of crystal filters at low signal levels is determined by surface defects associated with the resonator manufacturing processes and is not subject to analytical prediction. In-band modulation occurs when two closely spaced signals within the filter passband cause IM products that are also within the filter passband. It is most prevalent in transmit applications where signal levels are high, typically between -10 dBm and +10 dBm, but it can also occur in some receiver applications. The IM performance at high signal levels is a function of both the resonator manufacturing processes and nonlinear elastic properties of quartz. The latter is dominate at higher signal levels, and can be analyzed.

Input & Output Impedance 

Impedances presented by the filter to the outside world. They normally have both resistive and reactive components and change with frequency. The impedances may be expressed in terms of VSMR, return loss, resistance and reactance or magnitude and phase angle. Sometimes a user may wish to specify return loss of VSWR limits. Under these circumstances it is important to remember that all commonly used crystal filter designs are based on reflective rather than absorptive theory. This is demonstrated by the stopband products by an ideal lossless filter. Since a lossless filter can have no resistance to absorb power, it must attenuate by reflecting power. For example, at the 3 dB passband edge, half of the incident power is reflected; the return loss has already reduced to 3dB and the VSWR is 5.8:1. From this it can be appreciated that constant impedance is impossible to achieve except by the incorporation of loss pads or compensation networks. The problem is usually exacerbated when the effects of dissipation are included. Specifications on return loss and VSWR are best restricted to the flat portion of the filter response in the center of the passband and should make allowance for filter component tolerances as well as non-ideal terminations.

Vibration-Included Sidebands 

May appear on a crystal filter output signal when the filter is subject to acceleration forces due to vibration. Quartz crystal resonators, being piezoelectric devices, convert mechanical to electric energy. Therefore, the resonant frequency of a crystal is modulated at the frequency of vibration. The peak deviation of this frequency modulation is determined by the acceleration sensitivity of the crystal and the amplitude of vibration. If all crystals in a filter deviate by the same amount and the same time (i.e. in phase with each other) the filter response will oscillate about the nominal center frequency at a rate equal to the frequency of vibration. Because the insertion phase shift of the filter is a function of frequency, as the filter changes frequency the phase shift imposed on a CW signal will also change, i.e., the CW signal will be phase modulated at the frequency of vibration. In most instances, the crystal frequency is deviated a fraction of a ppm. Filter bandwidths are comparatively wide with a corresponding low insertion phase slope; consequently, the phase-modulated sidebands are often of no concern. However, narrowband spectrum clean up filters may require special attention. Sideband generation is minimized by minimizing the acceleration sensitivity of the resonator and by control of mechanical resonances within the filter structure. Reduction of resonator acceleration sensitivity is a current research topic in a number of organizations

Phase Noise 

Can be introduced by a crystal filter under static conditions as well as under vibration. It is associated with resonator defects and can be minimized by proper processing and is generally confined to the passband. Crystal filters can, however, improve the phase noise floor for crystal oscillators

Noise Bandwidth 

The noise bandwidth is the band width of an ideal filter which would pass the same amount of white noise as the filter under test. The noise bandwidth indicates power at the filter output, hence often serves as a performance measure for comparing filters. The noise band width is primarily controlled by the passband. The Butterworth and Chebychev family of filters, because of their more rapid transition from passband to stopband, have a smaller noise bandwidth than do that flat delay type filters. 

遥遥领先MTRONPTI晶体滤波器词汇表

线性相位
滤波器家族，包括高斯滤波器、贝塞尔滤波器及其导数，都在频带边缘缓慢衰减，因此具有减小的或最小的延迟峰值。这些滤波器的圆形通带之所以出现，是因为当频率偏离标称中心频率时，功率被反射到源。因此，这些SAW滤波器的回波损耗在远离中心频率处较差。经典网络理论表明，对元素值变化的敏感性伴随着较差的回波损耗。因此，该系列要求对组件进行更严格的控制，并且延迟性能经常偏离理论预测。这种敏感性还导致制造成本增加。一般来说，如果极点的数量受到限制，这些设计的工作更具可预测性。

互调（IM）
当筛选器以非线性方式操作导致事件信号混合时发生。这种混合产生的新频率称为互调产物。它们通常是三阶乘积，入射信号电平增加1dB，IM乘积增加3dB。当MTRONPTI麦特伦皮滤波器阻带中的两个入射信号（通常为-20至-30 dBm）在滤波器通带中产生IM产物时，会发生带外互调。当输入信号同时出现在与滤波器通带相邻的第一和第二通道中时，这种IM在接收器应用中最为普遍。晶体滤波器在低信号电平下的IM性能由与谐振器制造工艺相关的表面缺陷决定，并且不受分析预测的影响。当滤波器通带内的两个紧密间隔的信号导致IM产物也在滤波器通带中时，会发生带内调制。它在信号电平较高的发射应用中最为普遍，通常在-10 dBm和+10 dBm之间，但也可能出现在一些接收机应用中。高信号电平下的IM性能是谐振器制造工艺和石英非线性弹性特性的函数。后者在较高的信号电平下占主导地位，并且可以进行分析。

输入和输出阻抗
过滤器向外界提供的阻力。它们通常同时具有电阻和无功分量，并随频率变化。阻抗可以用VSMR、回波损耗、电阻和电抗或幅度和相位角来表示。有时，用户可能希望指定VSWR限制的回波损耗。在这种情况下，重要的是要记住，所有常用的贴片晶体滤波器设计都是基于反射而非吸收理论。理想无损滤波器的阻带积证明了这一点。由于无损滤波器可能没有吸收功率的电阻，因此它必须通过反射功率来衰减。例如，在3dB通带边缘，一半的入射功率被反射；回波损耗已经降低到3dB，VSWR为5.8:1。由此可以理解，除了通过结合损耗焊盘或补偿网络之外，不可能实现恒定阻抗。当包括耗散的影响时，这个问题通常会加剧。回波损耗和VSWR的规格最好限制在通带中心的滤波器响应的平坦部分，并且应考虑滤波器部件公差以及非理想终端。

包含振动的侧带
当滤波器由于振动而受到加速力时，可能会出现在晶体滤波器输出信号上。石英晶体谐振器是一种压电装置，可将机械能转换为电能。因此，晶体的谐振频率被调制为振动频率。这种频率调制的峰值偏差由晶体的加速度灵敏度和振动幅度决定。如果滤波器中的所有晶体都偏离相同的量和时间（即彼此同相），陶瓷滤波器响应将以等于振动频率的速率围绕标称中心频率振荡。因为滤波器的插入相移是频率的函数，所以当滤波器改变频率时，施加在CW信号上的相移也将改变，即，CW信号将以振动频率进行相位调制。在大多数情况下，石英晶振频率偏离一个ppm的分数。滤波器带宽相对较宽，具有相应的低插入相位斜率；因此，相位调制的边带通常是无关紧要的。然而，窄带频谱清理滤波器可能需要特别注意。通过最小化谐振器的加速度灵敏度和通过控制滤波器结构内的机械谐振来最小化边带的产生。谐振器加速度的降低敏感性是许多组织当前的研究主题。

相位噪声
可以通过晶体滤波器在静态条件下以及在振动条件下引入。它与谐振器缺陷有关，可以通过适当的处理将其最小化，并且通常局限于通带。然而，晶体滤波器可以改善晶体振荡器的相位噪声基底。

噪声带宽
噪声带宽是理想滤波器的带宽，该滤波器将通过与被测试滤波器相同量的白噪声。噪声带宽表示滤波器输出处的功率，因此通常用作比较滤波器的性能度量。噪声带宽主要由通带控制。Butterworth和Chebychev滤波器家族从通带到阻带的转换速度更快，因此其噪声带宽比平坦延迟型滤波器更小。