遙遙領先MTRONPTI晶體濾波器詞匯表

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.

遙遙領先MTRONPTI晶體濾波器詞匯表

線性相位
濾波器家族，包括高斯濾波器、貝塞爾濾波器及其導數(shù)，都在頻帶邊緣緩慢衰減，因此具有減小的或最小的延遲峰值。這些濾波器的圓形通帶之所以出現(xiàn)，是因為當頻率偏離標稱中心頻率時，功率被反射到源。因此，這些SAW濾波器的回波損耗在遠離中心頻率處較差。經(jīng)典網(wǎng)絡理論表明，對元素值變化的敏感性伴隨著較差的回波損耗。因此，該系列要求對組件進行更嚴格的控制，并且延遲性能經(jīng)常偏離理論預測。這種敏感性還導致制造成本增加。一般來說，如果極點的數(shù)量受到限制，這些設計的工作更具可預測性。

互調（IM）
當篩選器以非線性方式操作導致事件信號混合時發(fā)生。這種混合產(chǎn)生的新頻率稱為互調產(chǎn)物。它們通常是三階乘積，入射信號電平增加1dB，IM乘積增加3dB。當MTRONPTI麥特倫皮濾波器阻帶中的兩個入射信號（通常為-20至-30 dBm）在濾波器通帶中產(chǎn)生IM產(chǎn)物時，會發(fā)生帶外互調。當輸入信號同時出現(xiàn)在與濾波器通帶相鄰的第一和第二通道中時，這種IM在接收器應用中最為普遍。晶體濾波器在低信號電平下的IM性能由與諧振器制造工藝相關的表面缺陷決定，并且不受分析預測的影響。當濾波器通帶內的兩個緊密間隔的信號導致IM產(chǎn)物也在濾波器通帶中時，會發(fā)生帶內調制。它在信號電平較高的發(fā)射應用中最為普遍，通常在-10 dBm和+10 dBm之間，但也可能出現(xiàn)在一些接收機應用中。高信號電平下的IM性能是諧振器制造工藝和石英非線性彈性特性的函數(shù)。后者在較高的信號電平下占主導地位，并且可以進行分析。

輸入和輸出阻抗
過濾器向外界提供的阻力。它們通常同時具有電阻和無功分量，并隨頻率變化。阻抗可以用VSMR、回波損耗、電阻和電抗或幅度和相位角來表示。有時，用戶可能希望指定VSWR限制的回波損耗。在這種情況下，重要的是要記住，所有常用的貼片晶體濾波器設計都是基于反射而非吸收理論。理想無損濾波器的阻帶積證明了這一點。由于無損濾波器可能沒有吸收功率的電阻，因此它必須通過反射功率來衰減。例如，在3dB通帶邊緣，一半的入射功率被反射；回波損耗已經(jīng)降低到3dB，VSWR為5.8:1。由此可以理解，除了通過結合損耗焊盤或補償網(wǎng)絡之外，不可能實現(xiàn)恒定阻抗。當包括耗散的影響時，這個問題通常會加劇。回波損耗和VSWR的規(guī)格最好限制在通帶中心的濾波器響應的平坦部分，并且應考慮濾波器部件公差以及非理想終端。

包含振動的側帶
當濾波器由于振動而受到加速力時，可能會出現(xiàn)在晶體濾波器輸出信號上。石英晶體諧振器是一種壓電裝置，可將機械能轉換為電能。因此，晶體的諧振頻率被調制為振動頻率。這種頻率調制的峰值偏差由晶體的加速度靈敏度和振動幅度決定。如果濾波器中的所有晶體都偏離相同的量和時間（即彼此同相），陶瓷濾波器響應將以等于振動頻率的速率圍繞標稱中心頻率振蕩。因為濾波器的插入相移是頻率的函數(shù)，所以當濾波器改變頻率時，施加在CW信號上的相移也將改變，即，CW信號將以振動頻率進行相位調制。在大多數(shù)情況下，石英晶振頻率偏離一個ppm的分數(shù)。濾波器帶寬相對較寬，具有相應的低插入相位斜率；因此，相位調制的邊帶通常是無關緊要的。然而，窄帶頻譜清理濾波器可能需要特別注意。通過最小化諧振器的加速度靈敏度和通過控制濾波器結構內的機械諧振來最小化邊帶的產(chǎn)生。諧振器加速度的降低敏感性是許多組織當前的研究主題。

相位噪聲
可以通過晶體濾波器在靜態(tài)條件下以及在振動條件下引入。它與諧振器缺陷有關，可以通過適當?shù)奶幚韺⑵渥钚』⑶彝ǔ＞窒抻谕◣АＨ欢w濾波器可以改善晶體振蕩器的相位噪聲基底。

噪聲帶寬
噪聲帶寬是理想濾波器的帶寬，該濾波器將通過與被測試濾波器相同量的白噪聲。噪聲帶寬表示濾波器輸出處的功率，因此通常用作比較濾波器的性能度量。噪聲帶寬主要由通帶控制。Butterworth和Chebychev濾波器家族從通帶到阻帶的轉換速度更快，因此其噪聲帶寬比平坦延遲型濾波器更小。

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