All quartz crystals have spurious resonances (unwanted resonance) besides the main resonance frequency. They are represented in the equivalent circuit diagram(Fig.5) by additional resonant circuits in parallel with R1, L1, C1. The ratio of spurious resonance resistance R NW to resonance resistance Rr of the main wave is generally specified in the attenuation constant dB and designated spurious attenuation a NW : a NW =-20 · lg
For oscillator crystals 3 to 6 dB are normally sufficient. For filter crystals attenuations greater than 40 dB are often required. This can only be achieved by special design techniques and involves the use of very small values of the dynamic capacity C1.
The achievable attenuation decreases with higher frequency and with higher orders of overtone. It is found generally that plano-parallel quartz resonators have better spurious attenuation than plano-convex resonators. In specifying spurious resonance parameters it is necessary to give an indication of both the acceptable attenuation level desired and their frequencies relative to main resonance frequency.
Resonance in the region of +40 to +150 KHZ for plano parallel resonators biconvex or plano-convex resonators.
In the passive measurement method indicated above, spurious resonance attenuations up to 20 to 30 dB can be measured. With higher attenuations, Co compensation is necessary.
所有石英谐振器均有寄生（在主频率之外的不期望出现的）振荡响应。他们在等效电路图中表现为附加的以 R1 、 L1 、 C1 形成的响应回路。
寄生响应的阻抗 R NW 与主谐振波的阻抗 Rr 的比例通常以衰减常数 dB 来表示 , 并被定义为寄生衰减 a NW =-20 · lg
对于振荡用晶体, 3 至 6dB 是完全足够的 . 对于滤波用晶体 , 通常的要求是超过 40dB. 这一规格要求只有通过特殊设计工艺并使用数值非常小的动态电容方能达到 .
可达到的衰减随着频率的上升和泛音次数的增加而减小 . 通常的平面石英晶片谐振器比平凸或双面凸晶片谐振器的寄生衰减要良好 . 在确定寄生响应参数时 , 应同时确定一个可接受的寄生衰减水平以及寄生频率与主振频率的相对关系 .
在 AT 切型中 , 对于平面晶片 ," 不和谐的响应 " 只存在于主响应的 +40 至 +150KHZ 之间 , 对于平凸或双面凸的晶片 , 寄生则在 +200 至 +400KHZ 之间 .
在以上的测量方法中 , 寄生响应衰减至 20 至 30dB 时是可以测量的 , 对于再高一些的衰减 .C0 的补偿是必需的 .