了解石英晶体振荡器的操作

在本系列的第一部分中，我们研究了一些习惯的重要指标特征频率deviations of quartz crystals。现在，我们将更仔细地研究这些设备的操作。

We’ll first examine the reactance vs. frequency curve of a typical crystal. Armed with this knowledge, we’ll take a look at two different oscillator topologies and discuss how the circuit architecture forces the crystal to oscillate at a particular frequency.

Based on this discussion, we’ll be able to look at the definition for parallel and series crystals—two technical terms that can sometimes cause confusion.

晶体具有两个共振频率

这equivalent electrical circuitfor a crystal is shown in Figure 1.

图1。图像提供STMicroelectronics.

Based on this model, we can find the reactance vs. frequency curve of a typical quartz crystal unit as depicted in Figure 2:

图2。图像提供柏。

为了深入了解晶体的操作，让我们假设晶体是理想的，R_mis negligible. Hence, in the lower branch of the crystal electrical model, we have L_m一种nd C_min series.

When L_m一种nd C_m在串联谐振,阻抗取消e一种ch other out. At this frequency, the impedance of the lower branch and consequently, the total impedance across the crystal drops to zero. This corresponds to f_sin Figure 2 that is commonly referred to as the series resonant frequency of the crystal. Note that C_o不会影响此频率的价值。

Just above f_s, the reactance of L_m比C大_m我们观察到晶体表现出诱导行为。该有效电感的电抗（L的系列组合_m一种nd C_m) increases with frequency and at a certain frequency (f_一种），它变得等于C的电抗_oin the crystal model. At this point, we effectively have a parallel LC in resonance and the total impedance of the crystal approaches infinity. The frequency f_一种称为抗谐振频率。该频率总是高于串联谐振频率。

At What Frequency Is the Crystal Going to Oscillate?

我们看到晶体具有两种共振模式。在两个f_s一种nd f_一种, the impedance of the crystal is resistive. At f_s，电阻很小；但是，在抗谐振频率下，晶体接近无穷大的等效阻抗。

现在要问的问题是，在振荡器电路中使用时晶体将在哪个频率下振荡？

答案是，这取决于振荡器拓扑。

在振荡频率下，振荡器的回路增益必须等于或大于一个，其相移应为2π的积分倍数（正反馈）。这些条件决定了晶体的振荡频率。

例如，考虑图3所示的振荡器。

图3

In this case, the phase shift of the amplification stage is an integral multiple of 2π. Hence, at the oscillation frequency, the phase shift caused by the crystal and R₁应该为零。可以在晶体具有纯电阻阻抗的频率下实现此零相移（F_s一种nd f_一种).

At f_s, the impedance of the crystal is minimal and, hence, the voltage divider created by the crystal and R₁has a larger gain as depicted in the above figure. Therefore, with the above arrangement, the circuit can oscillate at f_s。

Another oscillator topology, commonly known as the Pierce-Gate oscillator, is depicted in Figure 4.

Figure 4.示例Pierce-Gate振荡器。图像提供Ramon Cerda。

With this topology, the amplifier provides a phase shift of 180°. Hence, the network of R_s, C₂, C₁，晶体应提供180°的额外相移以满足振荡相条件。随着放大器输出信号通过反馈路径传播，它经历了从晶体和C的某些相移₁combination. The amount of this phase shift depends on the signal frequency.

Below f_s，晶体充当电容器，并从x进行相移₁一种nd C₁is near 0°. At f_s, the crystal has resistive impedance and this phase shift is about 90°. Above f_s, the crystal exhibits inductive behavior, and the phase shift can be close to 180°.

在实践中，r提供的相移_s一种nd C₂is less than 90° and hence, the combination of X₁一种nd C₁需要提供90°以上。这就是为什么晶体需要在其感应区域的某个地方运行的原因（在F之间_s一种nd f_一种在图2中）。

Parallel-Resonant and Series-Resonant Oscillators

这一种bove discussion shows that a quartz crystal can oscillate at any frequency between the series resonant frequency (f_s）和抗谐振频率（f_一种）取决于振荡器拓扑。

许多常见的振荡器电路，例如Pierce，Colpitts和Clapp Style振荡器，在F之间运行该区域的晶体_s一种nd f_一种。该区域通常称为“平行共振区域”，迫使晶体在该区域中运行的振荡器称为“平行谐振振荡器”。

迫使晶体在F处运行的振荡器_s一种re not very common. These oscillators are referred to as “series-resonant oscillators”. It is worthwhile to mention that the anti-resonance point is not used in oscillator designs.

Parallel and Series Resonant Crystals

这re are two technical terms in the crystal industry that can occasionally cause confusion: “parallel-resonant crystal” (or simply parallel crystal) and “series-resonant crystal” (or series crystal).

Parallel crystals are intended for use in parallel-resonant oscillators. Since a parallel-resonant oscillator operates the crystal somewhere between fs and f_一种, the nominal frequency of a parallel crystal is a frequency in this range, i.e., in the crystal’s “area of parallel resonance”.

另一方面，系列晶体旨在用于串联振荡器中。因此，晶体的标称频率与其串联谐振频率相同（f_s).

Is there any physical difference between these two types of crystal?

我们知道，每个晶体都有其特定的串联谐振频率和“平行共振区域”。我们可以在这两个共振条件下的任何一个中操作给定的晶体。因此，平行晶体和串联晶体的物理结构没有差异。

这se two terms are only about the conditions under which the crystal will oscillate at its nominal frequency.

Do they specify with what type of oscillator topology the crystal will hit its nominal frequency? Is it a parallel-resonant oscillator or a series-resonant type?

Load Capacitance

Load capacitance refers to the amount of external capacitance that the crystal should “see” across its terminals. With series resonant oscillators, there are no reactive components in the oscillator feedback path (please see the example oscillator shown in Figure 3). That’s why, for series crystals, the load capacitance is not important (and is not specified).

但是，使用平行晶体，负载电容是一个关键参数。在这种情况下，晶体用于其电抗曲线的电感​​区域。并且，晶体形成具有外部负载电容的LC储罐。因此，负载电容的值起着关键作用并确定振荡频率。

A parallel crystal is actually calibrated at the factory to oscillate at its nominal frequency when connected to its specified load capacitance. To achieve the nominal frequency, our application board should provide the same load capacitance.

在下一篇文章中，我们将更详细地讨论负载电容的效果。

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