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When two crystal oscillators of different frequencies are close together, the following situations may occur:
I. Electromagnetic interference
1. Frequency pulling phenomenon: A crystal oscillator is a piezoelectric crystal oscillator that generates an oscillation signal of a certain frequency when it is working. When two crystal oscillators are close together, the electromagnetic fields between them will affect each other. If the oscillation intensity of one crystal oscillator is large enough, the electromagnetic field it generates may interfere with the oscillation frequency of the other crystal oscillator. For example, a strong alternating electromagnetic field generated by a high-frequency crystal oscillator will induce additional charges on the piezoelectric crystal of another crystal oscillator, thereby changing its oscillation frequency. This phenomenon is very unfavorable in some circuits that require extremely high frequency accuracy, such as frequency sources in high-precision communication equipment. - To use a simple analogy, it is like two adjacent tuning forks. When one tuning fork vibrates strongly and makes a sound (equivalent to the crystal oscillator generating an oscillation signal), the other tuning fork may produce a weak resonance due to the vibration of the air, thereby changing its own vibration frequency.
2. Increased noise: Due to the superposition and interference of the electromagnetic fields of the two crystal oscillators, additional electromagnetic noise will be introduced into the circuit. This noise may affect the normal operation of the circuit modules related to the crystal oscillator. For example, in a system containing an analog signal processing circuit, the noise generated by the crystal oscillator may be coupled to the analog signal line, causing the quality of the analog signal to deteriorate, resulting in signal distortion or reduced signal-to-noise ratio. From a spectrum perspective, this noise will appear within a certain frequency range, interfering with the originally relatively pure crystal frequency signal, making the spectrum messy.
3. Signal crosstalk: The signals of two crystal oscillators may crosstalk with each other. The output signal of a crystal oscillator may enter the circuit path related to another crystal oscillator through electromagnetic coupling, capacitive coupling or inductive coupling. For example, on a printed circuit board (PCB), if the wiring is unreasonable and the signal lines of the two crystal oscillators are too close, signal crosstalk will occur. This crosstalk may cause the receiving circuit to receive the wrong signal, resulting in incorrect operation or wrong judgment of the system. For example, in a digital circuit system, the crosstalk signal may be mistakenly identified as a valid clock signal or data signal, causing confusion in the logic of the system.
II. Mechanical vibration (in some vibration-sensitive scenarios)
Micro-vibration affects each other: The oscillation of a crystal oscillator is essentially a mechanical vibration (at the piezoelectric crystal level). When two crystal oscillators are close to each other, their mechanical vibrations may affect each other. The vibration of one crystal oscillator may be transmitted to another crystal oscillator through a medium such as a circuit board or a casing, thereby changing the vibration characteristics of the other crystal oscillator. This situation is particularly important in some high-precision measuring instruments that are sensitive to vibration. For example, in high-precision atomic clocks and other equipment, tiny vibration interference may affect the frequency stability of the crystal oscillator, and thus affect the timing accuracy of the entire device.
