Crystal oscillators form a vast family, encompassing various types such as simple packaged clock oscillators (SPXO), voltage-controlled crystal oscillators (VCXO), temperature-compensated crystal oscillators (TCXO), oven-controlled crystal oscillators (OCXO), and digital temperature-compensated crystal oscillators (MCXO or DTCXO). Each type exhibits unique performance characteristics, including critical indicators such as phase noise and jitter.
- What are Phase Noise and Jitter? Jitter refers to the deviation in timing of an event from its ideal timing. It is measured in units of fs (femtoseconds, 10^-15 seconds) or ps (picoseconds, 10^-12 seconds). When measured with instruments, it manifests as the frequency domain characteristics of a signal, termed "phase noise." Essentially, these two terms describe the same phenomenon but in different ways.
- ?Jitter: Jitter is classified into deterministic jitter (DJ) and random jitter (RJ). DJ typically has a limited amplitude and is expressed in units of time, while RJ follows a Gaussian distribution and is measured in RMS (Root Mean Square) values. Oscillator jitter is often caused by noise and leads to frequency instability. Choosing low-noise oscillators is crucial for applications such as precision electronic instruments, wireless positioning, high-speed target tracking, and space communications.
- ?Phase Noise: Phase noise is the manifestation of jitter on measurement instruments. It is typically defined as the ratio of single-sideband power at a frequency offset fm within a 1Hz bandwidth to the total signal power, expressed in dBc/Hz. Without phase noise, an oscillator's entire power is concentrated at f0 (e.g., 10MHz), resulting in a power spectrum as a single straight line centered at f0, with a pure sine wave signal. However, due to inherent signal instability, sidebands emerge.
The sources of phase noise primarily include:
- Crystal quality (Q value): Higher-frequency crystals exhibit higher close-in phase noise due to their lower Q values and wider sidebands.
- Crystal peripheral circuits: Including ICs, RC elements, pins, etc.
- Signal output (white noise).
Reducing phase noise is a key aspect of improving oscillator performance. From the perspectives of crystal quality, crystal peripheral circuits, and signal output, the following methods can be adopted:
- Crystal Quality (Q value): Selecting high-quality crystals with uniform lattice structures and minimal defects enhances frequency stability and reduces phase noise.Optimizing crystal preparation processes ensures physical properties and structural quality, contributing to higher Q values and reduced phase noise.
- Crystal Peripheral Circuits: Utilizing low-noise amplifiers as oscillator circuit drive sources helps reduce noise contributions.Minimizing impedance mismatches in oscillator peripheral circuits, including input/output impedances and power supply impedances, preserves signal transmission quality and reduces noise effects.Implementing effective electromagnetic shielding measures minimizes external environmental interference, thereby reducing oscillator phase noise.
- Signal Output (White Noise):
- Filtering the oscillator output signal path with appropriate filters removes high-frequency noise components, reducing the impact of white noise on oscillator performance.
- Ensuring load matching for oscillator output signals prevents signal reflection and interference, improving signal purity and further reducing phase noise.
- Optimizing the layout of oscillator output signals minimizes signal transmission losses and interference during signal transmission, thereby reducing phase noise.
By adopting comprehensive measures, it is possible to effectively reduce oscillator phase noise, enhancing its performance stability and reliability to meet the diverse requirements of clock signal stability in various applications.
