Quartz crystals can enable low jitter oscillators for high performance markets by offering high Q, stable resonance and a broad range of frequencies. Applying the correct type for the appropriate reasons can help optimize high performance designs. Below are the types of quartz oscillators and when they are most advantageous in high bandwidth applications.
1. 3rd Overtone
The third overtone quartz crystal oscillator takes advantage of the oscillation mode that resonates at three times the frequency of the fundamental. Catching the 3rd overtone and maintaining oscillation with a low noise oscillator circuit can be tricky. The quartz blank must be of very high quality, high purity and well polished. Otherwise, the oscillator circuit may not oscillate.
However, driving the quartz at this high of a frequency does triple the typical frequency range. The third overtone (typically from about 20MHz to 80MHz in most standard crystals) can easily extend the frequency up to about 200MHz. In high performance applications, starting from a higher base frequency enables lower multiplication and ultimately adds less noise as the clock propagates to all the necessary subsystems. These devices can be manufactured at lower cost and in smaller footprints. The simple oscillator circuit produces little excess noise which enables a well performing low jitter and low noise design. If size and low noise is important, then the 3rd overtone may be the way to go, in the instance that you don’t need anything greater than 200MHz.
2. Phase locked loop solutions
As the base oscillator frequency of crystals has an upper limit near 80MHz without advancing to boutique technologies, many circuits have been designed to multiply the frequency of the base fundamental and increase the range. The most commonly used is the phase locked loop (PLL).
PLLs are specialized feedback systems that employ a divider in the feedback path. Since the transfer function of the closed loop is the inverse of the feedback, the system becomes a multiplier. This sounds like an easy way to achieve a higher frequency from a low frequency base. While this technique is common, it does come at a cost.
The PLL itself can be complex which adds to the cost compared to a basic oscillator die. Additionally, it will add noise. PLLs multiply the base phase noise of the crystal and then introduce a noise floor known as the PLL hump. This floor tends to become the limiting factor in high performance systems.
Lastly, the PLL consumes power. More power is required when the multiplication is higher, the noise is lower and the multiplication is more granular. Despite adding cost, power, size, complexity and noise, the PLL has become useful in high performance systems, especially when high frequencies are necessary. Frequencies beyond 200MHz are considered boutique. A good PLL design with ultra low phase noise below 130fs such as the AX5 or AX7, can easily support some of the lowest jitter requirements while delivering any frequency from 50MHz to 2.1GHz.
3. High frequency fundamental
As the quartz crystal is pushed to higher and higher frequencies of resonance, there is a point when it cannot be considered a common crystal technology. While most quartz crystals operate in fundamental mode (contrasted with 3rd overtone), oscillation beyond 100MHz requires nano scale and lithographic technologies that are considered boutique. The result is a lower yielding fabrication method that produces thin crystals. The boutique technology is not flexible and requires the generation of fine tuning for each frequency blank. It also introduces cost. HFF technology may not be considered mainstream. Although, some low noise applications above 200MHz have benefited from employing that technology.
Most ultra low jitter solutions can be addressed with either low cost and low noise 3rd overtone technology or frequency flexible PLL technology. The selection depends mainly on frequency range. Selecting the appropriate technology improves system design while maintaining lowest possible power consumption and smallest possible footprint.
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