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Oscillators designed for 5G and ultra-fast data transmission


By Andy Treble at Euroquartz


Although the Covid-19 crisis has affected day-to-day business, oscillator manufacturers have continued to push the boundaries of oscillator design to achieve these higher clock speeds. There are many challenges for the oscillator designer, but one of the main issues is how to generate higher frequencies while keeping both phase noise and phase jitter low.


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The performance of standard CMOS clock oscillators degrades above 50MHz when the waveform starts to deteriorate from a square wave to a distorted shape depending on the loading of the output. As clock frequencies rise, gate transition times are too slow for fast propagation with current consumption tending to increase proportionately as a result while phase noise performance declines. To achieve higher speed solutions, differential output devices such


he advent of 5G and high-speed fibre-optic data networking has generated a requirement for faster clock oscillators running at ever higher frequencies.


as Low Voltage Differential Signalling (LVDS) and Low Voltage Positive Emitter Coupled Logic (LVPECL) have been developed but these each have their own issues. Figure 1 below shows the difference between a CMOS output and the various types of differential output. A comparison of the basic LVDS signalling levels with those of PECL shows that LVDS output devices exhibit half the voltage swing


compared to that of PECL types and these are approximately one tenth of that of the traditional CMOS levels. Both LVDS and PECL oscillators also have limitations when it comes to phase noise and jitter in high-speed circuits.


Figure 1 shows the difference between a CMOS output and the various types of differential output


58 MAY 2021 | ELECTRONICS TODAY


The latest designs are moving towards newer techniques such as High- speed Current Steering Logic (HCSL) and Current Mode Logic (CML) outputs, which offer additional benefits and advantages over previous output formats.


The processing of increased amounts of


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