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PCI Express Applications Drive Development of HCSL Output Oscillators
Peripheral Component Interconnect – or PCI as it is better known – is a serial expansion bus standard for connecting computers to one or more peripheral devices. PCI Express is the latest version of this standard and is essential for modern video streaming, high-definition systems and high-resolution camera systems.
L
ike practically every other type of electronic equipment, PCI Express requires an external reference clock to transmit the data.
The typical clock requirement for this would be a frequency of 100MHz, stability ±300ppm maximum and High Current Steering Logic (HCSL) output. The reference clock frequency has remained the same for all PCIe generations – from PCIe1.0 to PCIe4.0 – however the low jitter requirements have become much tougher to support the higher transfer rates in the latest versions. Automotive computer platforms and architectures closely
resemble those used in data centres and, as PCI Express has been successful in cloud and data centre applications, it is no surprise to see the protocol being used in automotive applications. To provide an indication of the power and speed required for the
latest digital cameras, a 25 Megapixel camera operating at 100 frames per second will produce a data stream approaching tens of gigabits per second. In this example, the video data transfer could reach 100Gps and the image acquisition systems require a flexible interface allowing data to be handled with rapid throughput. The growth in AI systems is another area requiring optical systems
capable of producing huge volumes of high-definition data and PCI Express is designed to offer much faster data transfer speeds than the earlier PCI standard. Obviously, for processors to achieve such speeds, the system
requires a fast clock oscillator that also exhibits low phase jitter to prevent loss of data. Differential oscillators can supply frequencies as high as 1GHz, offering increased robustness against power supply noise and
34 OCTOBER 2021 | ELECTRONICS TODAY
reducing common mode noise coupling in the system. This is due to balancing of the negative and positive corresponding waveforms. Differential signalling uses two complementary voltage signals to
transmit one signal. The differential information signal requires a pair of conductors: one carrying the signal and the other carrying the inverted signal. The output waveform from an HCSL differential oscillator can be seen in figure 1 below.
Figure 1
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