The relentless pursuit of speed and capacity across the electronics industry is changing the rules for high-end oscilloscopes. While bandwidth used to be the “banner spec” that customers and vendors focused on when evaluating oscilloscopes, the level of precision required to capture and analyze today’s fastest serial and optical signals means measurement accuracy or signal integrity has now emerged as the most important factor.
But what is measurement accuracy?
Bandwidth is a simple way to compare one instrument to another – the one with the highest bandwidth must be the best, right? To be sure, bandwidth is important and for high-speed applications high-bandwidth is a necessity. But the true aim of the oscilloscope is to represent a signal of interest as accurately as possible, and that is a more complex story involving the fundamental design of the instrument, probe architecture and connectivity accessories, and parameters beyond bandwidth including rise time, sample rate and jitter noise floor.
A summary of the key parameters engineers should evaluate when choosing an oscilloscope are shown in table 1.
Table 1: Market drivers - the need for better signal integrity.
High speed signals are susceptible to signal integrity issues because they involve very fast edges and very narrow unit intervals or bit times. As the data rate for a communication link increases two things happen: The UI narrows and the rise time of the signal decreases. For example, in looking at a 5 Gb/s pulse compared to an 8 Gb/s pulse the UI width drops from 200 ps to 125 ps. This results in 38% less margin or error budget in a design. This makes the job of the receiver even tougher as it tries to distinguish a one from a zero at very fast data rates with less margin. Rise time also decreases from about 30 ps to just over 28 ps. An 8 GB/s signal is shown in figure 1.
Fig 1: An 8 GB/s signal.
Complicating the issue is the fact that multiple signal integrity issues can be introduced as a transmitted signal makes its way to the receiver. These signal integrity issues can include attenuation as the signal travels across a circuit board or from the Si die to the package pins and onto the circuit board. Attenuation in a channel is a serious issue that must be addressed. Losses in PCB material such as FR-4 increase with trace length as well as with data rate. As signal amplitude shrinks, noise and reflections are becoming a bigger factor. Customers need to employ de-embedding strategies in the receiver to open closed eyes.
With the advent of Gen 3 serial standards, 8-10 Gb/s is becoming the norm. In the optical communications market, with the push to 4 x 25G (100GbE) Ethernet, designers need the ability to test signals with bit rates as high as 32 Gb/s. High-speed FPGA and wideband RF are also pushing the limits. For these high-end applications, the Tektronix DPO/DSA73304D offers the industry’s most accurate measurement capabilities.