Insertion Loss: A Bigger Concern in High-Speed Digital


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A design may be 50 ohm nominal impedance, and the PCB is tested to show that it has the correct impedance within some tolerance. The tolerance is sometimes ± 10%, other times ± 5% or less. In the past, one of OEM customers’ main concerns when dealing with their PCB fabricators was characteristic impedance. Many times, a PCB design is considered controlled impedance and the PCB fabricator is held to a specification for impedance control.
 
As digital rates continue to increase, there are more requests for fabricators to perform insertion loss and other types of electrical testing. Understanding the basics of digital signaling can help explain why these new electrical tests are desired for ensuring a quality PCB for high-speed digital applications.

Digital pulses are made up of analog sine waves. In order to make a digital pulse, sine waves of different frequencies are added together to form the digital pulse. Each of these sine waves is a high-frequency RF wave at a specific frequency. As an example, when a 10 Gbps pulse is generated from a combination of the RF waves, the frequencies used will be approximately 5 GHz, 15 GHz, 25 GHz and 35 GHz added together. Actually there can be more, but this is a simple example of showing the adding of sine waves using the fundamental harmonic (5 GHz), 3rd harmonic (15 GHz), 5th harmonic (25 GHz) and the 7th harmonic frequency at 35 GHz.


Figure 1 shows how the digital pulse can be formed from adding these sine waves.

Coonrod-Fig-1.jpg 

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