Phase 101 Webinar

Summary

The webinar is the first in a two-part series on phase stable assemblies and electrical length changes within cable assemblies. In this session, Dave Slack shares his insights on the importance of phase in cable assemblies. He explains what it is, why it matters, and how it can be properly specified for precise performance requirements in aerospace engineering, space technology, and more.

Watch the video or read the session notes below.

Session Notes

What is Phase?

Phase is a key parameter for detection and measurement in many RF/microwave systems including radar, direction-finding (DF) systems, and missile defense systems. Phase must be accurately controlled in the components within those systems, such as coaxial cables and connectors.

The concept of phase starts with the fact that a microwave signal propagates in the form of a sine wave. For every cycle of a sine wave, 360 degrees of electrical length is accumulated. If 50 cycles per second accumulate, it is a low frequency, with few cycles per unit of time, and a relatively long wavelength. At a higher frequency, millions or billions of cycles per second will accumulate, and the wavelengths are exponentially shorter.

Frequency, time delay, and physical properties including dielectric constant and propagation velocity all affect electrical length. Environmental factors are also very important, such as temperature fluctuations, flexure, handling, twisting, pulling, crushing and more.

Why Do Phase-Stable Assemblies Matter?

Phase-stable cable assemblies are important in today’s increasingly sophisticated electronic systems. In aerospace engineering and space technology applications, phased array antennas, synthetic aperture radars, and direction finding are all phase-sensitive uses. For example, electronically steered antennas use a variety of radiating elements, and then vary their phase relationships to control the radiation pattern, so they can switch from a search radiation pattern to a tracking radiation pattern or shift direction very quickly. All these elements are fed by transmission lines; beam accuracy depends upon the phase relationships between those cables. Phase is also responsible for precision in some of the more time-sensitive satellite applications like GPS systems, mobile cellular, military radar and more.

Specifying Phase-Stable Assemblies

There are two ways to spec a phase-stable cable assembly. One is to specify it in terms of an absolute quantity or to talk about it in relative terms. For the absolute electrical spec, you would determine that the cable assembly is 5,271 degrees +/- 1, for example. You can buy a cable and specify that as an absolute length in terms of time delay, 5.1 nanoseconds +/- 0.1, etc. It’s very convenient and easy. Then there’s the relative way of specifying cables, which is one cable assembly relative to another. Whether it’s 10,000 degrees or 20,000 degrees all that matters is the cables are the same electrical length within a specific tolerance.

As frequencies get higher and higher beyond the UHF frequency range, a million degrees can accumulate easily, making the absolute electrical length measurement a challenge. At high frequencies, tight tolerances and really short wavelengths, a relative measurement can be much more precise using a relative measurement.

Sometimes specs are requested that we think are impossible, but at Times, we roll up our sleeves and gather around the whiteboard to figure it out. What was impossible just a few years ago is typical today. And that’s going to continue what is impossible today will be typical tomorrow.

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