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Bench Talk for Design Engineers

Bench Talk


Bench Talk for Design Engineers | The Official Blog of Mouser Electronics

RF Power Transistors and the Mystery of Video Bandwidth Barry Manz

If you use RF power transistors, you may have come across a new term of late on the datasheets: video bandwidth (VBW). Now, anyone who uses spectrum analyzers knows about this key metric, but I’ve been poring over power transistor datasheets for years, and this one seemed to appear out of nowhere.

Not wanting to sound like the last person in the electronics industry who doesn’t know how VBW and power transistors are related, I turned to the all-knowing source that would ensure my anonymity: Google. After all, a Google search always turns up something no matter how bizarre or obscure the topic. In this case, I got nothing, or at least nothing useful.

What I did find were 42.7 million hits covering VBW and spectrum analyzers, broadcast television, video in general, network bandwidth requirements, VBW vs. resolution bandwidth, VBW vs. instantaneous bandwidth (IBW), and others. I finally posed the question to several people in the industry who, after a group huddle, sent me an email citing Microwaves 101, EE Times, and assorted other sources, none of which actually answered the question. Discouraged, I called a person (whose name I dare not reveal) whose mind is a virtual Library of Congress on topics ranging from the “actual” height of the Himalayas to all things RF.

His answer was: “In old-school analog TV broadcast signals, one had the audio bandwidth and the video bandwidth sections of the transmission signal, which were filtered and split inside of the TV receptor. Somehow we got stuck with the VBW term in RF. That is how humans work, incredibly and wonderfully flawed.”

In short, VBW and IBW are the same thing. Broadly defined, VBW (or IBW) is the maximum amount of spectrum a device or amplifier can process while maintaining a symmetrical and constant intermodulation product, ensuring that clipping and increased intermodulation distortion do not occur. The instantaneous bandwidth of the amplifier typically needs to be at least three times as wide as the operational bandwidth in order to address intermodulation terms up to the ninth order.

The questions become why is VBW appearing now and what’s the point of using it at all? Manufacturers could do themselves and their customers a favor by sticking with instantaneous bandwidth and ditching VBW, which traditionally had never been applied to RF power transistors.

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Barry Manz is president of Manz Communications, Inc., a technical media relations agency he founded in 1987. He has since worked with more than 100 companies in the RF and microwave, defense, test and measurement, semiconductor, embedded systems, lightwave, and other markets. Barry writes articles for print and online trade publications, as well as white papers, application notes, symposium papers, technical references guides, and Web content. He is also a contributing editor for the Journal of Electronic Defense, editor of Military Microwave Digest, co-founder of MilCOTS Digest magazine, and was editor in chief of Microwaves & RF magazine.

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