Cure RF squegging with a Neutrodyne circuit

Some headlines write themselves, huh? Squegging is when an RF amplifier or MHz-class switching regulator starts cycling on and off. In an audio amp it is called “motorboating” since that is the sound it makes. FET amplifiers are subject to this, like old tube amplifiers. Both have a high-impedance input, the tube grid or the FET gate. A FET gate is capacitive, so any charge that gets put on it will be stored by the gate, moving the bias point of the FET too high, and causing squegging. The Neutrodyne circuit comes from 1920 vacuum tube amplifiers. It is one of the ways you can tame squegging. High Frequency Electronics magazine has a nice article about squegging (pdf). The best way to show it is a figure in the article, who I hope the fine legal team at Summit Technical Media will let me show you.

Squegging-amplifier

Squegging is when the input of a FET or vacuum tube floats up momentarily and shuts down oscillations. This make the output cycle on and off, called motorboating (courtesy High Frequency Electronics).

Trust me; you really want to click over to the article since it has the schematics of a FET amplifier that will start to motor boat, as well as several ways to fix it. The whole magazine is pretty good. While you are at it, think of signing up for a print copy of the magazine. You need to be an engineer or tech worker, since the magazine is audited by BPA, so the advertisers know they are reaching tech people and not random idiots.

Remember that these tips apply to high frequency switching converters. And regulators are getting up into RF ranges. I remember seeing an 8-MHz switching regulator from Micrel years ago when I worked at EDN magazine. You might be using one of these fast regulators for some extreme size problems. These high speeds do cause less efficiency, as the gate charge is getting shunted to ground, but the inductor you need with these fast converters is miniscule. That Micrel part still manages 90% max efficiency, but you can use a 0.47µH inductor. That is one tiny inductor.

So I assume the Micrel folks have solved any squegging problems in their part, but it is still a good principle to understand should you run across it. It’s like sub-harmonic oscillations in switchers with a duty cycle greater than 50% (pdf page 10, pdf page 5, pdf page 72. It might befuddle you if you have never heard of it and don’t know the steps you need to take to solve it.

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