Cavendish Kinetics offers a new variable capacitor

It’s like coming back to a place you used to know and finding that everything has changed. A few years ago, when MEMS was just a(n) FLA to me, I decided to try to figure out a bit more of what it was about. Someone I was working with at the time used to be associated with a MEMS company – one working on monolithic CMOS/MEMS – and so that was what I studied first.

I didn’t do anything with that specific story at the time, but now, a few years later, I’ve come back to that company – and everything has changed.

The company is Cavendish Kinetics. I’ve recently mentioned them briefly here and there with respect to both MEMS yields and smartphone antenna tuning. But those mentions didn’t get into the guts of what they do.

My memory may be failing me, but I recall switches that actually relied on that perennial MEMS bugaboo stiction to hold a state. The Cavendish Kinetics goal then was to create a non-volatile memory that could survive the harsh conditions that will play havoc with a standard transistor-based memory. Like out in space.

Problem is, that’s a pretty small market. So, as has happened so many times in the MEMS world, they went back to the drawing board for a new approach and a new market. And that approach is to use those switches not as memory elements, but as variable capacitors. And it’s not like they simply reused their old switch; they had to retarget that as well.

The idea is to create an RF tuning circuit that will repurpose a given antenna for different bands. Right now, either a phone has to have multiple antennas, or it must make one antenna do for multiple frequencies. But in that single-antenna case, the antenna will be optimized for only one frequency; the rest will be inefficient.

Retuning the antenna means reoptimizing for the specific band being used. And this is done with variable capacitors. But it has to be done in a manner that’s consistent with RF signals (analog, high voltages, high frequencies) and consumer-friendly yields. Cavendish Kinetics is trying to retune an antenna from 700 MHz to 2.5 GHz, which covers 41 different bands.

Any RF circuit is going to be judged by at least one thing: quality. Or loss. Or Q. Or any of the other various parameters used to judge what a “good” circuit is. This may sound trivial, but according to Cavendish Kinetics, the “insertion loss” factor that is often touted and spec’ed isn’t so meaningful. The number is relevant only when driving a 50-Ω line – which phone antennas mostly aren’t.

What Cavendish Kinetics focuses on instead is “equivalent series resistance,” or ESR. This models the circuit as having a simple resistor in series with the RF path. Higher ESR means more resistance and more loss. They say that one typical approach, BST (barium-strontium-titanate) variable capacitors add as much as 3 Ω, meaning a loss of 3.5 dB, knocking out 60% of the data rate gains you were hoping for by doing the tuning in the first place. On the other end of the spectrum, a high-quality discrete cap can add as little as 0.25 Ω.

via A New Variable Capacitor.

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