Ask the RF Experts - Questions and Answers

Q) How is antenna tuning different from impedance matching?  Isn’t it the same thing – to improve the RF signal under different conditions?

A) At a high level, yes, both techniques improve RF performance under different conditions.  The big difference is lies in what the circuits are doing.  Impedance matching an antenna provides a way to reduce the negative effect of operating the antenna when the actual impedance differs from the one it was designed for.  Mismatch correction can compensate for head-hand effects or having a metal surface nearby or other environmental situations that degrade the impedance match.  IM cannot improve the efficiency profile of an antenna, so whatever the efficiency is, it rolls off as the transmitted/received frequency diverges from the design impedance.

Antenna tuning, on the other hand, changes the resonant response of the antenna so that if operated in a different band, the resonance can track and can maintain optimal efficiency.  AT can also compensate for some degree of impedance mismatch if that is design into the antenna as well.

The difference between the two approaches can result in several dB of performance.  AT almost always is a better solution since the designer can approach the problem as a narrow-band antenna (with higher gain) and move the resonance to another frequency without suffering the efficiency roll-off that a broadband antenna has.

The difficulty with AT is that the components need  to sit in the aperture of the antenna or at least be coupled to the aperture.  Lossy components can degrade the antenna and reduce efficiency.  The traditional approach using switches with passive components works pretty well as long as the switch has low insertion loss – some have IL as low as 0.5dB in the frequency of interest.  Using high Q passives (usually capacitors, but inductors work, too), creates an acceptable solution so long as the IL doesn’t kill the higher band performance.

The Cavendish tunable capacitors behave like a bank of 32 high Q capacitors selected with a switch with zero insertion loss.  So the designer can have the best of both worlds – a tuned antenna and high performance with the loss of a switch.

Q) Why do your components have a high quality factor?

A) Quality factor measure how much energy the device consumes instead of passes on.  The MEMS devices are nearly ideal parallel plate capacitors and so mimic the way high quality passive capacitors work electrically.  The capacitive switches are always locked into place with an electro-static charge and so there is nothing moving as the circuit operates and very, very little energy is dissipated.  Other technologies (BST, MIM-caps with solid state switches) have leakage paths and parasitics that rob the circuit of energy and thus have lower Q.

The CK tunable capacitors have the highest quality factors in the industry as measured at the “user side” of the bumps on the die.  Since no packaging is require, a bare, bumped die delivers the same Q to the designer’s circuit.  Also, since these are the smallest devices in the industry, the board parasitics due to routing are greatly reduced since the traces have a smaller distance to travel to the next device.  One CK device takes less board area than two 0402 capacitors.  It’s that small!