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Optimizing at the Cellphone Antenna

So you run across an announcement from Imec that they’ve presented a circuit at ISSCC that involves tuning to match changing antenna impedances in cell phones. And if you’ve been hanging out in the same places I have, you might have the following reaction: “Hey, this is what some MEMS guys are doing as well! Fight! Fight! Fight!”

Turns out it’s not quite that simple. I’d feel like it was just another day of me discovering yet another knowledge gap, but I wasn’t the only one conflating two concepts. Conversations with Cavendish Kinetics and WiSpry helped me pry the two issues apart.

You may recall coverage we’ve done of both WiSpry and Cavendish Kinetics on the MEMS side and Peregrine on the SoS side. They make capacitor arrays that can be configured as variable capacitors. The first two guys use MEMS switches; Peregrine uses electrical.

What problems do they solve? They help tune the antenna as conditions drift on the phone (grip changes etc.). In fact, even this conflates two things, as we covered before: both matching changing antenna impedances and recentering the resonance of the antenna to ensure that the strongest possible signals enter the phone.

Critically, that frequency centering can’t be done too tightly: full duplex on your cellphone is achieved by placing receive and transmit on two slightly different frequencies so that they don’t interfere with each other. So the antenna response has to be wide enough to provide good performance at both frequencies.

Meanwhile, Imec is actually working on something different. The duplexer in the phone separates out the receive and transmit signal paths. Filters are typically implemented using relatively bulky surface acoustic wave (SAW) or thin-film bulk acoustic resonator ((T)FBAR) devices. Imec wanted to try instead to use an electrical-balancing (EB) technique, with the initial goal of simplifying the circuit.Imec_ISSCC_image.png 

The high-level concept they’re exploring was actually used in the original telephone: taking a combined full duplex signal, consisting of mixed receive and transmit, and subtracting out the transmit signal (which you know because you’re at its origin) to get the pure receive signal. While that was a simple thing on old phones at voice frequency, it’s apparently not so easy up in the many-megahertz range with changing conditions. The three challenges in particular are isolation between receive and transmit, linearity, and insertion loss – and managing these in the face of changing antenna impedance.

In this particular project, Imec tackled the linearity and insertion loss as a step towards a commercially-viable EB duplexer. This is distinct from optimizing the antenna; they’re complementary solutions.

That’s all well and good, but there’s more promise if this all works out. If you can effectively isolate receive and transmit through subtraction, then you no longer need to place the signals on different frequencies. In fact, there was a paper presented at ISSCC by Columbia University that claims to have done just that: run receive and transmit at the same frequency. (Unfortunately I missed ISSCC this year due to illness and didn’t see the presentation; I received no response to a request for more information from Columbia).

And here’s the big payoff: if we can combine signals at one frequency, then instead of receive on one and transmit on the other, we can have both on both – we’ve doubled our capacity just like that.

So a relatively obscure-sounding topic (believe me, reading the Imec paper got me lost pretty quickly) may turn out to have rather astonishing consequences. We might eventually return to the old telephone approach that used to work so well.

The Imec paper can be found here (behind a paywall).

 

(Image courtesty Imec)

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