RF Integration: Can We Get It All?

We had a chat session recently where we discussed the integration of RF devices as it pertains to analog integration.

We discussed how 10 years ago, it was difficult to integrate RF circuitry onto a chip. Now, it's a lot easier and far more practical. We talked about some of the more well-known and familiar subsystems that are part of the software defined radio (SDR). It's possible and practical to put most of the UHF receiver section on a chip.

This, of course, pertains directly to cellphone applications. There are certainly other receiver applications — radio equipment for the military or safety forces — but if you're looking at high production volumes, then you're looking at cellphones. These high volumes are the reason you would want to integrate as much as possible onto the chip to reduce manufacturing cost. That cost includes the parts themselves and PC board real estate (i.e., materials). Also included is the effort to build, and more importantly, test the board assembly (labor). So the more you can integrate, the lower the overall cost.

The receiver section is the easy part. For the transmitter, it's pretty easy to put most of the circuitry on a chip — until you get to the RF power output stage and the antenna. With existing technology, it's not practical. The high-power output stage will be problematic because, well, it's a high-power RF source (probably a couple of watts) very close to the receiver circuitry. There are a few tricks you can do to minimize the inevitable crosstalk — active noise cancellation techniques, for example, though that doesn't completely solve the problem.

Even with crosstalk mitigated, the other difficult portion of the circuitry to integrate is the antenna tuner. Ideally, you'd like to have voltage variable capacitors (and perhaps inductors) to allow matching the antenna's impedance to the RF output stage. The antenna's impedance changes depending on proximity to surrounding objects and to the transmitting frequency.

Using the existing technology — the voltage variable capacitor or varicap — won't work here. The varicap is actually just a reverse-biased diode. The higher the DC bias (fed to the diode through an inductor to provide isolation), the farther apart the areas of conduction in the P- and N-layers are. Therefore, the capacitance is lower. So, vary the voltage, vary the capacitance.

Unfortunately, the applied RF voltage is high enough that it will muck this up by affecting the capacitance dynamically. The result will be some very bad waveform distortion.

Another choice that seems promising at first is the MEMS capacitor devices. With these, you can actually have a capacitor is the classic sense — two conductive plates that can be moved with a control voltage to put them closer together or farther apart. Sadly, at the RF power levels, arcing may occur across the capacitor.

A related possible version of this uses the MEMS device as a relay — a set of contacts that can be opened or closed. Here, you could select various fixed capacitor and inductor values, switching them in and out to get the desired value. But you would have a similar problem as above — arcing across the contacts as they opened.

To date, we don't have these problems solved — but it's only a matter of time before these design issues are successfully addressed. We can then integrate the complete transmitter chain, including the final RF stage and the antenna tuner.

Are you working on any designs where you are integrating large portions of RF circuitry?

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4 comments on “RF Integration: Can We Get It All?

  1. Netcrawl
    July 27, 2013

    @Brad that was great! I hope we can, integration is an esential of enabling mobile devices to reach mass adoption, its industry's way of reducing costs and complexity of radio design, The elctronic industry requires a massive technological leap from system design to circuit level, with one one goal- to make radio integration a reality.

    But we still have some major challenges to face here (integration) we lack tools for testing designs, today's tools are not enough, I'm not saying its not working but it still in infancy need some additional stuff here. 



  2. Brad_Albing
    July 29, 2013

    @Netcrawl – thanks for mentioning the tools aspect. I'll go into that in more detail later.

  3. Davidled
    August 1, 2013

    In the Mobile Phone, PCB size is very restricted due to the space of phone. But once customer hand and head is closer to antenna, that cause the antenna to detune from the main frequency, losing signal from base station with signal attenuation. At the time, base station drive phone to increase transmission power to reach the signal. Consequently, battery drain is increased. To avoid this issue, power amplifier might be updated as gain is increased, but there is no room in the PCB of phone. So, as alternative method, sensor located at PA is developed and by sensor, antenna is re-tuned to the target frequency. It is realized that RF performance and development is not easy in the restricted PCB space.

  4. Brad_Albing
    August 6, 2013

    @DaeJ – yes, that problem of the antenna performance being degraded as you move the phone nearer to your body is significant. Two of our bloggers addressed the issue in different ways. See these:

    Is Cellular RF PA Integration Finally Here? Part 2

    Digital Tuning for RF Analog Front End

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