The Metal Frame & External Antenna with Dynamic Tuning

While I’ve discussed material choices before in the context of mobile devices, the new Moto X requires a new depth of understanding in order to really appreciate the amount of work Motorola has done to enable the industrial and material design desired. In order to avoid issues with the metal frame detuning internal antennas and maintain radio performance, Motorola has developed their own custom antenna tuner that is supposed to be even better than the QFE15xx antenna tuner that Qualcomm has made as a part of their RF360 package.

Of course, at this point the iPhone 4’s “death grip” issue has been discussed to death. In short, due to a lack of antenna diversity, it was possible to easily detune the phone’s antenna and significantly decrease signal reception by putting a finger on the gap between two parts of the metal frame. Understanding how an antenna tuner can help to resolve this situation requires an understanding of impedance and how it relates to antennas.

The first and inevitable question is what impedance is. To briefly summarize this topic, impedance is essentially resistance in an AC circuit. Impedance in an AC (alternating current, or what comes out of most power outlets) is determined by resistors, capacitors, and inductors present in the circuit. In antennas, what’s really happening is that electromagnetic waves in the air are causing the antenna to resonate, and as a result the waves are converted in electrical signals. While this is easy enough to understand, the crucial portion of this is where the antenna connects to the rest of the system. Antennas inherently have an impedance determined by natural resonant frequencies, the height above the ground, and the conductors used to construct the antenna. For the most part though, this is relatively easy to tune for at the factory such that the impedance mismatch is small.

The major problem is that the real world is not just the inside of a factory. As mentioned before, the hand detunes the antenna due to its capacitive effects. This means that the impedance changes. For those still following along with the physics, the reason why an impedance mismatch causes reception to worsen is because the electrical signal is still in the form of a wave in the AC circuit, parts of the wave will reflect just like how some light is reflected when crossing from one medium to another, which is why water can appear to be a mirror from one side but a window from another.

Now that we’ve gone over the physics, let’s get back to the Moto X. Motorola has developed their own custom antenna tuner. While Qualcomm has their own antenna tuner, the major differentiator is that this antenna tuner actually detects capacitance changes at the antenna and adjusts impedance accordingly. In practice, the antenna is retuned incredibly quickly, with next to no hesitation. Motorola demonstrated this by showing two Moto Xs that were identical except one had the antenna tuner disabled. The Moto X without this antenna tuner rapidly dropped from ~23 dBm output power to ~7 dBm output power. The unit with the antenna tuner managed to achieve around ~15 dBm output power after detuning. Remember, decibels are a logarithmic scale so this represents around a 6.3x increase in power output.

In addition, in discussions with Motorola’s engineering team they claimed that the new Moto X improves upon the receive sensitivity of the first Moto X. This is no small feat as the original Moto X was known to have some of the best radio reception amongst its peers. Once again, this makes sense as even though polycarbonate is RF transparent there will always be some level of reflection, just like how there is reflection with impedance mismatches, and I would once again refer to our article on materials in mobile devices to get a better understanding of this subject.

Introduction & The New Moto X Moto Voice, Display, Actions, Attentive Display, Sound, Camera, and Final Words
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  • ehfeng - Friday, September 5, 2014 - link

    Agreed. Great article, one that few, if any, other publications would be capable of producing.
  • kpkp - Friday, September 5, 2014 - link

    Little bug:
    "and the power button has a ridged pattern to it to distinguish it from the power button"
  • JoshHo - Friday, September 5, 2014 - link

    Bug fixed, always seems like there's one typo that gets through...
  • VulgarDisplay - Friday, September 5, 2014 - link

    I have LOVED my original Moto X so far. I would buy this Gen 2 phone, but the original is still absolutely fast enough for everything I use it for, and the battery lasts me for 10 hours of heavy usage.

    Maybe Gen 3 will be time for a new phone.
  • ankushrajput - Friday, September 5, 2014 - link

    I'm a new fan of Anandtech. I had read Anand's words of praise for Joshua in his signing off article and after reading this article, I realize he was so right. I love the amount of detail in your articles. Just wish you do more reviews of mobile devices, including mid-range devices. And yes, I'm waiting for an in-depth review of the new Moto devices launched today.
  • CrystalBay - Friday, September 5, 2014 - link

    Yeah good preview JoshHo !
  • Frenetic Pony - Friday, September 5, 2014 - link

    This phone looks sexy as hell, and considering every phone now has these specs at least that's a differentiatior.
  • sr1030nx - Friday, September 5, 2014 - link

    I would love to see this running Windows phone.
  • lostboykev - Friday, September 5, 2014 - link

    This was a great first impression. You're the only one I've come across that noted the camera sensor. Not only that but went in depth with everything new an similar about the phone. Back to the camera. This is a no buy for me. I have the original moto x. Which has a bigger sensor and less megapixels allowing greater individual pixel size. Not to mention how old this Sony sensor is. I cant justify the phone being a camera enthusiast. Even if it can perform better, not sure since I've used the LG G3, I still can't live with that 1.1µ (micro) pixel size. I'm sticking with bigger is better; in fact newer is better too!
  • modulusshift - Friday, September 5, 2014 - link

    You have the original Moto X, and think it's somehow better than the G2's camera? I don't think you've ever seen pictures from the other smartphones from last year, the Moto X had the worst camera performance besides the Nexus 5.

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