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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  • Lord of the Bored - Saturday, September 6, 2014 - link

    " Now named Moto Voice, the new Moto X now allows for any five syllable phrase to be used. "

    So I can activate my phone by saying "go go gadget" now? THAT is a feature that will move units!
    Reply
  • tremblingwater - Saturday, September 6, 2014 - link

    Hey Josh,

    I'm so glad you guys exist. So far, its been site after site all showing the same thing.

    The crucial aspects that matter are never discussed or explored.

    Recently you did a piece on smartphone audio. I am anxiously waiting a follow up on that and would love a comparison with the new Moto X and Sony Experia Z3.

    Audio quality is something that is hardly explored and considering its one of the main things we all do on a phone, those articles influence my next purchase heavily.

    I hope you can explore the amps and audio solutions for the headphone output, soon!
    Reply
  • Bhairava - Monday, September 8, 2014 - link

    Wow, a great piece of article! Thank you. Reply
  • zachrohlfs - Monday, September 8, 2014 - link

    Stopped reading when I saw the increased screen size. Sorry but I want to work a phone with one hand not have to always be using two like with my Razr MAXX. When will device manufacturers realize that this behemoth needs to fit in my pocket and I have a wallet and keys to contend with. You want to be able to use this for business and not for nothing but I will have a laptop out if I really need horsepower. Reply
  • Tiger33 - Monday, September 8, 2014 - link

    Wow excellent write up! - this phone is looking very promising.. Waiting to see how motomaker works in the UK... so far we only have the brown version available from Phone 4 U (launch date of 6th October) any ideas as to when motomaker is due? Reply
  • pjcamp - Monday, September 8, 2014 - link

    Missing info: 16 GB memory and no SD. So if you carry around a lot of media, or you use something like Sygic to navigate out of cell service range, forget it. You can get 32 GB but only direct from Motorola, and still no SD.

    I understand this is the last device from the Googlerola period so maybe they will change in the future. However, I need a phone now, and SD is not optional to me, so this is not the one. Sad. I was looking forward to it, and rumors had it that SD would be included as on the G series. Goes to show you can't trust rumors. I understand that not everyone cares about SD, but it is so cheap I don't understand why every vendor doesn't include it for those of us who do.

    Right now, Sony (surprisingly) Z3 is looking like the device for me. Just waiting for a review of it.
    Reply
  • theoilman - Sunday, September 14, 2014 - link

    check out Oppo Find 7 and Find 7a. they're two of the top phones in the market and they have SD and replaceable batteries. if I needed a phone right now I'd get one of them. Reply
  • vishwacs111 - Wednesday, September 10, 2014 - link

    From what I know Moto X (1st Gen) does not support MHL or any type of HDMI out. I was wondering if Moto X (2nd gen) does. You have claimed they both do. Are you sure? Reply

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