The Rise and Fall of the RF Switch for Smartphone Antenna Tuning
An RF MEMS tuner creates no resistance for the RF signal, therefore causing virtually zero insertion loss.
The evolution of smartphones is nothing short of awesome, putting supercomputers with brilliant screens, high-res cameras and stereo speakers in your pockets. They are supposed to give instant access to anything, anytime, anywhere. But smartphones not only have to be feature laden, they also need to be functional. And as OEMs are one-upping each other with bigger, slimmer (6.5mm as of MWC 2015) and glitzier (shiny metal with screens curved around the edge) devices, the radio performance is degrading, leaving the user wanting stronger signals.
Figure 1. Evolution of Smartphone capabilities and degradation of Smartphone radio efficiency.
Figure 1 shows the shifting design paradigms that have led to a decrease in radio performance:
- Giant screens that extend beyond the edge and squeeze antennas into the frame
- Batteries dominate the interior of the device, leaving little volume for the antenna
- Metal frames that look and feel good but end up shielding the RF signals
Unfortunately, not all that looks good works well. To understand how these design trends impact radio performance we need to take a look inside the Smartphone.
Tracking the Signal Loss Pathway inside the Smartphone
Radio performance is best defined as the “efficiency of the RF transmission”, which can be summed up as the over-the-air power transmitted by the antenna (in form of electromagnetic waves) divided by the electrical power consumed by the power amplifier to generate that transmission. Figure 2 highlights the tremendous amount of signal strength that is lost both at the antenna and inside the smartphone. And the loss of signal strength means that the power delivered by the power amplifier has to increase in order to be able to transmit the same amount of power at the antenna, significantly reducing the radio efficiency, in some cases by as much as 75%.
Figure 2. Simplified Smartphone RF block diagram showing the loss of signal strength in the receive path.
To identify the contributors to the signal loss we start at the antenna, where the available network signal strength represents 100% of the power theoretically available for signal processing inside the modem chip. Right after the antenna however, the signal strength can easily drop to 40% or less, representing a 60% loss of signal strength at the antenna alone. Continuing its path towards the modem, the signal passes through a number of switches and filters, all of which further weaken the signal to the point where the total RF signal loss adds up to 75% and more before the signal reaches the modem chip.
What’s wrong with the antenna?
At ‘the height of cellular telephony’ just 10 years ago (when cell phones were just phones) antenna efficiencies of 65% were common for 800MHz low-band and 2GHz high-band antennas, meaning that only 35% of available signal strength was lost. Today in the age of Smartphones the antenna efficiency is often less than 40%, meaning that 60% of the power available at the antenna is lost. What happened?
Antenna Design Physics 1.0 tells us that an antenna can only absorb the energy of a RF signal if the wavelength of that signal roughly corresponds to the length of the antenna, in order to create an efficient resonance. An 800MHz frequency signal translates into ~16 inch wavelength, and typical 800MHz antennas are cleverly designed to operate at only a quarter of that wavelength. But an 800MHz antenna still requires a length of ~4 inches to operate efficiently, and a shorter ~1.5 inch antenna for the 2GHz spectrum is also needed. To make matters worse the LTE standard requires two independent antennas to operate at every frequency to support MIMO operation, bringing the total number of independent antenna elements to 4, ignoring the need for WiFi, GPS and NFC antennas …. Finding room for 4 cellular antennas inside a Smartphone that is dominated by batteries and electronics, is nearly impossible, forcing designers to utilize the top and bottom sections of the smartphone together with portions of the frame. Not hard to see why antenna efficiency isn’t at the levels it used to be.
To make matters worse we turn to Antenna Design Physics 2.0, which are equally clear. An antenna can only function efficiently over a bandwidth that is equivalent to ~10% of the frequency of the spectrum band it is operating in. This is problematic since the global LTE spectrum allocations require antennas to function over a wide-swath of spectrum bands, 41 to be precise, ranging from 700MHz up to 2.7GHz. 10 years ago, at that famous ‘height of cellular telephony’, the global spectrum band plan for the 2G/3G networks consisted of only five spectrum bands, 2 at 850/900MHz, 2 at 1.8/1.9 GHz and 1 at 2.1GHz. Considering just the low-band requirements for a 2015 LTE antenna, which is required to operate at 700/800/900MHz, it becomes clear that such a low-band antenna can only have <100MHz of effective bandwidth, while having to cover at least 250MHz of spectrum. And the 1.7 – 2.1GHz high-band antenna, which cannot have more than ~200MHz of effective bandwidth, has to cover 400MHz of spectrum.
Taken together the Physics of antenna size compared to the Smartphone space constraints and the Physics of antenna bandwidth compared to the LTE spectrum requirements explain the antenna dramatic efficiency degradation experienced in today’s Smartphones.
If physics is the problem can physics be the solution?
The antenna’s radiated performance challenges are based on well understood physics, and so is the solution: antenna tuning. To the RF engineer, antennas are nothing other than ‘resonating circuits’ composed of a series of parallel inductors and capacitors, which have a specific resonating frequency. But only if the frequency of the electro-magnetic radio waves matches the resonating frequency of the circuit is the antenna able to receive the desired radio signal. So if a smartphone must receive signals in different spectrum bands (with varying wavelengths), the only solution is to manipulate the composition of the resonating circuit such that its resonating frequency shifts to the desired target frequency of the radio signal. Figure 3 below shows the antenna efficiency envelope that is created by a highly efficient narrow-band antenna that can be tuned (blue curve), in comparison to an inefficient, wideband antenna that is not tuned (red curve).
Figure 3. Comparison of the antenna efficiency of a wide-band antenna with a tunable narrow-band antenna.
Tunable antennas offer obvious, significant performance advantages over wideband antennas, and the industry has embraced this concept to try to solve the antenna performance degradation experienced in so many recent smartphones. Looking for example at public teardown reports of Samsung Galaxy smartphones and Apple iPhones (https://www.ifixit.com/Teardown/), it appears that both use antenna tuning technology. RF frontend market research reports from Mobile Experts or Strategy Analytics estimate that there will be approx. 500 million antenna tuners shipped in 2015.
The only problem with today’s tuning solution is that until now OEMs had to use RF switches to facilitate the desired antenna tuning. This solution is known as “RF band switching” and while it provides some antenna performance improvements it leaves two major challenges, as shown on the left-hand side of Figure 4 below:
- RF switches are inherently lossy components when inserted into the RF signal path, creating almost as much “insertion loss” as the performance gain enabled by the band switching (red curve vs blue curve).
- RF switches only provide a limited number of states to switch the antenna resonance, typically two, which is not enough to cover all the required spectrum bands efficiently.
Figure 4. Antenna efficiency comparison of RF switch and RF MEMS based antenna tuning solutions.
The right-hand side of Figure 4 shows how to “properly tune” a narrow-band antenna over a wide bandwidth by using digitally variable, effectively loss-less RF MEMS capacitors to shift the antenna’s resonating frequency. The main advantages of RF MEMS based antenna tuning are:
- RF MEMS tuners are virtual loss-less components when inserted into the RF signal path, creating almost zero “insertion loss” and thereby preserving the entire tuning gain.
- RF MEMS tuners provide 32 tuning states that seamlessly tune the efficient narrow-band antenna resonance across the entire tuning spectrum.
As stated before, the reason that RF switches perform so poorly when used for aperture tuning is largely driven by their high insertion loss, which permanently reduces the antenna efficiency by 1-2dB. Therefore it is critical to understand what causes these losses in the RF switch, and why RF MEMS tuners are virtually loss-less.
An RF switch uses semiconductor technology to create different routes for the RF signal. These routes run through traces of “doped” Metal-Oxide material that change its conducting properties in the presence of an external voltage. The external voltage is realized through the so called field-effect-transistors (“FETs”). This gives an RF switch the ability to selectively direct the RF signal path using the corresponding FETs to apply voltage to the semiconducting material (Figure 5 below, left hand side). Unfortunately, the semiconducting material exhibits high resistance, typically greater than 1.5Ω, depending on the switch configuration. Having to traverse a region of such high resistance weakens the RF signal transmission, which explains the permanent 1-2dB insertion loss exhibited by RF switches.
Figure 5. Comparison of the insertion loss contributors of an RF switch compared to an RF MEMS tuner.
An RF MEMS tuner, on the other hand, creates no resistance for the RF signal, therefore causing virtually zero insertion loss. This is accomplished by keeping the RF signal path in metal at all times, directly connecting the antenna directly to the RF pin of the variable Metal MEMS tuner. Considering that the RF MEMS tuner has the capability to fine-tune across 32 stages RF MEMS Tuners give the RF designer the perfect tool to use Tuning Physics to overcome Antenna Physics and design highly efficient Smartphone antennas that can cover all LTE spectrum bands in very space constrained environments.
And just as the limitations of RF band switching are becoming clearer proven, precise, loss-less and highly reliable RF MEMS tuners are given designers the option to deliver antenna tuning solutions that can provide ~2-3dB of extra RF performance, effectively doubling the signal strength and data rates for our otherwise awesome Smartphones. It seems that time has come to “ditch the RF switch”.