The need for what’s called antenna tuning—either by adjusting the antenna itself or via a matching circuit between power-amplifier output and the antenna—is almost as old as wireless itself. Even in those early days, electromagnetic theory and hands-on practical experimentation showed that effective power transfer and optimal antenna performance, measured by several parameters, require that the source impedance and the load impedance be complex conjugates.
The problem has not gone away, but instead has morphed into a new and more-challenging form. Traditionally, antenna-matching circuits were built into smaller, lower-power radio designs; in other cases, they were and still are offered as commercial units in external enclosures. This was due to the high-power ratings spanning tens, hundreds, or even thousands of watts, along with the physically larger values needed at the lower frequencies of tens or several hundred megahertz.
Some of these external tuners were designed for a single band, while others for multiband use cases, such as amateur or ham radio, which had front-panel switchers to enable adjustment settings for the different bands in use (Figure 1).
Figure 1 This variable L-network random wire antenna tuner is designed for manually matching the low output impedance of transmitter (up to 200 watts) to the high impedance of a random wire (or vice versa) from 2 to 30 MHz. Source: MFJ Enterprises
Many of them are one-off, hand-crafted works combining artful form and required function (Figure 2).
Figure 2 Many amateur-radio enthusiasts prefer to design and fabricate their own antenna-matching units for their bands of interest and power levels, such as this one covering 3 to 30 MHz and handling up to 150 watts. Note the toroidal transformer with multiple windings. Source: http://pa-11019.blogspot.com/2011/
Newer antenna tuners incorporate autonomous self-controlled auto-tuning using an internal processor or allow an external PC to do so via an USB port.
New applications, new approaches
But as the saying goes, times have changed. Now the tuning battle is over 5G and even 4G phones supporting multiple bands and embedded antennas such as the widely used planar inverted-F antenna (PIFA). Smartphones are relatively low-power devices operating above a gigahertz with multiple bands, which must be supported with seamless band transitions and handoffs. The associated LC values are small, which simplifies the challenge in some ways, but also makes it harder in other ways.
Complicating the situation, the matching values are not static but are dynamic in routine use as user’s hand changes location and angle, and phone’s position moves with respect to the head and body. Certainly, expecting the user to tune and optimize the antenna-matching circuit in use is simply not an option.
Fortunately, there are now solutions to this dilemma via antenna-tuner ICs. These ICs address the issue by allowing digital setting of up to 16 capacitance values, thus changing the electrical characteristics just enough to optimize the matching or get close enough. Among the vendors are Peregrine Semiconductor (PE64909), Qorvo (QM13025), Skyworks Solutions (SKY59272-707LF), and Infineon (BGSC2341ML10).
Unlike lower-frequency matching circuits with capacitance in the tens of picofarads, and even extending to the microfarads range, these ICs allow tweaking of very small capacitance shifts. For example, the Peregrine Semiconductor’s PE64909 device is a digitally tunable capacitor for 100-3,000 MHz (Figure 3).
Figure 3 The PE64909 antenna tuner IC has a simple function and schematic (above), but its equivalent-circuit model is more complicated (below). Source: Peregrine Semiconductor
In operation, a system processor can use a four-bit code to select one of 16 capacitance values via its 3-wire, SPI-compatible serial interface, serving from 0.6 pF to 2.35 pF (a 3.9:1 tuning ratio) in discrete steps of 117 femtofarad (fF). That’s clearly a modest dynamic range and a small step size, but it’s enough for the application.
Qorvo notes that there are two ways to use capacitance to adjust the antenna appearance (Figure 4).
Figure 4 Antenna tuning can be accomplished via aperture tuning or impedance tuning, each with distinct tradeoffs in attributes and capabilities. Source: Qorvo
Aperture tuning optimizes the total antenna efficiency from the antenna terminal’s free space, and it can do so across multiple bands. It can provide advantages with respect to antenna efficiency for both transmit and receive communications, improving total radiated power (TRP) and total isotropic sensitivity (TIS) by 3 dB or more in some situations.
Impedance tuning maximizes power transfer between the RF front-end and the antenna, and it increases the TRP and TIS by minimizing mismatch loss between the antenna and antenna front-end. It also helps to compensate for environmental effects such as a person’s hand position on a smartphone.
Beyond antenna tuning
According to Qorvo, “Today, aperture tuning is the primary method used in handsets to overcome reduced antenna area and efficiency. Mid-tier and higher-end smartphones use a combination of aperture and impedance tuning to support the ever-broadening range of frequency bands, especially for 5G.”
These ICs are somewhat analogous to the widely used digital potentiometers (digipots), except those typically have 256 or more steps spread over a fairly wide kilohm range along with a much-larger relative step size. All this makes me wonder if commercially available digi-inductors will be coming soon as well.
Beyond antenna tuning, I’d like to think that creative engineers are already looking at these parts and finding unforeseen uses for them. Historically, that’s been the reality as components which originally targeted one class of situations are soon adopted and adapted to address other problems.
Perhaps these tunable pico-farad capacitors will be used to compensate for or cancel circuit parasitics in a balanced or differential topology. Or perhaps they will be used for precise calibration and measurement in some Wheatstone-bridge type of arrangement…you never know.