What Are Tuning Elements?

Tuning elements are the physical components within an electronic circuit whose variable electrical parameter, typically capacitance, inductance, or resistance, allows the circuit's resonant frequency or impedance to be adjusted. They are the mechanism by which a tunable filter, oscillator, or matching network shifts its operating point in response to an analog voltage, a digital code word, a mechanical displacement, or a magnetic field. The choice of tuning element determines the tuning range, speed, linearity, power consumption, and circuit complexity of the system in which it is embedded.

The history of tuning elements maps closely onto the history of frequency-agile electronics. Nineteenth-century spark-gap transmitters relied on variable air capacitors and adjustable inductors controlled by hand. Twentieth-century receiver design standardized on the ganged variable capacitor, a precision mechanical assembly that simultaneously tuned multiple LC circuits while maintaining their relative alignment. Contemporary designs replace these mechanical assemblies with electronically controlled semiconductor and MEMS devices that respond in microseconds and occupy a fraction of a square millimeter on a chip.

Varactor Diodes

The varactor diode is the most widely used electronic tuning element from VHF through lower microwave frequencies. A varactor is a p-n junction operated under reverse bias; the depletion region width, and hence the junction capacitance, decreases predictably as the reverse voltage increases. A typical varactor changes capacitance by a factor of three to ten over its voltage range, which corresponds to a resonant frequency shift of roughly one and a half to three times in an LC circuit. Varactors are available as discrete components and as integrated structures in CMOS, silicon-germanium, and gallium arsenide processes. Their principal limitations are nonlinearity under large signals, which produces intermodulation products, and relatively modest Q values compared to fixed-value capacitors of the same capacitance. The ARRL technical file on resonance and tuning methods illustrates how varactor characteristics interact with the Q and linearity requirements of broadcast and communications receiver designs.

Mechanical and MEMS Tuning Elements

Mechanically variable capacitors and inductors achieve the lowest loss of any tuning element and remain in use in high-power transmitter tank circuits and precision laboratory instruments where low insertion loss and high power handling outweigh the disadvantage of slow manual adjustment. MEMS (microelectromechanical systems) tuning elements bring the low-loss characteristics of mechanical devices to integrated circuit dimensions. MEMS variable capacitors use electrostatically actuated movable plates whose gap or overlap area changes capacitance in response to an applied voltage. MEMS switches route signals among fixed-value components in switched-filter tuning banks. Compared to varactors, MEMS devices offer higher Q, better linearity, and lower dc power, making them attractive for the front-end filter banks of reconfigurable radios, as documented in work on reconfigurable RF impedance tuners.

Magnetically Tunable Elements

Yttrium iron garnet (YIG) spheres and ferrite tuning elements exploit magnetically tunable resonance to achieve tuning ranges of multiple octaves with high Q. A YIG sphere placed in a dc magnetic field resonates at a frequency proportional to the field strength; sweeping the field from a few hundred to a few thousand oersteds tunes the resonance from approximately one to several tens of gigahertz. YIG-tuned oscillators and filters are used in microwave test instruments and electronic warfare receivers where wide tuning range and low phase noise are simultaneous requirements. Voltage-tunable ferroelectric materials, such as barium strontium titanate (BST), change permittivity with applied electric field and provide electrically controlled capacitance at millimeter-wave frequencies where varactors and MEMS devices become impractical, as shown in research indexed on IEEE Xplore on automated resonator tuning.

Applications

Tuning elements have applications in a wide range of disciplines, including:

• VHF and UHF voltage-controlled oscillators in phase-locked synthesizers
• Electronically tuned bandpass filters in satellite and cellular receivers
• Wide-range microwave signal generators using YIG-tuned oscillators
• Impedance matching networks in RF power amplifier modules
• MEMS-based reconfigurable antenna front ends