A Brief Introduction to the Types of RF Phase Shifters

Phase Shifters are a critical component in many RF and Microwave systems. A phase shift module is a microwave network module which provides a controllable phase shift of the RF signal and are used in phased arrays. Phase shifters are used to change the transmission phase angle of a two-port network. Applications include controlling the relative phase of each element in a phase array antenna in a RADAR or steerable communications link and in cancellation loops used in high linearity amplifiers. But what is a phase shifter? And more to the point, what is phase?

Phase Shifter Basics

Phase is the position of a point in time on a waveform cycle. The phase of a single tone is meaningless until it is compared to another signal. Phase then is the fraction of a wavelength difference between two signals and can vary from 0 to 360 degrees. Phase can also be an expression of relative displacement between two corresponding features of two waveforms having the same frequency. In sinusoidal waves “phase” has two meanings: one is the initial angle of a sinusoidal function at its origin, sometimes called phase offset or phase difference, and the other is the fraction of the wave cycle that has elapsed relative to the origin.

Phase shift is any change that occurs in the phase of one quantity, or in the phase difference between two or more quantities. Phase difference is the difference, expressed in degrees or time, between two waves having the same frequency and referenced to the same point in time. Two oscillators that have the same frequency and no phase difference are said to be in phase. Two oscillators that have the same frequency and different phases have a phase difference, and the oscillators are said to be out of phase with each other. Phase Range (degrees) is the phase shift range of the device so that, based on the way the device is configured, it will only be able to provide a phase shift within this range.

Phase Shifter properties include insertion Loss (dB) (loss/gain) and amplitude.  The loss of signal from the input of the phase shifter to the output of the device is called insertion loss. Effective phase shifters provide low insertion loss in all phase states and require less amplification and lower power to overcome the losses. Systems using phase shifters must not experience amplitude changes in signal level as phase states are changed and therefore must have equal amplitude for all phase states.

Types of RF Phase Shifters

Phase shifters can be controlled magnetically, mechanically, or electrically using analog signals or digital bits.

Mechanical Phase Shifter –controlled manually with a knob and the phase from the input to the output is adjusted by turning a knob.

Analog Phase Shifter –controlled by a voltage level and phase shift change is based on the tuning voltage specified for the phase shifter. Analog phase shifters provide a continuously variable phase often controlled by a voltage. These analog phase shifters can be controlled with tuning diodes that change capacitance with voltage, or nonlinear dielectrics such as barium strontium titanate, or ferro-electric materials such as yttrium iron garnet. Analog phase shifter advantages are lower loss and lower cost of parts.

Digital Phase Shifter –digitally controlled, programmable, or can be controlled via a computer interface. Digital, in this case, means two-state devices, where the states have different insertion phases at microwave frequencies. Most phase shifters digitally controlled as they are less susceptible to noise on voltage control lines. Digital phase shifter advantages include immunity to noise on control lines, a more uniform performance, the ability to achieve flat phase over wide bandwidth, are less susceptible to phase pulling when embedded in networks that are not perfectly impedance-matched, ease of assembly versus analog, and have a potentially higher power handling and linearity.