Below is an outline of all of our material on phase shifters. This collection of material is easily the best free phase shifter resource on the planet. And it keeps getting better!
An important characteristic of phase shifters (We recommend that you read this first!)
On this page we have:
Active versus passive phase shifters - new for June 2012!
Phase Shifter Search Tool (on everything RF)
Other pages include:
Types of phase shifters
180 degree hybrid phase shifters (such as rat-races)
Reflection phase shifters using circulators (coming soon)
Time delay units (TDUs) new for January 2011!
Multi-state phase shifter simulations in Agilent's ADS (new for January 2010!)
Multi-state phase shifter simulations part II (new for January 2012!)
MMIC phase shifter examples
Example 3: Hittite HMC543 X-band phase shifter (new for January 2012!)
Applications of phase shifters
Residual phase noise measurement
Phase shifters are used to change the transmission phase angle (phase of S21) of a network. Ideal phase shifters provide low insertion loss, and equal amplitude (or loss) in all phase states. While the loss of a phase shifter is often overcome using an amplifier stage, the less loss, the less power that is needed to overcome it. Most phase shifters are reciprocal networks, meaning that they work effectively on signals passing in either direction. Phase shifters can be controlled electrically, magnetically or mechanically. Most of the phase shifters described on this web site are passive reciprocal networks; we will concentrate mainly on those that are electrically-controlled.
While the applications of microwave phase shifters are numerous, perhaps the most important application is within a phased array antenna system (a.k.a. electrically steerable array, or ESA), in which the phase of a large number of radiating elements are controlled to force the electro-magnetic wave to add up at a particular angle to the array. For this very purpose, phase shifters are often embedded in TR modules. The total phase variation of a phase shifter need only be 360 degrees to control an ESA of moderate bandwidth. Networks that stretch phase more than 360 degrees are often called time delay bits or true time delays (part of a TDU), and are constructed similar to the switched line phase shifters that are described below.
Phase shifters can be analog or digital. Analog phase shifters provide a continuously variable phase, perhaps controlled by a voltage. Electrically controlled analog phase shifters can be realized with varactor diodes that change capacitance with voltage, or nonlinear dielectrics such as barium strontium titanate, or Ferro-electric materials such as yttrium iron garnet. A mechanically-controlled analog phase shifter is really just a mechanically lengthened transmission line, often called a trombone line. Analog phase shifters are a mere side-show and will not be covered here in depth at this time. If you are interested in more information on any of these analog phase shifter topics, let us know and we will try to accommodate you.
Most phase shifters are of the digital variety, as they are more immune to noise on their voltage control lines. Digital phase shifters provide a discrete set of phase states that are controlled by two-state "phase bits." The highest order bit is 180 degrees, the next highest is 90 degrees, then 45 degrees, etc., as 360 degrees is divided into smaller and smaller binary steps. A three bit phase shifter would have a 45 degree least significant bit (LSB), while a six bit phase shifter would have a 5.6 degree least significant bit. Technically the latter case would have a 5.625 degree LSB, but in the microwave world it is best to ignore precision that you cannot obtain. If you can't comprehend this point, you might want to consider a different career such as accounting.
Someone asked us about the relative advantages of analog and digital phase shifters. We provide a list below, the two technologies we are considering are discrete varactor diode (analog) and GaAs MMIC (digital). We don't have a reference for this information, it is based on personal observation. Feel free to use Microwaves101 as THE reference for phase shifter trade studies.
Analog phase shifter advantages
- Lower loss
- Lower cost
Digital phase shifter advantages
- Immunity to noise on control lines
- More uniform performance, unit-to-unit
- Ability to achieve flat phase over wide bandwidth
- Less susceptable to phase pulling when embedded in networks that are not perfectly impedance-matched
- Easier to assemble
- Potentially higher power handling and linearity
The convention followed for phase shifters is that the shortest phase length is the reference or "off" state, and the longest path or phase length is the "on" state. Thus a 90 degree phase shifter actually provides minus ninety degrees of phase shift in its "on" state.
One of the first phase shifter MMIC designs used dual-gate FETs in a way that it was non-reciprocal. You can learn of it here:
Vorhaus, J. L. et al, "Monolithic Dual-Gate GaAs FET Digital Phase Shifter", IEEE transactions on Microwave Theory and Techniques, Vol. MTT-30, No. 7, July 1982.
With an active phase shifter, it might be possible to counteract the loss of the phase shift elements and eliminate an amplifier stage. Seems worth exploring, right?
In the 1980s the idea of an active phase shifter quickly fell beside the wayside, because a passive reciprocal phase shifter is more versatile and requires fewer SPDT switches to route the transmit and receive signals through the phase shifter.
An important note on active phase shifters... you must consider both the gain and the noise figure of your phase shifter when you are analyzing the performance of the next higher assembly. Vorhaus' paper did not report the noise figure of his 1982 active phase shifter.
Since the beginning of RF MEMS, there has been about a billion dollars spent to try to develop a phase shifter with low loss such that a PESA is enabled and the inventors make a killing. In the beginning, it seemed possible that a three-bit Ka-band phase shifter could be made with 1 dB loss. Then the dark ages of MEMS began, when reliability was uncovered to be a major problem, and hence the MEMS Tree of Woe was born and all involved took the acronym off of their business cards. More recently, promises have been reduced to maybe 2.5 dB loss for three bits at Ka-band, 2 dB at X-band. This level of loss spells "game over" for MEMS phase shifters.
We'll keep an eye on the technology, and one of these days make a page of content about it.
If you know of any phase shifter topologies not covered on one of our phase shifter pages that should be described here, drop us a line and we will add your knowledge to this chapter! Want to donate a photo of a phase shifter you designed? Send it our way and we might get your 15 minutes of fame if we decide to put it on this page
Check out what Wikipedia offers on phase shifters, it's slim pickings... they do link back to us, but don't consider us as a "reference". Someone ought to teach those eggheads some manners...1 1 1 1 1 1 1 1 1 1 Rating 4.63 (4 Votes)