Phase shifters are used to change the transmission phase angle (phase of S21) of a network. Ideally phase shifters provide low insertion loss, and approximately equal 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.

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. 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 line stretchers, and are constructed similar to the switched line phase shifters to be 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 non-linear 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.

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.

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.

Attenuators are passive resistive elements that do the opposite of amplifiers, they kill gain. What is the need of suppressing the gain? The answer is, suppose your design specification calls for 10 dB gain, with a 1.2:1 maximum VSWR, you search the amplifier vendors, and locate an amplifier in your frequency band, but it has 14.5 dB gain and a lousy 2.5:1 match on the input. By adding an attenuator to the input, you can bring the gain down to 10 dB, and you will be improving the input match.

Two things to consider while adding an attenuator :

1. Don’t add an attenuator to an amplifier’s input if you are concerned with the amplifier’s noise figure, every dB of attenuation you put on the input will raise the noise figure by the same amount.

2. Don’t add an attenuator to a power amplifier’s output without considering what it will do to your output power, or what the RF output power of the power amp might do to your attenuator.

There are five common attenuator topologies used in microwave circuits, the tee, the pi, the bridged tee, the reflection attenuator and the balanced attenuator. The tee, pi and bridged tee each require two different resistor values, while the reflection and balances attenuators need only a matched pair of resistors. This allows both the reflection and balanced topologies to be used as variable attenuators with a single control voltage or control current. There are two variations of the reflection attenuator, depending on whether the terminations R1 are less than or greater than the system characteristic impedance Z0.

The bridged tee can be thought of as a modified pi network. The attraction to the bridged tee comes when you are making a variable attenuator, with PIN diodes or FETs. Here are two reasons you might consider it over the pi. First, it only needs two variable resistors (pi and tee need three). Second, the bridged tee uses the full range of resistor values, from zero to infinity, for both R1 and R2. For the pi attenuator, R1 does never goes below Z0 (50 ohms) so some of the diodes’ useful resistance range is wasted. Finally, the bridged tee has a tendency to match itself to Z0 at high attenuation values, because of its two fixed resistors. In practice, the pi may give you higher attenuation range. The resistor R2 can be a “sneak path” in the bridged tee because the diode (or FET) never reaches zero ohms.

Microstrip is a planar transmission line, similar to stripline and coplanar waveguide. Microstrip was developed by ITT laboratories as a competitor to stripline.

Microstrip transmission lines consist of a conductive strip of width “W” and thickness “t” and a wider ground plane, separated by a dielectric layer (a.k.a. the “substrate”) of thickness “H” as shown in the figure below. Microstrip is by far the most popular microwave transmission line, especially for microwave integrated circuits and MMICs. The major advantage of microstrip over stripline is that all active components can be mounted on top of the board. The disadvantages are that when high isolation is required such as in a filter or switch, some external shielding may have to be considered. Given the chance, microstrip circuits can radiate, causing unintended circuit response. A minor issue with microstrip is that it is dispersive, meaning that signals of different frequencies travel at slightly different speeds (usually not a big deal, but this property is what causes the asymmetric frequency of bandpass filters, for example).

Effective Dielectric Constant

The effective dielectric constant eff of microstrip is calculated by:

The effective dielectric constant is a seen to be a function of the ratio of the width to the height of the microstrip line (W/H), as well as the dielectric constant of the substrate material.

Note:- There are separate solutions for cases where W/H is less than 1, and when W/H is greater than or equal to 1. These equations provide a reasonable approximation for eff (effective dielectric constant). This calculation ignores strip thickness and frequency dispersion, but their effects are usually small.