November 2010 Expert Advice

Combining various print transmission lines is one way resolve complexity of RF and microwave system in order to more effectively realize miniaturization, trade-off design, and the design of novel components with better electrical performance. Such combinations depend on the type, the configuration, and the performance of transmission lines used (see Table 1). These combinations have to provide very low return loss, wide bandwidth, and very high transmission efficiency (low loss). Table 1 represents performance of various print transmission lines which can be used in combinations.

Table 1. Performance of Various Print Transmission Lines

A balun provides a combination of an unbalanced and a balanced transmission line. An unbalanced transmission line transmits signals between a conductor and a ground plane, whereas a balanced line transmits signals between two conductors. Commonly used unbalanced lines are SL, SSL, ML, CPW while balanced lines are CS, BS, SLL, FL. Baluns are important components of many wireless applications such as push-pull amplifiers, balanced mixers, antenna feed networks, frequency multipliers, and balanced modulators, among others. In a typical RF or microwave mixer, LO and RF signals, being unbalanced, require conversion into a balanced signal. Baluns provide these transformations. In modern RF and microwave antenna arrays, passive balanced antenna elements (dipoles, loops, yagis) should be integrated with the active beam forming structure which is typically an unbalanced line. Design of baluns generally includes the consideration of amplitude and phase balance performance, all-ports matching, and isolation between the two balanced outputs. A balun should deliver equal current amplitude through its two output ports with a 180-deg phase difference, return loss should be minimized to ensure proper matching, and the insertion loss should be low. Baluns are required for proper connection of parallel antenna lines to a transceiver with a 50 ohm unbalanced input/output. Besides impedance matching, baluns must provide good isolation, minimum loss for balanced to unbalanced transformation, small size, and low cost.

The simplest printed three-port splitter can have a series or a parallel connection of one input and two outputs of different print transmission lines. The case where input ML and two output lines are nonsymmetrical SLLs that are realized on two sides of the dielectric substrate. The slotline arms and the microstrip arm are connected in parallel. The another splitter includes an input top slotline with an open end and two output MLs on the bottom substrate side. The microstrip arms and the slotline arm are connected in series. In the T-junction between an input ML with a quarter-wavelength open stub and two output CPWs, the center conductor of the CPW has a via hole. In a T-junction between input CPW and two ML outputs, the CPW has a quarter-wavelength shorted stub.

The combinations of regular and coupled print lines uses in the broadband ring coupler with phase overturn (Λ-ring). A microstrip ring coupler includes the three-quarter wavelength section is replaced with a one-quarter-wavelength coupled line section (phase inverter) with two diagonal grounded ends. The shorted parallel coupled line section provides a 180⁰ phase delay. This hybrid ring, which uses a combination of single and coupled lines, provides 20 dB of return loss and isolation greater than 25 dB within the bandwidth of over 40%. The switched-line phase shifter includes two SPDT switches, one regular reference line of length 3L, and two other parallel coupled lines of equal length L=Λ/4 , directly connected to each other at one end. This network is a modification of the well known Schiffman phase shifter. Phase shift function is determined by the phase difference of signals transmitted through the coupled section of length L and the reference line of length 3L. In the octave band, this section provides a nearly constant phase shift with respect to the regular line with electrical length 3 .

The connection of regular and irregular lines can be used in phase shifters. The term “irregular line” will stand for coupled conductors with strong magnetic coupling, with minimum influence of the RF ground plane on the parameters of the line (ideally, the absence of ground plane in the coupling area). Strong magnetic coupling is realized without magnets or ferrites. In irregular lines, strong magnetic coupling between the lines provides for miniature dimensions and an increased bandwidth. The RF ground has to be apart from the irregular line and close to the appropriate input/output lines, that is, there should be no RF ground plane in the area of coupled conductors. The electrical length of the irregular lines is equal to the electrical length of the regular line. The very small physical length of the irregular lines depends on the coefficient of magnetic coupling.

The combination of a high-Q line with small size transmission lines provides the trade-off design (loss vs. physical dimensions and cost) of RF and microwave components and subsystems (front-end, preselector, etc.). SSL and ML both have their own particular advantages. Although the microstrip circuit occupies a smaller area on the substrate and provides the easiest fabrication (low cost), it has high insertion loss (see Table 1) and poor temperature stability. The high-Q SSL provides low insertion loss but has the disadvantage of large physical dimensions and higher cost, especially for the low frequency application (less than 2 GHz). For some applications, a combination of these two lines provides a trade-off design with a better performance.

The combination of low- and high-loss transmission lines provides implementation of distributed RF and microwave attenuators and terminations. In most conventional microstrip designs, metal conductor thickness t_c and ground plane thickness t_G should be greater than approximately three times the skin depth in order to minimize ML loss. The distributed attenuator or termination can be realized by using a high-loss uniform microstrip line, which is several wavelengths long. This line has a film with a high surface resistance R_S. The surface resistance R_S should be chosen as a compromise between the required conductor loss and input matching of the attenuator. To increase the surface resistance, the conductor thickness t_c of the line is chosen to be significantly less than δ. The input line for termination and input/output lines of the attenuator are regular low-loss ML with conductor width equal to that of the high-loss ML. The special ML with ground plane lossy aperture (GPLA) can be used for implementation of distributed resistors and attenuators with high surface resistance R_S. To increase the surface resistance for a termination or an attenuator, the thickness of the ground plane in the corresponding area is chosen to be significantly less than the skin layer thickness T_PGLA less than δ. This film has low thickness of metallization which is chosen to be significantly less than δ. Another alternative is to use a low-conductivity material. Nichrome and tantalum are widely used due to their good stability and low TCR. For high attenuation values with limited dimensions, the conductor of attenuator or resistor can be given a meander or a spiral line shape.

The combination of high-Q transmission lines and a transmission line with elliptical or circular polarization (SLL, CPW) can be used for realization of non-reciprocal devices. The presence of both longitudinal and transverse RF magnetic field in SLL and CPW provides elliptical polarization that is useful for nonreciprocal ferrite circulators and isolators. For matching to standard 50-Ohm inputs/outputs, an impedance-transforming network should be introduced in each of isolator/circulator input/output. Quarter-wavelength transformers based on high-Q transmission lines (see Table 1) are commonly used for these purposes.

The multilayer techniques are used for RF and microwave IC to significantly increase the density of modules, reduce weight, improve reliability, and reduce system cost. Circuit miniaturization can be achieved by multilayer three-dimensional vertically integrated circuits using components based on various transmission lines. The most popular combination in a multilayer design is ML-SL.

This is a quick review of transmission lines. If you have any questions, please use the comments box and Leo will get back to you with an answer.