Design of a Miniaturised Asymmetrical Power Splitter Using Low Impedance Artifical Transmission Lines

Abstract

Transmission lines are the basic building blocks of any RF and microwave circuits. The width of a microstripline increases as the characteristic impedance is lowered for a given substrate. Wide microstriplines suffer from spurious and higher order modes at higher frequencies and may not behave as transmission lines. This means the lower limitation for a realisable microstripline is about 10 ohm. In this project microstriplines with characteristic impedances of 7 ohm and 25 ohm at a frequency of 2 GHz were designed and realised using the artificial transmission lines (ATL) concept. Detailed theoretical analysis and circuit and EM simulations were used for the design and implementation of the ATLs. Taconic TLY-5 substrate was used for the PCB fabrication. The substrate thickness was 0.787 mm and the dielectric constant was 2.2. The measured results were de-embedded and compared with the simulation results. The detailed procedure of modelling and de-embedding of an SMA connector is also given.

The 25 ohm ATL was realised using microstriplines only, where as microstriplines and chip capacitors were used in realising the 7 ohm ATL. The measured characteristic impedance of the 25 ohm ATL was 24.4 ohm and the measured electrical length of the 25 ohm ATL was 180 degrees at 2.1 GHz. To realise a 25 ohm ATL with 90 degrees electrical length, the half-wavelength 25 ohm ATL geometry was cut into half and one of the half geometries was EM simulated. The EM simulated electrical length of the 25 ohm ATL was 90 degrees at 1.9 GHz. The measured characteristic impedance of the 7 ohm ATL was 5.9 ohm and the measured electrical length of the 7 ohm ATL was 90 degrees at 1.8 GHz.

The main advantage of an ATL is size reduction. A 25 ohm meandered microstrip line (substrate thickness = 0.787 mm, dielectric constant = 2.2) with 180 degrees electrical length at 2 GHz has a size of 34 mm x 15 mm. The 25 ohm ATL with 180 degrees electrical length at 2.1 GHz was realised in a size of 22 mm x 19 mm. The design of the 25 Ω ATL resulted in 18 percent reduction in area compared to the meander line. A 7 ohm conventional microstripline (substrate thickness = 0.787 mm, dielectric constant = 2.2) with 90 degrees electrical length at 1.8 GHz has a size of about 28 mm x 27 mm. The 7 ohm ATL with 90 degrees electrical length at 1.8 GHz was realised in a size of 7 mm x 8.4 mm which is only 8 percent of the conventional 7 ohm microstripline area.

In general, a spacing of 3h where h is the substrate thickness is required between the adjacent microstriplines. In this project detailed investigations were done to see if the spacing can be reduced without any detrimental coupling affects and a spacing of 0.6 mm was used. This reduction in spacing has resulted in reduced size of the ATL.

For an asymmetrical power splitter based on the Wilkinson topology, the power splitter output power split ratio depends on the square of the characteristic impedances of the quarter-wavelength arms. In this project an asymmetrical power splitter was designed and realised using a 7 ohm ATL and a 25 ohm ATL as the quarter-wavelength arms. The desired centre frequency of the power splitter was 2 GHz and the measured centre frequency was 1.6 GHz. At the centre frequency the phase difference between the output ports of the power splitter will be zero. The simulated power split ratio was 10.1 dB and the measured power split ratio was 13 dB. The power split ratio calculated using the measured characteristic impedances of the ATLs (24.4 ohm and 5.9 ohm) will be 12.4 dB which is very close to the measured power split ratio.