digital phase shifter
E.Yu. Zameshaeva, P.A. Turalchuk, D.V. Kholodnyak, M.D. Parnes, R.G. Shifman, I.B. Vendik
Modern microwave devices should meet the requirements of compact size, low cost and reproducibility. For example, 6-bit phase shifter (PS) and 5 bit attenuator show the possibility of successful application of technology of printed circuit boards with surface mounting components to the design of miniature microwave devices, meeting the above mentioned requirements.
The SPDT switch NEC uPG2214TK was used to provide digital control. In the frequency range up to 6GHz the insertion loss of the opened channel of the switch does not exceed 0.5 dB, blocking in the closed channel is better than -20 dB, decoupling level is not less than 22 dB, return loss in the input channel is -25 dB, and power handling is 27 dBm.
In order to miniaturize a phase shifter transmission line sections should be replaced with the equivalent circuits based on lumped components. The presented scheme of a broadband phase shifter uses artificial LC-based transmission line sections with positive and negative dispersion. Switching is implemented from the transmission line section with positive dispersion and linear dependence of the propagation constant on frequency and the artificial transmission line section with negative dispersion and the inverse dependence of the propagation constant on frequency. In the presented design one channel is realized as a traditional transmission line section with positive dispersion and another one as an artificial transmission line section with negative dispersion. Both channels have the same value but different sign of the electrical length at the central frequency. Also these transmission line sections have nearly the same slope of phase characteristics, that makes it possible to obtain a small phase shift error in wide frequency band.
In the bits providing the phase shift of 45º, 90º, and 180º, the transmission line section with positive dispersion was realized with 2 sections of misrostrip line separated with the series capacitor. Another channel presented with the transmission line section with negative dispersion was designed using one T-shaped network implemented with surface mounted LC-components.
The smaller bits providing the phase shift of 5.625º, 11.25º and 22.5º, were realized as two reactive networks of different electrical length switched alternately. These reactive networks were presented with two microstrip line sections of different length in 5.625º bit. In 11.25º bit series connection of inductance and capacitor were used in reactive networks. The 22.5º bit was realized as two identical sections of 11.25º. The order of bits was optimized to minimize the insertion loss and input/output VSWR in the whole device.
The device was implemented on the printed circuit Rogers RO3210 (εr=10.2, h=0.625mm) with miniature surface mounted components (inductors by Coilcraft and capacitors by ATC) and has the size of 27 mm × 16 mm. The measurements of the manufactured device show insertion loss less than 2 dB and VSWR less than 1.8 in every bit. Phase shift error does not exceed 6º.
The 5-bit attenuator with the attenuation discrete of 1 dB was designed using the same technology of printed circuit board with the surface mounted components. The order of bits was chosen in order to minimize the VSWR and the attenuation errors of the device. The bits were designed as switching channels. The first channel was realized as a transmission line section separated with the blocking capacitor in two parts. The second channel is presented with a resistive T-shaped or П-shaped network. Switching was implemented with SPDT switch described earlier.
The 5-bit attenuator was implemented on printed circuit board Rogers RO3210 with surface mounted resistors by Panasonic and has the size 29 mm 16 mm. The device was tested in the frequency band of 0.5 6 GHz. The attenuation error is less than 2 dB for the 16-dB bit, for other bits the attenuation error does not exceed 0.75 dB. VSWR of the device is less than 1.8.