MULTI-PORT NETWORKS FOR RF MEASUREMENT AND WIDEBAND TRANSMITTER DESIGN

Abstract

This thesis describes different multi-port networks for applications such as radio frequency (RF) measurement and harmonic less wideband transmitter design. In the first work, a low-cost network analyzer is proposed for measurement of complex S-parameter of RF passive circuits through scalar voltage readings. The multi-port network (MPN) being used has an RF input port, four test ports and two power detection ports. A novel calibration technique is proposed which requires only half the number of power detectors than a standard six-port reflectometer (SPR) for S11 measurement. The MPN has been modelled through mathematical equations and Neural Network (NN) based model. The NN based model resulted better accuracy and relatively wideband operation while measuring the reflection coefficient. The full 2-port S-parameter of a device under test (DUT) can be measured by using two such MPNs. The proposed Alternate Network Analyzer (ANA) works between 1.8-2.2 GHz. The amplitude and phase errors are found to be less than 0.5 dB and 5°, respectively, when compared to commercial vector network analyzer (VNA) for 1-port DUT. The second work discusses different ultrawideband (UWB) balun architectures and their use in differential mode measurement of balanced networks. Balanced transmission lines are usually characterized in terms of mixed-mode S-parameter (MMS), which requires differential and common mode stimulus. A standard two-port VNA cannot measure the MMS directly. Thus, a measurement technique has been proposed for characterizing a balanced transmission line with the help of baluns. A wideband planar Guanella balun has been designed from 0.5-2.6 GHz. The measured amplitude and phase imbalance of this balun is found to be less than 0.75 dB and 7°, respectively. A transmission line transformer (TLT) based coaxial balun has been proposed for 0.3-2.7 GHz band. The lower operating frequency of this balun has been further reduced by encapsulating the coaxial cable with ferrite beads. The operating frequency of this modified balun is found to be 0.03-2.6 GHz. All these baluns are analyzed in terms of their MMS. In the third work, a polyphase (multi-path multi-phase) network is proposed for in-band harmonic cancellation of RF Power amplifier (PA). Due to its non-linear behaviour, the PA generates various harmonics and intermodulation distortion (IMD) products, which may interfere with other usable channels and must be suppressed before transmission. In-band harmonic suppression of wideband PA is a challenging task and requires an expensive switched filter bank (SFB). Thus, a multioctave filterless RF transmitter has been designed with in-band harmonics suppression using polyphase network. A novel passive polyphase network is developed for harmonic suppression within the operating band. Three wideband continuous 10W class F PAs are used in the setup. The output power of the overall system varies between 41.7-42.4 dBm and the gain varies between 4.4-7.7 dB from 0.65 to 2.05 GHz. The signal measured at the output has the 2nd and 3rd harmonics suppressed by more than 30 dB in the desired band. Finally, some design techniques have been proposed for the miniaturization and bandwidth enhancement of RF hybrid couplers. Out of the many miniaturization techniques available in the literature, the slow wave structure is a promising one. However, its miniaturization level is limited by the maximum realizable line impedance. To overcome this limitation, a geometrically progressive stub-loaded slow wave line (GPSWL) has been proposed which helps in improving the miniaturization level, as well as suppresses spurious passbands. Both 90° and 180° hybrid couplers are widely used in various RF circuits. Thus, a rat race coupler (RRC) has been designed using the GPSWL resulting 20% more size reduction than the conventional slow wave technique. A bandwidth enhancement scheme of branch line coupler (BLC) and RRC is also developed using stepped impedance line (SIL). This gives moderate bandwidth enhancement without increasing the circuit size.