High power waveform measurement system enabling characterisation of high power devices including memory effects

The increasing demand for higher data rates in wireless communication systems has led to the more effective and efficient use of all allocated frequency bands. In order to use the whole bandwidth at maximum efficiency, one needs to have RF power amplifiers with a higher linear level and memory-less performance. This is considered to be a major challenge to circuit designers. In this thesis the linearity and memory are studied and examined via the behaviour of the inter-modulation distortion (IMD). A major source of the in-band distortion can be shown to be influenced by the out-of-band impedances presented at either the input or the output of the device, especially those impedances terminated the low frequency (IF) components. Thus, in order to regulate the in-band distortion, the out of-band distortion must be controllable. This has necessitated the development of an upgraded measurement system, where, for the first time, the IF measurement system power is scaled up and extended from approximately 2W to approximately 100W. This was made possible by the design of high power IF bias tee and its integration with a high power IF test-set. The investigation of the influence of out of-band distortion, particularly that at the low frequencies, generally referred to as base-band memory effects, on in-band distortion in high power LDMOS devices, has been made possible by the development of this pioneering, high-power modulated waveform measurement system since it allows for the observation and control of all relevant frequency components (RF, IF and DC). This measurement system is capable of handling IF and RF power levels in excess of 100W with bandwidths ranging from approximately 10 kHz to approximately 12GHz, which makes it particularly appropriate for the characterisation of devices used in base-station mobile communications system applications These subsequent measurements demonstrate that the bandwidth, over which the base-band impedances must be controlled, should be extended beyond the generally accepted value of twice to at least four times the modulated bandwidth. Moreover, the measurement system permitted an intensive investigation of the base-band impedance terminations variations on inter-modulation distortion allowing an optimum to be found that minimise overall in-band distortion (circuit linearisation technique). Hence, indicating that it may be possible to meet the 3rd Generation Partnership Project (3GPP) standards for the maximum allowable adjacent channel leakage ratio (ACLR) in mobile terminals without using any extra linearisation techniques such as predistortion. These important observations have significant implications for modern PA linearisation techniques, as well as requiring careful consideration when designing PA bias networks