irect conversion receivers, the second frequency is identical to the receive frequency but with a 90 degree phase offset. The frequency synthesiser takes the stability of a frequency reference such as a quartz crystal and translates that reference to the required frequency to be demodulated (receive path) or modulated (transmit path). The transmitter power amplifier amplifies the signal to be transmitted. 4.5.1 RF Device Functionality and Useful Inventions Over the Past 100 Years For the past 100 years radio devices have been required to oscillate, resonate, filter, switch and amplify. The efficiency with which these tasks are performed defines the overall efficiency of the radio system. Fleming's thermionic valve in 1904 and Lee de Forest's triode valve in 1907 were major moments in radio device development. These devices, combined with resistors, inductors, diodes and capacitors provided the basis for Marconi's development of tuned circuits during the First World War. In retrospect, the discovery of the piezoelectric effect by Pierre and Jacques Curie in 1880 was probably at least as significant. The Curie brothers discovered that when pressure was Telecommunications Economies of Scale applied to certain crystals, an electrical voltage was generated. Conveniently for the radio industry, this proved to be a bidirectional effect. Applying electrical voltage to certain crystals would cause them to vibrate at a specific frequency. In 1917, Paul Langevin used quartz crystals in a sonar device for submarine detection and from then on quartz became the basis for detecting and creating specific audio and radio frequencies. In the Second World War, similar research in the US, Japan and the Soviet Union showed that certain classes of ceramics exhibited piezoelectric behaviour. Courtesy of two world wars we were provided with a choice of quartz crystals and/or ceramic-based devices as the basis for providing accurate frequency and time referencing in radio products. The invention of the transistor in 1947 and the integrat