Transient analysis of dynamically switched frequency shift keyed transmitters

Abstract

A dynamically switched matching network for transmitting a wideband frequency shift keyed (FSK) signal from an electrically small monopole antenna is studied. The effects of switch synchronization, frequency deviation, the antenna's impedance, and Q-factor at the two carrier frequencies are studied. It is found that the average energy per bit of the transmitted FSK waveform is maximum when the loss of reactive energy between states transition is minimised by switching it at the instant of peak voltage across the antenna's terminals. However, a switching network topology that stores more energy is shown to be less sensitive to the synchronization condition. In addition, we show that this dynamically switched system transmits more bit energy when the carriers are closely spaced due to a smaller change in stored energy levels between states. Results are presented using a detailed analytical study and full wave simulation. Near field measurements from a monopole antenna circuit model show that under proper conditions, an FSK signal can be transmitted at a rate 7 times greater than the bandwidth of the transmit antenna with minimal signal quality degradation.