Abstract
The object of the present project was to investigate some of the problems of using ferro-electric material for permetric amplification. The material was used in the form of a cylindrical rod. Analysis shows that a percentage change of dielectric constant do not necessarily give the same percentage change of reactance of a dielectric rod. For a particular value of frequency and k, the relative dielectric constant, the diameter of the rod which gives a large percentage change of reactance is fixed. With very large dielectric constant (> 5000), this value of the diameter becomes very small. The required condition can be expected to be satisfied, in practice, by varying the dielectric constant, keeping the other parameters constant. With this in view, samples of different percentage compositions were made with barium titanate and polythene. Several methods have been discussed for measuring the non-linearity of the samples. The microwave bridge is a quick and sensitive method for the detection of the non- linearity. The primary interest being in the shift of the resonant frequency of a cavity, detail measurements have been carried out in the cavity. A cavity was designed in which the frequency condition for parametric amplification, i.e., Wp = Ws + Wi, was satisfied. The output at the signal frequency was observed while the cavity, was subjected to high microwave power at the pump frequency. With high average pump power, the cavity tuning at Fs, the signal frequency changed and on returning the output at Fs was larger than before. The effect was not instantaneous but slow indicating heating effect. This effect was also noticed with a sample which did not satisfy the above frequency condition. Perturbation measurements showed that the field at the sample, inside the cavity, was small. This is one of the possible factors which contributed to the absence of the parametric amplification. An attempt has been made to measure the quality of these ferroelectric samples in a way similar to that used for finding the quality of the variable capacitance diode. A value of 2 for change of susceptance conductance can be obtained with some of the- compositions with a d.c. bias field of 9.6 kV/cm. Finally, the complex permittivity of some of the: compositions have been measured in a cylindrical cavity operating on E010 - mode at 3800 Mc/s. It is found that the dielectric constant of the compositions vary from 48 to 11 with 93 to 78 per cent of barium titanate in the composition. The measured low frequency value of the dielectric constant of barium titanate is 6470. Small signal measurements at microwave frequencies show that the incremental dielectric constant of the compositions is dependent on their previous history.