Design of complex microstrip circuits by measurement and computer modelling

Measurement techniques have been developed which allow the complex admittance existing at a nominal microstrip open circuit to be accurately characterised. This information, together with the line loss and wavelength, has enabled the electrical defining plane of a shunt stub to be determined. By using a circuit-design technique based on computer modelling it is possible to allow for all these factors. When this approach was applied to the design of a filter with 13 elements, the computer element lengths, when fabricated in microstrip, produced a filter with a bandpass characteristic to within 1% of the computer-modelled response. The need for cut-and-try development is therefore virtually eliminated. The technique of exact computer modelling has also been applied to oscillator circuits, using varactortuned Gunn devices. These circuits are extremely complicated, since they employ waveshaping of the voltage applied to the device. The second harmonic is trapped in a carefully controlled independent resonant circuit. For certain devices, this technique can increase the microwave output power by a factor of 4 or 5. Without the use of cut-and-try methods, devices have operated only 5MHz away from the designed output frequency of 6GHz.