Wide-band active variable tuning circuits

Active variable tuning circuits having a constant bandwidth or a constant Q value are described. Their resonant frequency can be varied continuously over a considerable range maintaining high performance. A generalised immittance convertor (GIC) is used as variable reactance and a new compensation technique using negative capacitance is proposed. The validity of the theory is confirmed by experiments employing ganged variable resistances as variable elements in the tuning circuits. The variable resistances are replaced by periodically switched resistances to obtain circuits which can change their resonance frequency linearly with voltage and are suited for integrated circuits. As a practical application, low frequency spectrum analysers are described.