An oscillator circuit is disclosed which can be used to produce oscillation using a piezoelectric crystal, with a frequency of oscillation being largely independent of any shunt capacitance associated with the crystal (i.e. due to electrodes on the surfaces of the crystal and due to packaging and wiring for the crystal). The oscillator circuit is based on a tuned gain stage which operates the crystal at a frequency, f, near a series resonance frequency, f.sub.S. The oscillator circuit further includes a compensation circuit that supplies all the ac current flow through the shunt resistance associated with the crystal so that this ac current need not be supplied by the tuned gain stage. The compensation circuit uses a current mirror to provide the ac current flow based on the current flow through a reference capacitor that is equivalent to the shunt capacitance associated with the crystal. The oscillator circuit has applications for driving piezoelectric crystals for sensing of viscous, fluid or solid media by detecting a change in the frequency of oscillation of the crystal and a resonator loss which occur from contact of an exposed surface of the crystal by the viscous, fluid or solid media.

A relaxation oscillator of reduced complexity is described which can be constructed as part of a silicon integrated circuit. The current controlled oscillator includes complementary field effect transistors operating in enhancement mode. The drain of one FET is connected to the gate of the other FET and vice versa. The resulting CMOS circuit functions as a four-layer diode. A resistor is connected between the drains of both transistors. A storage capacitor is connected between the sources of both transistors. A current source is connected to charge the storage capacitor such that the frequency of an oscillator output signal is determined by the current generated by the current source.

A variable capacitance network is disclosed, comprising a plurality of capacitance arms connected in parallel with each other between first and second terminals of the network. Each capacitance arm has a varactor and a series capacitor in series with the varactor A control input applies a common control signal to the junctions between the varactors and their associated series capacitors, to allow for simultaneous control of each varactor.

A variable capacitance network is disclosed, comprising a plurality of capacitance arms connected in parallel with each other between first and second terminals of the network. Each capacitance arm has a varactor and a series capacitor in series with the varactor A control input applies a common control signal to the junctions between the varactors and their associated series capacitors, to allow for simultaneous control of each varactor.

A radio frequency oscillator containing a means of frequency band selection and a tuning mechanism permitting frequency variation within each of the bands. The oscillator typically consists of a transistor, a frequency determining circuit, one or more voltage controlled variable capacitance diodes for frequency tuning and frequency range switching means to step change the operating frequency. The frequency determining circuit can consist of a lumped inductor, a microstrip line or a combination of a microstrip line and a lumped inductor. The frequency determining circuit is typically connected to the base of the transistor. In a preferred embodiment the frequency range switching is accomplished using an RF switching diode, used to short out one or more portions of the tuned microstrip line in accordance with the required frequency step change. The circuit also incorporates additional circuitry to support frequency modulation.