Integrated circuits involve using voltage-controlled oscillators which are mostly in the form of ring oscillators or LC-oscillators. Ring oscillators are distinguished by a high degree of frequency tunability. That advantage however is impaired by a strong phase noise and a severe phase jitter. In the case of LC-oscillators frequency tunability is predominantly implemented by means of variable capacitors, for example capacitor diodes. Those oscillators admittedly involve a lower level of phase noise and a lesser degree of phase jitter, but frequency tunability is in most cases seriously reduced.
Japanese patent application 093 215 38 A describes a voltage-controlled LC-oscillator circuit in which a part of the inductance is short-circuited by means of a switching transistor for given periods of time, whereby the inductive component is reduced at times in such a way that alternate operation of the frequency in two frequency bands is possible.
Apart from the switching operation which is substantially slower than the period duration in the desired frequency range, such an arrangement does not permit continuous tuning of the frequency in a wide frequency range.
A similar principle is also described in: A. Kral et al “RF-CMOS-Oscillators with Switched Tuning”, Custom Integrated Circuits Conference (CICC'98), pp. 555-558. In the case of a fully integrated CMOS-oscillator for a frequency range of between 1 and 2 GHz a tuning range of about 26% is achieved by switching between a plurality of discrete inductance values.
Besides the use of switching elements which adversely influence phase noise and phase jitter, that arrangement is seen to suffer from the disadvantage that, in spite of the circuit being of a high degree of complexity, it was only possible to achieve a relatively limited frequency tuning range. In addition the switching of discrete inductance values means that it is only possible to achieve quasi-continuous frequency tuning which has to be supplemented by capacitive tuning.
In integrated radio systems the oscillator must enjoy a relatively great tuning range in order to compensate for technology and temperature fluctuations and to cover the receiving and transmitting band respectively.
With operating voltages in modern technologies becoming smaller and smaller the available voltage range for the control voltage of the voltage-controlled oscillator (VCO) is becoming progressively smaller. That means that the necessary sensitivity of the oscillation frequency of the oscillator in relation to control voltage variations increases. The consequence of this is that, upon integration of the VCO into a phase-locked loop (PLL) the noise of the control voltage causes severe phase noise. That problem is becoming more acute with down-scaling of the technology, which goes hand-in-hand with the reduction in the supply voltage.
Therefore the object of the invention is to propose a voltage-controlled oscillator with an LC-resonant circuit, with which the disadvantages of the state of the art are overcome and with which continuous frequency tunability in a wide range can be achieved in particular with a low level of phase noise and a low level of phase jitter.