| 摘要 |
A variable frequency oscillator, such as a VCO or DCO, comprises an inductor Lt and a first variable capacitor Ct coupled across the inductor Lt. The capacitance Ct and inductance Lt may be used to set the centre frequency of the oscillator. A second variable capacitor C is coupled across a part L of the inductor, the inductance of the part L being a proportion of the first inductance Lt. The second capacitor C may be smaller than Ct and may be used for fine tuning. It is shown that, in a conventional oscillator (fig.2), the gain Kv, i.e. frequency step size with respect to frequency (d o/dC), is dependent on the cube of the frequency ( o3). According to the invention, the relationship between the inductances Lt and L is chosen to reduce this non-linearity and substantially to minimise the variation of gain Kv over an operating frequency range, i.e. to provide an almost constant d o/dC. The oscillator may find application in systems that use PLLs for the direct modulation of a transmit signal, e.g. in FM or polar modulation. |
