(1). Field of the Invention
The invention relates to a band-rejection filter (notch filter) for suppressing at least one unwanted frequency component, (called noise component hereinafter) present in an analogue information signal. For that purpose it comprises a forward path and a negative feedback path coupled thereto for selective negative feedback of the said noise component. This negative feedback path comprises a bandpass filter whose output signal is subtracted from the information signal in the forward path and which has a very high gain factor for the noise component to be suppressed.
Such a band-rejection filter can, for example, be used as a servo-control loop for controlling the position of a setting element in an optical reading arrangement for disc-shaped information carriers, for example as described in Reference 1. It can alternatively be used in a decoding circuit for stereo multiplex signals, as proposed in, for example, reference 2.
(2). Description of the Prior Art
In order to provide adequate suppression of the noise component, the band-rejection filter must be adjusted accurately, which is time-consuming and expensive. To obviate this, the band-rejection filter can be implemented with the aid of digital filters and a digital difference producer might alternatively be employed for subtracting the digital output signal of the digital bandpass filter from the information signal. However, such an implementation of the band-rejection filter requires the use of a digital-to-analogue converter. As is known, such converters introduce however a significant amount of quantization noise, while the band-rejection filter realized thus with the aid of digital circuits dissipates, in addition, a considerable power. A band-rejection filter implemented in this way has however the advantage that it can completely be constructed as an IC (Integrated Circuit), but it requires a large substrate surface area.
A band-rejection filter which has indeed the advantages, but not the disadvantages of the above-described filter formed from digital circuits is proposed in Reference 2. More specifically, it is proposed therein to form the band-rejection filter with the aid of the switched capacitors. The input of this bandpass filter is then coupled to the filter output via a switching arrangement. The signal stream to this bandpass filter is periodically interrupted by this switching arrangement. Also the negative feedback signal and the analogue information signal are each applied to an adding device via a switching arrangement.
The said switching arrangements are all controlled by a control signal consisting of a sequence of control signal periods having a duration T.sub.s and occurring at a rate f.sub.s and produced by a control signal generator. Each control signal period is divided into a tracking phase and an interrupt phase. More specifically, such a switching arrangement allows the signal applied to it to pass unobstructedly during each tracking phase, whereas during each interrupt phase the switching arrangement does not allow signals to pass.
Should the sum signal produced by the adding device be prevented from being lost when the switching devices do not allow signals to pass, then a hold circuit can be provided at the output of the adding device.
Only analogue signals are now processed in this band-rejection filter of the switched capacitor type. This means that neither an analogue-to-digital converter nor a digital-to-analogue converter is required therein and that consequently no quantization noise is introduced. Partly as a result thereof the consumed power and the required substrate surface area on integration can be small. In practice it has been found that with this band-rejection filter a noise frequency suppression of approximately 30 dB can be achieved, which is sufficient for many uses.