Signal processing arrangement comprising a filter
A signal processing arrangement (REC) comprises a filter (PPF) with variable filter elements (FE). A switching circuit (SWCT) switches a filter element (FE 1 ) and, subsequently, another filter element (FE 3 ) from a filter state to an adjustment state and back again to the filter state. A filter element that is in the filter state contributes to a suppression of unwanted signals. A filter element that is in the adjustment state affects a characteristic of a measurement signal (Sm). An adjustment circuit (ADCT) adjusts the filter element that is in the adjustment state so that the characteristic of the measurement signal is substantially equal to a target value (TV).
1. A signal processing arrangement comprising a filter for suppressing unwanted signals, the filter comprising a plurality of filter elements characterized in that the signal processing arrangement comprises:
a switching circuit for switching a first filter element of the plurality of filter elements and, subsequently, a second filter element of the plurality of filter elements from a filter state to an adjustment state and back again to the filter state, the first and second filter elements that are in the filter state contributing to the suppressing of unwanted signals, the first and second filter elements that are in the adjustment state affecting a characteristic of a measurement signal;
an adjustment circuit for adjusting the first and second filter elements that are in the adjustment state so that the characteristic of the measurement signal is substantially equal to a target value; and
a signal source for applying a stimulus signal to the first and second filter elements that are in the adjustment state, the first and second filter elements providing the measurement signal in response to the stimulus signal,
wherein the signal source is arranged to provide a DC stimulus signal and an AC stimulus signal so as to obtain a DC measurement signal and an AC measurement signal, respectively,
wherein the adjustment circuit being arranged to adjust the first filter element which constitutes a resistor based on the DC measurement signal, and to adjust the second filter element which constitutes a capacitor based on the AC measurement signal.
2. The signal processing arrangement as claimed in claim 1 , wherein the signal source is arranged to simultaneously provide the DC stimulus signal and the AC stimulus signal, the adjustment circuit comprising a DC branch for adjusting the resistor and an AC branch for adjusting the capacitor.
3. The signal processing arrangement as claimed in claim 1 , wherein the target value is derived from a duplicated circuit which is representative of the first and second filter elements that are in the adjustment state, the duplicated circuit receiving a duplicated signal, which is representative of the stimulus signal.
4. The signal processing arrangement as claimed in claim 1 , wherein the signal processing arrangement comprises an oscillator circuit, the first and second filter elements that are switched to the adjustment state forming part of the oscillator circuit, an output signal of the oscillator circuit constituting the measurement signal.
5. The signal processing arrangement as claimed in claim 1 , wherein the signal processing arrangement comprises a controller arranged to cause the switching circuit to successively switch first and second filter elements from the filter state to the adjustment state and then back again to the filter state, for an interval of time that coincides with an operational transition of the signal processing arrangement.
6. The signal processing arrangement as claimed in claim 1 , wherein the signal processing arrangement comprises a controller that is arranged to detect an interval of time during which a desired signal is absent, and that is arranged to cause the switching circuit to successively switch first and second filter elements from the filter state to the adjustment state and then back again to the filter state within the interval of time.
7. The signal processing arrangement as claimed in claim 1 , wherein the measurement signal has a frequency that lies in within a frequency range wherein the respective signal response characteristics of the respective first and second filter elements significantly vary as a function of frequency.
8. A method of processing a signal, the method comprising a filtering step in which unwanted signals are suppressed by means of a filter that comprises a plurality of filter elements, the method comprising:
a switching step in which a first filter element of the plurality of filter elements and, subsequently, a second filter element of the plurality of filter elements is switched from a filter state to an adjustment state and then back again to the filter state, the first and second filter elements that are in the filter state contributing to the suppressing of unwanted signals, the first and second filter elements that are in the adjustment state affecting a characteristic of a measurement signal;
applying a stimulus signal to the first and second filter elements that are in the adjustment state, the first and second filter elements providing the measurement signal in response to the stimulus signal, the stimulus signal simultaneously providing a DC stimulus signal and an AC stimulus signal, so as to obtain a DC measurement signal and an AC measurement signal; and
an adjustment step in which the first and second filter elements that are in the adjustment state is adjusted so that the characteristic of the measurement signal is substantially equal to a target value by adjusting the first filter element which constitutes a resistor based on the DC measurement signal, and adjusting the second filter element which constitutes a capacitor based on the AC measurement signal.
9. A signal display arrangement comprising a signal processing arrangement comprising a filter for suppressing unwanted signals, the filter comprising a plurality of filter elements characterized in that the signal processing arrangement comprises:
a switching circuit for switching a first filter element and, subsequently, a second filter element from a filter state to an adjustment state and back again to the filter state, the first and second filter elements that are in the filter state contributing to the suppressing of unwanted signals, the first and second filter elements that are in the adjustment state affecting a characteristic of a measurement signal;
an adjustment circuit for adjusting the first and second filter elements that are in the adjustment state so that the characteristic of the measurement signal is substantially equal to a target value;
a display device for displaying a signal that the signal processing arrangement has processed; and
a signal source for applying a stimulus signal to the first and second filter elements that are in the adjustment state, the first and second filter elements providing the measurement signal in response to the stimulus signal,
wherein the signal source is arranged to provide a DC stimulus signal and an AC stimulus signal so as to obtain a DC measurement signal and an AC measurement signal, respectively,
wherein the adjustment circuit being arranged to adjust the first filter element which constitutes a resistor based on the DC measurement signal, and to adjust the second filter element which constitutes a capacitor based on the AC measurement signal.