Signal processor with a signal strength detection circuit that is coupled to a loop of an analog to digital converter
A signal processing circuit has an analog to digital converter ( 31 ) for providing a digital signal to a processor ( 15 ) from an analog input signal that is susceptible to variations in signal power, e.g. from a radio front end ( 12 ). The device has a variable gain amplifier ( 13 ) controlled by a gain control signal based on detected signal strength. The analog to digital converter has a loop comprising a loop filter for processing the input signal. A signal strength detection circuit ( 32 ) is provided for generating the gain control signal, which signal strength detection circuit has loop signal detector for detecting the signal strength from the loop. Hence a received signal strength indicator RSSI is directly coupled to the analog to digital converter ( 31 ), avoiding the delay of signal strength detection in the digital processor.
1. Device for digitally processing an analog input signal that is susceptible to variations in signal power, the device comprising
an amplifier having a variable gain in dependence of a gain control signal for generating an amplified signal from the input signal,
an analog to digital converter for converting the amplified signal to a digital signal to be processed, the analog to digital converter including a loop comprising a loop filter, and
a signal strength detection circuit for generating the gain control signal, the signal strength detection circuit being coupled to the loop of the analog to digital converter and comprising loop detection means for detecting the signal strength from the loop.
2. Device as claimed in claim 1 , wherein the loop detection means comprises at least one level detector coupled to the loop filter.
3. Device as claimed in claim 2 , wherein the loop filter comprises a sequence of loop units and the loop detection means comprises a multitude of level detectors coupled to the sequence of loop units.
4. Device as claimed in claim 3 , wherein the loop detection means comprise a logical output circuit for generating the gain control signal in dependence of a combination of output signals of the level detectors.
5. Device as claimed in claim 4 , wherein the logical output circuit comprises different weighing factors for the output signals of the level detectors, the weighing factors being selected based on an expected amount of degradation of the digital output signal due to a corresponding loop unit exceeding a level as detected by the respective level detector.
6. Device as claimed in claim 3 , wherein the sequence of loop units comprises integrators and feed forward units.
7. Device as claimed in claim 1 , wherein the analog to digital converter is a sigma-delta converter and the loop comprises an analog loop filter coupled to a quantizer.
8. Device as claimed in claim 1 , wherein the loop detection means comprise level detectors arranged for detecting a loop signal level at a reference level below a clipping level of a preceding loop unit.
9. Mobile communication device, the device comprising a device for digitally processing an analog input signal as claimed in claim 1 , and a receiving unit for providing the analog input signal.
10. The device as claimed in claim 1 , wherein the signal strength detection circuit is integrated with the analog to digital converter to detect the signal strength of the input signal at the loop of the analog to digital converter.
11. The device as claimed in claim 1 , wherein the analog to digital converter is a sigma delta analog to digital converter that includes a sequence of loop units, wherein the loop detection means is coupled to the sequence of loop units.
12. The device as claimed in claim 11 , wherein the loop detection means is coupled to the last one of the loop units in the sequence of loop units.