Analog FIR filter

1. An analog finite impulse response (AFIR) filter comprising:

at least one variable transconductance block comprising an input for receiving an input voltage and being adapted to sequentially apply each of a plurality of transconductance levels to said input voltage during at least one of a plurality of successive time periods to generate an output current at an output of said variable transconductance block, the at least one variable transconductance block comprising a plurality of fixed transconductance blocks each receiving said input voltage and capable of being independently activated to supply said output current; and

a capacitor coupled to the output of the variable transconductance block to receive said output current and provide an output voltage of said filter.

2. The AFIR filter of claim 1 , wherein each fixed transconductance block comprises a control input for receiving an activation signal.

3. The AFIR filter of claim 1 , wherein the output of said at least one variable transconductance blocks is connected to an analog to digital converter.

4. The AFIR filter of claim 1 , comprising a plurality of said variable transconductance blocks adapted to operate out of phase with each other, and a plurality of said capacitors each coupled to the output of a respective one of said variable transconductance blocks to receive the corresponding output currents.

5. The AFIR filter of claim 4 , comprising a plurality of analog to digital converters coupled to said plurality of capacitors, and a digital multiplexer arranged to sequentially couple the outputs of said analog to digital converters to an output node to provide the output voltage of said filter.

6. The AFIR filter of claim 1 , wherein each of said plurality of fixed transconductance blocks comprises at least one switch adapted to select or deselect the fixed level transconductance block to provide said output current, the AFIR filter comprising a control block adapted to control said at least one switch.

7. The AFIR filter of claim 6 , wherein each of said plurality of fixed transconductance blocks has the same transconductance level, and wherein said control block is arranged to select a different number of said fixed level transconductance blocks to provide each of said plurality of transconductance levels.

8. The AFIR filter of claim 6 , wherein said plurality of fixed transconductance blocks each comprises at least one transistor having a control terminal coupled to receive said input voltage, and main current terminals coupled to provide said output current to said capacitor when the fixed transconductance block is activated.

9. The AFIR filter of claim 1 , wherein each of said plurality of fixed transconductance blocks has a fixed transconductance level equal to a respective one of said plurality of transconductance levels.

10. An electronic device comprising:

An input node for receiving an input voltage; and

the AFIR filter of claim 1 coupled to receive said input signal;

an analog to digital converter coupled to receive the output voltage of the filter; and

a digital signal processor coupled to the output of the analog to digital converter.

11. A hard disk drive comprising:

an input node for receiving an input voltage; and

the AFIR filter of claim 1 coupled to receive said input signal;

an analog to digital converter coupled to receive the output voltage of the filter; and

a digital signal processor coupled to the output of the analog to digital converter.

12. A mobile telephone comprising:

an input node for receiving an input voltage; and

the AFIR filter of claim 1 coupled to receive said input signal;

an analog to digital converter coupled to receive the output voltage of the filter; and

a digital signal processor coupled to the output of the analog to digital converter.

13. A radio receiver comprising:

an input node for receiving an input voltage; and

the AFIR filter of claim 1 coupled to receive said input signal;

an analog to digital converter coupled to receive the output voltage of the filter; and

a digital signal processor coupled to the output of the analog to digital converter.

14. The AFIR filter of claim 1 , further comprising a switch coupled in parallel with said capacitor, the switch being arranged to periodically reset said capacitor.

15. A method of filtering an input signal comprising:

sequentially applying, by at least one variable transconductance block, each of a sequence of transconductance levels to said input signal during one or more of a plurality of successive time periods to generate an output current, the at least one variable transconductance block comprising a plurality of fixed transconductance blocks each receiving said input voltage and capable of being independently activated to supply said output current; and

receiving, by a capacitor, the output current to generate an output voltage of said filter.

16. The method of claim 15 , wherein said step of applying said sequence of transconductance levels is repeated at first regular time intervals, the method further comprising performing said step of applying said sequence of transconductance levels at second regular time intervals offset with respect to said first regular time intervals.

17. The method of claim 16 , further comprising periodically resetting the voltage on said capacitor.

18. The method of claim 15 , wherein applying the sequence of transconductance levels comprises, for each transconductance level, selecting a number of said plurality of fixed transconductance blocks.