Circuit for reconstructing an analog signal from a digital signal and transmission system, particularly for WCDMA cellular telephony, including such circuit

1. Circuit for reconstructing an analog signal starting from a digital input signal, said circuit comprising:

a digital to analog converter suitable for receiving said digital signal and converting it into signal in analog format; and

a low pass-filter connected at the output of said converter for receiving said signal in analog format and providing as output said reconstructed analog signal;

wherein the low pass filter is a second order active filter continuous in time and current-coupled to the output of the digital-analog converter and wherein the digital-analog converter is a converter of the current-steering type functioning at a sampling frequency greater than the Nyquist frequency of said analog signal.

2. Circuit for reconstructing an analog signal starting from a digital input signal, said circuit comprising:

a digital to analog converter suitable for receiving said digital signal and converting it into signal in analog format; and

a low pass-filter connected at the output of said converter for receiving said signal in analog format and providing as output said reconstructed analog signal;

wherein the low pass filter is an active filter continuous in time and current-coupled to the output of the digital-analog converter, wherein the digital-analog converter is a converter of the current-steering type functioning at a sampling frequency greater than the Nyquist frequency of said analog signal,

wherein said low-pass filter includes at least an operational amplifier and wherein said amplifier represents the output stage of said digital to analog converter.

3. Circuit according to claim 2 , wherein said filter further includes feedback resistors of said operational amplifier and wherein said converter is such to convert said digital signal into said signal in analog format in the form of a current directly injected into said feedback resistors.

4. Circuit according to claim 3 , wherein said digital to analog converter includes a reference current generator including a resistor having a first resistance and wherein said feedback resistors have a second resistance accurately correlated with said first resistance.

5. Circuit according to claim 2 , wherein said sampling frequency is greater than about eight times said Nyquist frequency.

6. Circuit according to claim 3 , wherein said sampling frequency is greater than sixteen times said Nyquist frequency.

7. Circuit for reconstructing an analog signal starting from a digital input signal, said circuit comprising:

a digital to analog converter suitable for receiving said digital signal and converting it into signal in analog format; and

a low pass-filter connected at the output of said converter for receiving said signal in analog format and providing as output said reconstructed analog signal;

wherein the low pass filter is an active filter continuous in time and current-coupled to the output of the digital-analog converter and wherein the digital-analog converter is a converter of the current-steering type functioning at a sampling frequency greater than about six times the Nyquist frequency of said analog signal.

8. Circuit for reconstructing an analog signal starting from a digital input signal, said circuit comprising:

a digital to analog converter suitable for receiving said digital signal and converting it into signal in analog format; and

a low pass-filter connected at the output of said converter for receiving said signal in analog format and providing as output said reconstructed analog signal;

wherein the low pass filter is an active filter continuous in time and current-coupled to the output of the digital-analog converter and wherein the digital-analog converter is a converter of the current-steering type functioning at a sampling frequency greater than the Nyquist frequency of said analog signal, and wherein the converter has a differential output and the filter has a differential input and output.

9. Circuit for reconstructing an analog signal starting from a digital input signal, said circuit comprising:

a digital to analog converter suitable for receiving said digital signal and converting it into signal in analog format;

a low pass-filter connected at the output of said converter for receiving said signal in analog format and providing as output said reconstructed analog signal; and

means for regulating the output amplitude of the converter, wherein the low pass filter is an active filter continuous in time and current-coupled to the output of the digital-analog converter and wherein the digital-analog converter is a converter of the current-steering type functioning at a sampling frequency greater than the Nyquist frequency of said analog signal.

10. Circuit in accordance with claim 9 , wherein the regulating means for the output amplitude of the converter, the bias of the differential inputs of the filter and the common mode output voltage of the filter have a regulating unit in common.

11. Circuit for reconstructing an analog signal starting from a digital input signal, said circuit comprising:

a digital to analog converter suitable for receiving said digital signal and converting it into signal in analog format;

a low pass-filter connected at the output of said converter for receiving said signal in analog format and providing as output said reconstructed analog signal; and

means for regulating the common mode output voltage of the filter, wherein the low pass filter is an active filter continuous in time and current-coupled to the output of the digital-analog converter and wherein the digital-analog converter is a converter of the current-steering type functioning at a sampling frequency greater than the Nyquist frequency of said analog signal.

12. Circuit for reconstructing an analog signal starting from a digital input signal, said circuit comprising:

a digital to analog converter suitable for receiving said digital signal and converting it into signal in analog format;

a low pass-filter connected at the output of said converter for receiving said signal in analog format and providing as output said reconstructed analog signal; and

means for regulating the differential input bias of the filter, wherein the low pass filter is an active filter continuous in time and current-coupled to the output of the digital-analog converter and wherein the digital-analog converter is a converter of the current-steering type functioning at a sampling frequency greater than the Nyquist frequency of said analog signal.

13. Circuit for reconstructing an analog signal starting from a digital input signal, said circuit comprising:

a digital to analog converter suitable for receiving said digital signal and converting it into signal in analog format;

a low pass-filter connected at the output of said converter for receiving said signal in analog format and providing as output said reconstructed analog signal; and

means for regulating the cut-off frequency of the filter, wherein the low pass filter is an active filter continuous in time and current-coupled to the output of the digital-analog converter and wherein the digital-analog converter is a converter of the current-steering type functioning at a sampling frequency greater than the Nyquist frequency of said analog signal.

14. Circuit in accordance with claim 13 , wherein the filter comprises an operational amplifier and a network of resistors and capacitors and means for regulating the cut-off frequency comprise means for modifying the capacitance of at least one of the capacitors.

15. A wide-band transmission system, particularly for employment in cellular telephony systems that adopt the WCDMA standard, comprising:

means for generating two digital signals containing information to be transmitted,

means for converting into analog form the two signals comprising, for each signal to be converted, a digital-analog converter followed by a low-pass filter,

means for modulating a radio frequency carrier both in phase and quadrature with the two signals issuing from the low-pass filters, and

means for transmitting the modulated carrier in accordance with a predetermined emission mask,

wherein said low-pass filter and said digital-analog converter makes part of a circuit for reconstructing an analog signal starting from a digital input signal, said circuit comprising:

a digital to analog converter suitable for receiving said digital signal and converting it into signal in analog format; and

a low pass-filter connected at the output of said converter for receiving said signal in analog format and providing as output said reconstructed analog signal;

wherein the low pass filter is an active filter continuous in time and current-coupled to the output of the digital-analog converter and wherein the digital-analog converter is a converter of the current-steering type functioning at a sampling frequency greater than the Nyquist frequency of said analog signal.

16. A system in accordance with claim 15 , comprising a constant current generator that provides to each of the converters of the two signals to be converted a reference current on a resistor of predetermined resistance, wherein each low-pass filter comprises at least one resistor having a resistance correlated with the resistance of the resistor of the reference current generator.

17. A system in accordance with claim 16 , wherein said filter and said converter includes means for regulating the common mode output voltage comprising a resistor having a resistance correlated with the resistance of the resistor of the reference current generator and are common to both the converters.

18. Mobile communication device comprising a transmission system in accordance with claim 15 .