Conversion Circuit For Converting Complex Analog Signal Into Digital Representation

Abstract

A conversion circuit for converting a complex analog input signal having an in-phase (I) component and a quadrature-phase (Q) component is disclosed. It comprises a channel-selection filter configured to filter the complex analog input signal, thereby generating a channel-filtered I and Q components, and one or more processing circuits. Each processing circuit comprises four mixers for generating a first and a second frequency-translated I component, and a first and a second channel-filtered Q component based on two LO signals with equal LO frequency and a 90° mutual phase shift. Each processing circuit also comprises a combiner circuit for generating a first, a second, a third, and a fourth combined signal proportional to sums and differences between the frequency translated I and Q components. The first and the fourth combined signals form a first complex signal, and the second and the third combined signals form a second complex signal.

What is claimed is: 
     
         1 . A conversion circuit for converting a complex analog input signal having an in-phase (I) component and a quadrature-phase (Q) component resulting from frequency down conversion of a radio frequency (RF) signal to a frequency band covering 0 Hz into a digital representation, the conversion circuit comprising:
 a channel selection filter configured to filter the complex analog input signal, thereby generating a channel-filtered I component and a channel-filtered Q component, wherein the channel selection filter unit has a passband that covers 0 Hz; and   one or more processing circuits, each comprising:
 a first mixer configured to mix the channel-filtered I component with a first local-oscillator (LO) signal to generate a first frequency-translated I component; 
 a second mixer configured to mix the channel-filtered I component with a second LO signal to generate a second frequency-translated I component; 
 a third mixer configured to mix the channel-filtered Q component with the first LO signal to generate a first frequency-translated Q component; 
 a fourth mixer configured to mix the channel-filtered Q component with the second LO signal to generate a second frequency-translated Q component; 
 a combiner circuit configured to generate:
 a first combined signal proportional to a sum of the first frequency translated I component and the second frequency-translated Q component; 
 a second combined signal proportional to a difference between the first frequency translated I component and the second frequency-translated Q component; 
 a third combined signal proportional to a sum of the second frequency-translated I component and the first frequency-translated Q-component; and 
 a fourth combined signal proportional to a difference between the first frequency-translated Q component and the second frequency-translated I component; 
 wherein the first and the fourth combined signals form a first complex signal; and 
 wherein the second and the third combined signals form a second complex signal; and 
 
 a first analog-to-digital converter (ADC), a second ADC, a third ADC, and a fourth ADC configured to provide digital representations of the first complex signal and the second complex signal for forming the digital representation of the analog complex input signal; 
 wherein the first and the second LO signal of a given processing circuit have a common LO frequency associated with the processing circuit and a mutual 90° phase shift; and 
 wherein the RF signal is a cellular communication signal. 
   
     
     
         2 . The conversion circuit of  claim 1 , wherein the first mixer, the second mixer, the third mixer, and the fourth mixer are harmonic rejection mixers. 
     
     
         3 . The conversion circuit of  claim 1 :
 wherein the combiner circuit is an analog combiner circuit configured to generate the first combined signal, the second combined signal, the third combined signal, and the fourth combined signal in the analog domain;   wherein the first ADC is configured to convert the first combined signal to the digital domain;   wherein the second ADC is configured to convert the second combined signal to the digital domain;   wherein the third ADC is configured to convert the third combined signal to the digital domain; and   wherein the fourth ADC is configured to convert the fourth combined signal to the digital domain.   
     
     
         4 . The conversion circuit of  claim 1 :
 wherein the combiner circuit is a digital combiner circuit configured to generate the first combined signal, the second combined signal, the third combined signal, and the fourth combined signal in the digital domain based on digital versions of the first frequency-translated I component, the second frequency-translated I component, the first frequency-translated Q component, and the second frequency-translated Q component;   wherein the first ADC is operatively connected to the first mixer and is configured to convert the first frequency-translated I component to the digital version of the first frequency-translated I component;   wherein the second ADC is operatively connected to the second mixer and is configured to convert the second frequency-translated I component to the digital version of the second frequency-translated I component;   wherein the third ADC is operatively connected to the third mixer and is configured to convert the first frequency-translated Q component to the digital version of the first frequency-translated Q component; and   wherein the fourth ADC is operatively connected to the fourth mixer and is configured to convert the second frequency-translated Q component to the digital version of the second frequency-translated Q component.   
     
     
         5 . The conversion circuit of  claim 1 :
 wherein each of the first mixer, the second mixer, the third mixer, and the fourth mixer are implemented with a switchable resistor network operatively connected to an input circuit of at least one of the first ADC, the second ADC, the third ADC, and the fourth ADC; and   wherein a resistance of the switchable resistor network is switchably variable in response to either:
 the first LO signal for the first and the third mixer; or 
 the second LO signal for the second and the fourth mixer. 
   
     
     
         6 . The conversion circuit of  claim 5 , wherein the switchable resistor network is configured to provide a constant input resistance to the combination of the switchable resistor network and the input circuit. 
     
     
         7 . The conversion circuit of  claim 5 , wherein the switchable resistor network together with the input circuit forms an active RC integrator. 
     
     
         8 . The conversion circuit of  claim 1 , further comprising a plurality of the processing circuits. 
     
     
         9 . The conversion circuit of  claim 8 , wherein the plurality of processing circuits have mutually different associated LO frequencies. 
     
     
         10 . The conversion circuit of  claim 1 , wherein the channel selection filter comprises:
 a first low-pass filter configured to filter the I component of the complex analog input signal to generate the channel-filtered I component; and   a second low-pass filter configured to filter the Q component of the complex analog input signal to generate the channel-filtered Q component.   
     
     
         11 . The conversion circuit of  claim 1 , wherein each of the first ADC, the second ADC, the third ADC, and the fourth ADC have a bandwidth that is lower than that of the channel selection filter. 
     
     
         12 . A radio receiver circuit comprising:
 a quadrature mixer configured to generate a complex analog input signal by frequency down-conversion of a RF signal to a frequency band covering 0 Hz, wherein the RF signal is a cellular communication signal; and   a conversion circuit configured to convert the complex analog input signal, which has an in-phase (I) component and a quadrature-phase (Q) component resulting from the frequency down conversion, into a digital representation, the conversion circuit comprising:
 a channel selection filter configured to filter the complex analog input signal, thereby generating a channel-filtered I component and a channel-filtered Q component, wherein the channel selection filter has a passband that covers 0 Hz; and 
 one or more processing circuits, each comprising:
 a first mixer configured to mix the channel-filtered I component with a first local-oscillator (LO) signal to generate a first frequency-translated I component; 
 a second mixer configured to mix the channel-filtered I component with a second LO signal to generate a second frequency-translated I component; 
 a third mixer configured to mix the channel-filtered Q component with the first LO signal to generate a first frequency-translated Q component; 
 a fourth mixer configured to mix the channel-filtered Q component with the second LO signal to generate a second frequency-translated Q component; 
 a combiner circuits configured to generate:
 a first combined signal proportional to a sum of the first frequency translated I component and the second frequency-translated Q component; 
 a second combined signal proportional to a difference between the first frequency translated I component and the second frequency-translated Q component; 
 a third combined signal proportional to a sum of the second frequency-translated I component and the first frequency-translated Q-component; and 
 a fourth combined signal proportional to a difference between the first frequency-translated Q component and the second frequency-translated I component; 
 wherein the first and the fourth combined signals form a first complex signal; and 
 wherein the second and the third combined signals form a second complex signal; and 
 
 
 a first analog-to-digital converter (ADC), a second ADC, a third ADC, and a fourth ADC configured to provide digital representations of the first complex signal and the second complex signal for forming the digital representation of the analog complex input signal; 
 wherein the first and the second LO signal of a processing circuit have a common LO frequency associated with the processing circuit and a mutual 90° phase shift. 
   
     
     
         13 . The radio receiver circuit of  claim 12 :
 wherein the RF signal comprises a number of contiguous frequency bands, each carrying an associated information signal; and   wherein the conversion circuit is configured such that each of the information signals is represented, in its entirety, in the digital representation of one of the first complex signal and the second complex signal of one of the processing circuits of the conversion circuit.   
     
     
         14 . The radio receiver circuit of  claim 12 :
 wherein the RF signal comprises a number of contiguous frequency bands, each carrying an associated information signal; and   wherein the conversion circuit is configured such that at least one of the information signals is represented partly in a primary signal of the digital representations of the first complex signal and the second complex signal of one of the processing circuits of the conversion circuit, and partly in a secondary signal of the digital representations of the first complex signal and the second complex signal of one of the processing circuits of the conversion circuit.   
     
     
         15 . The radio receiver of  claim 14 , further comprising recombining circuitry configured to recombine at least one of the information signals from the primary signal and the secondary signal. 
     
     
         16 . The radio receiver of  claim 15 :
 wherein the radio receiver circuit is an orthogonal frequency-division multiplexing receiver circuit; and   wherein the recombining circuitry is configured to recombine the at least one of the information signals in the frequency domain.   
     
     
         17 . The radio receiver of  claim 12 , wherein the radio receiver is included in a mobile terminal. 
     
     
         18 . The radio receiver of  claim 12 , wherein the radio receiver is included in a radio base station.