Abstract:
A method of compensating a direct-current offset coupled into a receiving path of a receiver, the receiving path having at least one gain factor adjustable by a gain control, which method first determines an amount of a direct-current offset present at a selected end point in the receiving path. A compensation quantity is set accordingly. In case of a forthcoming adjustment of the at least one gain factor by the gain control, the compensation quantity is then scaled with a scaling factor to prevent a direct-current step at the selected end point due to this forthcoming adjustment. The final compensation quantity is fed at a selected summing node into the receiving path. The invention is also directed to a corresponding receiver, an electronic device comprising such a receiver, a hardware component for such a receiver and to a corresponding software program product.

Description:
FIELD OF THE INVENTION 
   The invention relates to a method of compensating a direct current offset coupled into a receiving path of a receiver, wherein in the receiving path at least one gain factor is adjustable by a gain control. The invention relates equally to a corresponding receiver, to an electronic device comprising such a receiver, to a hardware component for such a receiver and to a software program product for such a receiver. 
   BACKGROUND OF THE INVENTION 
   A receiving path in which at least one gain factor is adjustable by a gain control is well known, for example as a receiving path of a direct conversion receiver. 
   For illustration, a block diagram of an exemplary analog direct conversion receiver  10  is presented as  FIG. 1 . 
   The depicted receiver  10  comprises a low noise amplifier (LNA)  11  for amplifying received radio frequency (RF) signals, mixers  12  for down-converting the amplified RF signals, an analog signal processing component  13  for processing the down-converted signals, analog-to-digital converters (ADC)  14  for converting the processed analog signals into digital signals, and a digital signal processing component (DSP)  15  for a further processing of the digital signals. For processing the analog down-converted signal, the analog signal processing component  13  comprises an Nth-order low-pass filter (LPF), an automatic gain control (AGC), a direct-current (DC) offset cancellation, etc. For processing the digital signal, the DSP  15  comprises a decimation stage, an LPF, etc. The output of the DSP  15  constitutes the digital baseband (BB) output. 
   Implementing a receiver with a direct conversion architecture has the advantage that it is cheaper than other conversion architectures, like super-heterodyne architectures, since expensive band pass filter components for an intermediate frequency (IF) are not required in a direct conversion. Moreover, the receiver can be realized as a system on chip (SoC) solution, that is, the components of the receiver can be implemented on a single chip. 
   The receiver  10  can be integrated for example in a mobile terminal  16  for receiving and processing RF signals transmitted by a mobile communication network. 
   One known problem of direct conversion receivers is the direct current (DC) offset in the base-band. In the case of a relatively high DC offset and of a high signal gain, the base-band signal may be forced out of a desired voltage range, become distorted or even be clipped. 
     FIG. 2  is a schematic circuit diagram of a straightforward implementation of the front end of the receiver of  FIG. 1 , in which possible RF coupling paths that can convert to a DC offset in the baseband are indicated. For reasons of clarity, only one of an I branch and a Q branch is illustrated. The circuit of  FIG. 2  comprises an RF amplifier  21  with the LNA  11 , a Gilbert cell  22  as mixers  12 , and two LPF stages  25 ,  27  as analog baseband filter of the analog signal processing component  13 . Instead of the depicted second order LPF  25 ,  27 , a higher order LPF can be used as well. 
   The LNA  11  comprises two input terminals and two outputs terminals. The LNA  11  amplifies received RF signals RFin and outputs the amplified signals as voltages Urf+ and Urf−. The outputs terminals of the LNA  11  are connected to two signal input terminals of a down-conversion mixing component  23  of the Gilbert cell  22 . The mixing component  23  receives via two additional input terminals alternating local oscillator signals LO+ and LO−, which enable a down-conversion of input radio frequency signals RFin. The resulting baseband signals are output as voltages Ubb+ and Ubb− via a respective output terminal. 
   The first output terminal of the mixing component  23  is connected via a first input terminal of the first LPF stage  25  and a resistor R 1   a  to a first input of an operational amplifier  26  of the first LPF stage  25 , and a first output of operational amplifier  26  is connected to a first output terminal of the first LPF stage  25 . A capacitor C 1   a  on the one hand and a resistor R 2   a  on the other hand are arranged in parallel to each other between the first input and the first output of the operational amplifier  26 . 
   The second output terminal of the mixing component  23  is connected via a second input terminal of the first LPF stage  25  and a resistor R 1   b  to a second input of the operational amplifier  26 , and a second output of the operational amplifier  26  is connected to a second output terminal of the first LPF stage  25 . A capacitor C 1   b  on the one hand and a resistor R 2   b  on the other hand are arranged in parallel to each other between the second input and the second output of the operational amplifier  26 . 
   The first output terminal of the first LPF stage  25  is connected via a first input terminal of the second LPF stage  27  and a resistor R 3   a  to a first input of an operational amplifier  28  of the second LPF stage  27 , and a first output of the operational amplifier  28  is connected to a first output terminal of the second LPF stage  27 . A capacitor C 2   a  on the one hand and a resistor R 4   a  on the other hand are arranged in parallel to each other between the first input and the first output of the operational amplifier  28 . 
   The second output terminal of the first LPF stage  25  is connected via a second input terminal of the second LPF stage  27  and a resistor R 3   b  to a second input of the operational amplifier  28 , and a second output of the operational amplifier  28  is connected to a second output terminal of the second LPF stage  27 . A capacitor C 2   b  on the one hand and a resistor R 4   b  on the other hand are arranged in parallel to each other between the first input and the first output of the operational amplifier  28 . 
   The two LPF stages  25 ,  27  apply a second order low pass filtering on the baseband signals Ubb+ and Ubb− received from the Gilbert mixer  22 . The resulting low-pass filtered baseband signals are forwarded to the analog-to-digital converters  14  of  FIG. 1 . 
   In the receiver front end of  FIG. 2 , a DC offset at the output of the second LPF stage  27  may be created by several mechanisms. A DC offset may be created for instance by a local oscillator signal feed-through to the RF port of the mixing component  23 . This coupling path is indicted by an arrow labeled “I”. Alternatively or additionally, a DC offset may be created by a local oscillator signal feed-through to the RF port of the LNA  11  and by a subsequent down-conversion to DC. This coupling path is indicted by an arrow labeled “II”. Alternatively or additionally, a DC offset may be created by an intermodulation of an interferer in adjacent channels, due to the non-linearity (IIP 2 , IIP 3 ) of the LNA  11  and the Gilbert mixer  22  to DC. This coupling path is indicted by an arrow labeled “III”. Even though not indicated in the Figure, also the baseband filter blocks  25 ,  27  may contribute to the DC offset with their own contribution. 
   Altogether, the receiver may suffer from a DC offset caused by several sources. The signals from each source are amplified in the receiver path and contribute to the total DC offset of the receiver.  FIG. 3  is a simplified model of the DC offset coupling and contribution to the output of the receiver path. In this model, the frequency conversion is omitted and all signals are presented as baseband equivalent signals for the sake of simplicity. 
   The model comprises a first summing node  31 , to which the desired baseband equivalent signal RFin_eq and in addition a first equivalent disturbing DC signal Uo 1  are fed. The baseband equivalent signal RFin_eq corresponds in a real system to the received radio frequency signal RFin. The signal Uo 1  corresponds in a real system to a harmful RF signal, which appears as DC signal in the baseband. The output of the first summing node  31  is connected via a gain with a gain factor of A 1 , which is a property of the LNA  11  and the mixer  22 , to a second summing node  32 . A second equivalent disturbing DC signal Uo 2  is equally fed to the second summing node  32 . The signal Uo 2  corresponds in a real system to a harmful RF signal, which appears as DC signal in the baseband. The output of the second summing node  32  is connected via a gain with a gain factor of A 2 , applied by the first LPF stage  25 , to a third summing node  33 . A third disturbing DC signal Uo 3  is equally fed to the third summing node  33 . The output of the third summing node  33  is connected via a gain with a gain factor of A 3 , applied by the second LPF stage  27 , to the analog-to-digital converters  14 . 
   The total DC offset Uoffset_out resulting at the output of the second LPF stage  27  is then given by the following equation:
 
Uoffset_out=Uo1 *A 1* A 2* A 3+Uo2* A 2* A 3+Uo3* A 3.
 
   The total DC offset is thus influenced by changes in the different DC offset contributions and as well by gain changes. The DC offset contributions may vary on a sample-to-sample basis and may further depend on time, temperature, proximity, etc. 
     FIG. 4  is a schematic diagram illustrating a conventional DC compensation arrangement. The diagram comprises again the elements  31 ,  32 ,  33 , A 1 , A 2 , A 3  of the model of  FIG. 3 . The output of the second LPF stage  27  represented by the gain with a gain factor of A 3  is connected in an analog feedback compensation loop via an integrator  41  and a summing device  42  to the second summing node  32 . 
   In general, DC offset contributions of Uo 1 , Uo 2  and Uo 3  can thus be classified as those coupling in before the summing node  32  of the DC offset compensation, as those coupling in at the summing node  32  and those coupling in within the compensation loop, respectively. In practice, if the summing node  32  is at the mixer output, which is frequently the case, then DC contribution of Uo 1  is subject to the LNA and mixer gain having a gain factor of A 1  and its variations. Exemplary origins of Uo 1  are local oscillator signal feed-through via the LNA  11  and/or mixer non-linearity. DC contribution Uo 2  is present at the summing node  32  and is independent of variations in the LNA gain An exemplary origin of Uo 2  is the undesired local oscillator signal feed-through via the RF port of the mixers  22  and subsequent down-conversion to DC via the desired mixer action or undesired non-linearity. DC contribution Uo 3  occurs within the compensation loop, for example the DC offset resulting at the second filter stage  27 . Usually DC contributions Uo 1  and Uo 2  dominate. 
   The integrator  41  is used for estimating the DC offset at the output of the second LPF stage  27  by integrating the output signal over time. The integration result is then summed by the summing device  42  in opposite polarity to the second summing node  32  at the output of the mixers  12  represented by the gain with a gain factor of A 1 . The integration result could also be summed to another suitable node in the receiver path. 
   The corresponding frequency response equation indicating the DC offset created at the output is then given by:
 
U_out —   dc =((RFin_eq+Uo1)* A 1* A 2* A 3* s +Uo2* A 2* A 3 s +Uo3* A 3 s )/( s+A   2 * A   3 )
 
   The integration applied by the integrator  41  can be continuous, in particular in 3G (third generation) receivers, or synchronized, for instance to the frame rate of GSM (global system for mobile communications). The DC offset could also be estimated differently than by an integration, for example by a positive and negative peak detection, by envelope forming, by checking whether the output signal lies within a permitted range, etc. 
   As an alternative to the analog feedback loop, it is also known to measure the DC offset in the digital domain after the analog-to-digital converters  14  and to subtract the compensation signal either from an analogue summing node, for instance summing node  32 , via a digital-to-analog converter, or from a digital signal in the digital domain. In practice, many solutions use both techniques in parallel, in particular by carrying out a coarse compensation in the analogue domain and a fine compensation in the digital domain. In  FIG. 4 , a dashed line indicates by way of example that a feedback signal from a digital loop may be connected to a further input of the summing device  42 . 
     FIG. 5  is a schematic circuit diagram of the front end of a direct conversion receiver which is provided with an analog DC compensation loop. The receiver front end presented in  FIG. 5  comprises the same elements as the receiver front end presented in  FIG. 2 . In addition, the first output of the second LPF stage  27  is connected via a resistor R 5   a  to a first input of an operational amplifier  51  and the second output of the second LPF stage  27  is connected via a resistor R 5   b  to a second input of the operational amplifier  51 . A first output of the operational amplifier  51  is connected via a resistor R 6   a  in addition to the first input of the operational amplifier  26  of the first LPF stage  25 . A second output of the operational amplifier  51  is connected via a resistor R 6   b  in addition to the second input of the operational amplifier  26 . A capacitor C 3   a  is arranged between the first input and the first output of the operational amplifier  51 , and a capacitor C 3   b  is arranged between the second input and the second output of the operational amplifier  51 . The components  51 , R 5   a , R 5   b , R 6   a , R 6   b , C 3   a , C 3   b  function as inverting integrator  52 , which integrates the output of the second LPF stage  27  and which provide the inverted integration result to the input of the first LPF stage  25 . 
   In conventional DC compensation approaches, however, two problems exist. Firstly, the frequency response may be a function of the gain setting. Secondly, every time one of the gain factors A 1 , A 2 , A 3  is changed, a DC step is created. This results in contradictory requirements for the DC compensation loop. On the one hand, a fast compensation loop is preferred for enabling a fast settling due to the DC steps. On the other hand, a slow compensation loop is preferred in order to avoid an impairment, namely an attenuation, of the actually received signal. Such a prolonged DC settling can cause a momentary loss of received symbols and, ideally, should be avoided. 
   In order to reduce the negative effect of DC steps, a binary search, the use of faster time constants during settling, etc. have been proposed. 
   Also, from practical implementations of the DC offset compensation loop, it is known to use large off-chip capacitors in them. 
   SUMMARY OF THE INVENTION 
   It is an object of the invention to improve a DC offset compensation in a receiver. It is in particular an object of the invention to provide a possibility for preventing DC steps whenever an adjustable gain factor in the receiver is changed. 
   A method of compensating a direct-current offset coupled into a receiving path of a receiver is proposed. In the receiving path, at least one gain factor is adjustable by a gain control. The proposed method comprises determining an amount of a direct-current offset present at a selected end point in the receiving path and setting a compensation quantity accordingly. The proposed method further comprises, in case of a forthcoming adjustment of the at least one gain factor by the gain control, scaling the compensation quantity with a scaling factor to prevent a direct-current step at the selected end point due to the forthcoming adjustment of the at least one gain factor. The proposed method further comprises feeding the compensation quantity at a selected summing node into the receiving path. 
   Moreover, a receiver comprising a receiving path is proposed. In the receiving path, at least one gain factor is adjustable by a gain control, and a direct-current offset coupled into this receiving path is to be compensated. The proposed receiver comprises a DC estimation portion adapted to estimate an amount of a direct-current offset present at a selected end point in the receiving path. The DC estimation portion is moreover adapted to set a compensation quantity accordingly. The DC estimation portion is moreover adapted to scale the compensation quantity with a provided scaling factor, if any. The DC estimation portion is moreover adapted to feed the compensation quantity into the receiving path at a selected summing node. The proposed receiver further comprises a DC compensation control portion adapted to determine, in case of a forthcoming adjustment of the at least one gain factor, a scaling factor preventing a direct-current step at the selected end point due to the forthcoming adjustment of the at least one gain factor. The DC compensation control portion is moreover adapted to provide this scaling factor to the DC estimation portion. 
   In addition, an electronic device is proposed, which comprises such a receiver. 
   In addition, a hardware component for a receiver including a corresponding receiving path is proposed. The hardware component comprises a corresponding DC estimation portion and a corresponding DC compensation control portion as the proposed receiver. 
   Finally, a software program product is proposed in which a software code is stored which realizes the steps of the proposed method when running in a receiver. 
   It has to be noted that the direct-current offset may be a direct current offset and/or a direct-current voltage offset. 
   The invention proceeds from the idea that a feedback loop employed for compensating a DC offset in a receiving path can be supplemented by a scaling of a compensation quantity determined in the feedback loop. More specifically, a scaling can be performed each time a gain factor of the gain on the receiving path is to be adjusted. The scaling is performed such that a DC step due to the gain adjustment is prevented. Thus, the DC compensation is tied to the operation of the gain control. 
   It is an advantage of the invention that it enables a DC offset compensation without DC steps caused by gain steps in the receiver path. At the same time, the properties of a conventional DC compensation can be maintained. That is, under steady state conditions, the DC offset can be canceled after a sufficient period of time irrespective of the actual DC offset contributions. External capacitors are not required for preventing the DC step. Thereby, the component count is reduced and a large area consumption is avoided. Also the quality of the signal processing may be improved, as problems in soldering capacitors etc. are avoided. The implementation of the invention can be configured easily. 
   In one embodiment of the invention, the scaling factor is determined by first estimating a current amount of contributions to the direct-current offset. Then, a relative change in the direct-current offset is determined, which would have resulted with the estimated amount of direct-current contributions after a preceding gain adjustment. Finally, this relative change is selected as a scaling factor, as a similar relative change can be expected as well for the forthcoming gain adjustment. 
   In a further embodiment of the invention, the signal provided at the selected end point of the receiving path is normalized before it is used for determining the amount of a direct-current offset, in order to maintain a constant frequency response of the loop regardless of any changes in the gain factors. The signal is normalized more specifically by dividing it by a total gain applied by the receiving path between the summing node and the end point of the receiving path. 
   The amount of a direct-current offset at the end point of the receiving path can be determined in various ways. The determination can be based for example on an integration of a signal which is present at the end point of the receiving path. Moreover, it can be based on windowing a signal which is present at the end point of the receiving path, on detecting positive and negative peaks in this signal, and/or on an envelope forming of this signal. The invention can be used in particular in conjunction with a conventional digital DC compensation scheme, resulting in an increased settling speed and a greater accuracy despite of the AGC operation. 
   The employed time constants and operation characteristics can be implemented most efficiently in the digital domain and/or by software. This is particularly true for the DC offset estimation, for the scaling and for a possible normalization. 
   If the compensation quantity is determined and scaled in the digital or software domain, a respective final compensation quantity is converted into the analog domain before it is fed to the summing node. 
   The invention can be implemented in any receiver, in particular in any direct conversion receiver. The invention enables in particular an operation of a DC compensation and an AGC function in a direct conversion receiver without the DC steps at the base band which are usually associated with gain changes. This applies to gain changes in the base-band and as well to changes of a gain applied by an LNA. 
   Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. It should be further understood that the drawings are not drawn to scale and that they are merely intended to conceptually illustrate the structures and procedures described herein. 

   
     BRIEF DESCRIPTION OF THE FIGURES 
       FIG. 1  is a schematic block diagram of a direct conversion radio frequency receiver; 
       FIG. 2  is a schematic circuit diagram of one branch of a receiver front-end of a direct conversion radio frequency receiver, in which a coupling in of DC offset contributions is illustrated; 
       FIG. 3  is a model of DC offset contributions in a direct conversion radio frequency receiver; 
       FIG. 4  is a schematic block diagram illustrating a conventional DC offset compensation based on the model of  FIG. 3 ; 
       FIG. 5  is a schematic circuit diagram of a conventional analog DC compensation loop; 
       FIG. 6  is a schematic block diagram illustrating a DC compensation in a direct conversion radio frequency receiver according to an embodiment of the invention; 
       FIG. 7  is a flow chart illustrating the DC compensation in the receiver of  FIG. 6 ; 
       FIG. 8  is a finite state machine illustrating an initialization of the DC compensation in the receiver of  FIG. 6 ; 
       FIG. 9  is a finite state machine illustrating the determination of a first offset estimator in the receiver of  FIG. 6 ; 
       FIG. 10  is a finite state machine illustrating the determination of a second offset estimator in the receiver of  FIG. 6 ; 
       FIG. 11  is a finite state machine illustrating the determination of a third offset estimator and a scaling of a DC offset compensation quantity in the receiver of  FIG. 6 ; 
       FIG. 12  presents diagrams illustrating exemplary gain sequences used as basis for simulations; 
       FIG. 13  presents diagrams illustrating various voltages resulting in a direct conversion receiver with a conventional DC compensation; 
       FIG. 14  presents diagrams illustrating various voltages resulting in a direct conversion receiver with a DC compensation in accordance with the invention, which starts off with uninitialized offset estimators and which results in a DC stepless operation after some gain step changes; and 
       FIG. 15  presents diagrams illustrating various voltages resulting in a direct conversion receiver with a DC compensation in accordance with the invention, which employs initialized offset estimators and which results immediately in a DC stepless operation. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 6  is a schematic block diagram of a direct conversion receiver  60 . The receiver  60  may correspond to the receiver  10  of  FIG. 1 , and the front-end of the receiver  60  may correspond to the receiver front-end schematically presented in  FIG. 2 , except that a DC offset cancellation according to an embodiment of the invention is implemented in addition. Only those components which are relevant for the invention are depicted. 
   For the sake of simplicity, the frequency conversion is omitted and all signals are presented as baseband equivalent signals, similarly as in the model of  FIG. 3 . 
   The receiver  60  of  FIG. 6  comprises a receiver path with an LNA and mixer portion  61 , which is connected via a first LPF stage  62 , a second LPF stage  63  and an analog-to-digital converter  64  to a digital baseband section of the receiver (not shown). The LNA and mixer portion  61  applies a gain with a gain factor of A 1  to a baseband equivalent signal RFin_eq of an input RF signal. The first LPF stage  62  applies a gain with a gain factor of A 2  to an input signal. The second LPF stage  63  applies a gain with a gain factor of A 3  to an input signal. Each of the gain factors A 1 , A 2  and A 3  thus contributes to the total gain of the receiver path. In existing AGC solutions and receiver implementations, their values are known, as well as any changes. 
   With reference to the baseband equivalent model, an undesired DC offset contribution Uo 1  can be introduced at the input of the LNA and mixer portion  61 , as indicated by a first summing node  65 . An undesired DC offset contribution Uo 2  can further be introduced at the input of the first LPF stage  62 , as indicated by a second summing node  66 . An undesired DC offset contribution Uo 3  can further be introduced at the input of the second LPF stage  63 , as indicated by a third summing node  67 . Thus, various DC offset contributions Uo 1 , Uo 2 , Uo 3  of unknown value are weighted in the receiving path by known gain factors A 1 , A 2  and A 3 , as described with reference to  FIG. 3 . Only the second summing node  66  represents a real component, therefore, the first and the second summing nodes  65 ,  67  are only circled by dashed lines. 
   It has to be noted that, similarly as in the model of  FIG. 3 , the signals Uo 1  and Uo 2  correspond in a real system to harmful RF signals due to receiver non-idealities, which appear as DC signals in the baseband. Signal Uo 3  is a real DC offset component which contributes in the baseband to the total receiver DC offset. 
   For the DC offset cancellation, the output of the analog-to-digital converter  64  is connected in addition via a normalization portion  71 , a DC estimation portion  72  and a digital-to-analog converter  73  to the second summing node  66 . 
   Moreover, a gain and DC compensation control portion  74  has a controlling access to each of the gain factors A 1 , A 2  and A 3 , as well as to the normalization portion  71  and to the DC estimation portion  72 . The gain and DC compensation control portion  74  may form part of the DSP  15  depicted in  FIG. 1  or receive information from the digital baseband portion of the receiver, for instance from the DSP  15 . 
   The receiver path itself operates in a conventional manner, namely as described above with reference to  FIG. 1 . 
   The operation of the DC offset compensation according to the invention will now be described with reference to the flow chart of  FIG. 7 . 
   The digital signal U_out output by the analog-to-digital converter  64  is provided to the normalizing portion  71 . The normalization portion  71  normalizes the output of the analog-to-digital converter  64  by a multiplication with 1/(A 2 *A 3 ). This ensures that a constant frequency response of the loop is obtained regardless of the current values of the gain factors A 1 , A 2  and A 3 . 
   The DC estimation portion  72  determines the amount of a direct current in the signal U_out by integrating the normalized signal provided by the normalization portion  71  in the time domain. The determined direct current is used as a compensation quantity Uoq. In steady state and a settled situation, the compensation quantity Uoq is given by:
 
Uoq= A 1*Uo1+Uo2+1 /A 2*Uo3.
 
   The gain and DC compensation control portion  74  receives from the digital baseband information about the next gain adjustment which is to be performed. In addition, it may receive a specific DC compensation control signal. 
   When the gain and DC compensation control portion  74  receives from the digital base band section a specific DC compensation control signal, the determined compensation quantity Uoq is replaced by a compensation quantity Uoq comprised in the control signal by means of an “override” signal provided to the DC estimation portion  72 . Such an “override” signal may be used in particular for setting the DC offset compensation quantity to an initial value by bypassing the compensation loop during a power up period, for example by initializing it to zero or to some other desired value. This new compensation quantity Uoq is then converted by the digital-to-analog converter  73  into an analog direct current, which is subtracted from the output of the LNA and mixer portion  61  at summing node  66 . 
   When no specific DC compensation control signal is provided and no adjustment of a gain factor A 1 , A 2  or A 3  is forthcoming, in contrast, the compensation quantity Uoq determined by the DC estimation portion  72  is converted by the digital-to-analog converter  73  into an analog direct current, which is then subtracted from the output of the LNA and mixer portion  61  at summing node  66 . Thereby, the integrator  72  drives the DC offset at the output of the analog-to-digital converter  64  to zero. 
   The resulting frequency response indicating the DC offset created at the output will be
 
U_out —   dc =((RFin_eq+Uo1)* A 1* A 2* A 3* s +Uo2* A 2* A 3* s +Uo3* A 3* s )/( s +1).
 
   When the gain and DC compensation control portion  74  receives no specific DC compensation control signal from the digital base band, but a command to adjust at least one of the gain factors A 1 , A 2 , A 3 , it determines a scaling factor for the compensation quantity Uoq. The scaling factor is determined such that with the resulting compensation quantity Uoq, a DC step in the output U_out of the analog-to-digital converter  64  due to the change in gain is prevented. The determination of the scaling factor is based on estimates for the different offset contributions Uo 1 , Uo 2  and Uo 3  and will be explained in more detail further below with reference to  FIGS. 8 to 11 . 
   The gain and DC compensation control portion  74  then provides the scaling factor to the DC estimation portion  72 . The DC estimation portion  72  scales the determined compensation quantity Uoq with this scaling factor. 
   The gain and DC compensation control portion  74  moreover changes the gain factor or factors A 1 , A 2 , A 3  as required by supplying an A 1  control signal, A 2  control signal and/or A 3  control signal to the portion  61 ,  62 ,  63  applying the respective gain. The scaled compensation quantity Uoq is then converted by the direct-to-analog converter  73  into an analog direct current, which is subtracted from the output of the LNA and mixer portion  61  at summing node  66  as DC offset compensation. 
   The digital-to-analog converter  73  may be provided in addition with an “enable” signal. Such an “enable” signal can be used to select between a continuous operation and a periodical operation of the DC compensation. For example, in WCDMA and CDMA, the loop may be set to operate continuously by setting the “enable” signal to “on”, whereas in GSM, the loop may optionally be set to operate synchronously with the GSM frame rate. In the periodical mode, the loop is activated and deactivated by an alternating “enable” signal. 
     FIGS. 8 to 11  all present finite state machines which operate concurrently, and which have the purpose to scale the compensation quantity Uoq properly each time a gain change occurs. 
   In the final state machines of  FIGS. 8 to 11 , each change of gain factor A 1  is indicated by dA 1 , while no change of gain factor A 1  is indicated by XdA 1 , each change of gain factor A 2  is indicated by dA 2 , while no change of gain factor A 2  is indicated by XdA 2 , each change of gain factor A 3  is indicated by dA 3 , while no change of gain factor A 3  is indicated by XdA 3 . 
   A scaling factor which is suited to speed up the DC compensation settling can be determined based on the different DC offset contributions Uo 1 , Uo 2  and Uo 3  and on the different gain factors A 1 , A 2 , A 3 . While the gain factors and their changes are known, the DC offset contributions Uo 1 , Uo 2  and Uo 3  are not known in advance, and they may change during the operation due to a temporal evolution, due to a change in temperature, due to a change in proximity to disturbing sources, etc. Thus, they have to be estimated by the DC compensation control portion  74 . 
   To this end, the DC offset estimation calculations are first initiated in the gain and DC compensation control portion  74 . This is illustrated by the finite state machines of  FIG. 8 . 
   On the left hand side of  FIG. 8 , a finite state machine proceeds from a state WAITA 1  to record values of the gain factor A 1 . This state is re-entered whenever one of gain factors A 2  and A 3  are to be changed. Upon each entry of the state WAITA 1 , a variable Uoq_A 1 _prevA is set to Uoq. Moreover, a variable A 1 _prevA is set to the value of gain factor A 1  which is currently used. The variables Uoq_A 1 _prevA and A 1 _prevA are both stored in the gain and DC compensation control portion  74 . 
   When the gain factor A 1  is to be changed, a transition to a state RETURNA 1  is caused. Upon each entry of this state, a variable Uoq_A 1 _lastA is set to Uoq. A variable A 1 _lastA is set to the value of gain factor A 1  which is currently used. Further, a variable Uoq_A 1 _prevB is set to Uoq, and a variable A 1 _prevB is set to the value of gain factor A 1  which is currently used. The variables Uoq_A 1 _lastA, A 1 _lastA, Uoq_A 1 _prevB and A 1 _prevB are equally stored in the gain and DC compensation control portion  74 . 
   When the gain factor A 1  is to be changed again, a transition to a state NEXTA 1  is caused. Upon each entry of this state, the variable Uoq_A 1 _prevA is set to Uoq, and the variable A 1 _prevA is set to the value of gain factor A 1  which is currently used. Further, a variable Uoq_A 1 _lastB is set to Uoq, and a variable A 1 _lastB is set to the value of gain factor A 1  which is currently used. Also the variables Uoq_A 1 _lastB and A 1 _lastB are stored in the gain and DC compensation control portion  74 . 
   When the gain factor A 1  is to be changed again, a transition back to the state RETURNA 1  is caused. 
   In both states, RETURNA 1  and NEXTA 1 , a return to the state WAITA 1  is caused, in case one of the gain factors A 2  and A 3  is to be changed. 
   Thus, before each change of one of the gain factors A 1 , A 2 , A 3 , the current values of the compensation quantity Uoq and of the gain factor A 1  are stored, such that the last two values of gain factor A 1  and the respectively associated values of the compensation quantity Uoq are always available. 
   On the right hand side of FIG.  8 ., a state machine proceeds from a state WAITA 2  to record values of the gain factor A 2 . This state is re-entered whenever one of gain factors A 1  and A 3  are to be changed. Upon each entry of this state, a variable Uoq_A 2 _revA is set to Uoq. Moreover, a variable A 2 _prevA is set to the value of gain factor A 2  which is currently used. Variables Uoq_A 2 _prevA and A 2 _prevA are both stored in the gain and DC compensation control portion  74 . 
   When the gain factor A 2  is to be changed, a transition to a state RETURNA 2  is caused. Upon each entry of this state, a variable Uoq_A 2 _lastA is set to Uoq. A variable A 2 _lastA is set to the value of gain factor A 2  which is currently used. Further, a variable Uoq_A 2 _prevB is set to Uoq, and a variable A 2 _prevB is set to the value of gain factor A 2  which is currently used. The variables Uoq_A 2 _lastA, A 2 _lastA, Uoq_A 2 _prevB and A 2 _prevB are equally stored in the gain and DC compensation control portion  74 . 
   When the gain factor A 2  is to be changed again, a transition to a state NEXTA 2  is caused. Upon entry of this state, the variable Uoq_A 2 _prevA is set to Uoq, and the variable A 2 _prevA is set to the value of gain factor A 2  which is currently used. Further, a variable Uoq_A 2 _lastB is set to Uoq, and a variable A 2 _lastB is set to the value of gain factor A 2  which is currently used. Also the variables Uoq_A 2 _astB and A 2 _lastB are stored in the gain and DC compensation control portion  74 . 
   When the gain factor A 2  is to be changed again, a transition back to the state RETURNA 1  is caused. 
   In both states, RETURNA 2  and NEXTA 2 , a return to the state WAITA 2  is caused, in case one of the gain factors A 1  and A 3  is to be changed. 
   Thus, before each change of one of the gain factors A 1 , A 2 , A 3 , the current values of the compensation quantity Uoq and of the gain factor A 2  are stored, such that the last two values of gain factor A 2  and the respectively associated values of the compensation quantity Uoq are always available. 
   Altogether, always four stored gain factors and four associated compensation quantities Uoq are available. For the gain factor A 3 , no values are stored, as a change of gain factor A 3  does not cause any DC step due to the normalization. 
   The stored values of A 1 , A 2  and Uoq are used for estimating the DC offset contributions Uo 1 , Uo 2  and Uo 3 . 
     FIG. 9  is a finite state machine illustrating the estimation of Uo 1  in the gain and DC compensation control portion  74 . 
   As long as none of the gain factors A 1 , A 2  and A 3  is to be changed, an estimation is not required and a state WAITUo 1  is maintained. 
   As soon as the gain and DC compensation control portion  74  learns that one of the gain factors A 1 , A 2 , A 3  is to be adjusted, it determines whether the initiation of the offset estimation calculations changed thereupon to state RETURNA 1  or to state NEXTA 1 . 
   If the initiation entered the state RETURNA 1 , the DC offset contribution estimation for Uo 1  enters the state Uo 1 setA. In this state, the offset contribution Uo 1  is estimated to be:
 
Ûo 1=(Uoq —   A 1_prev A - Uoq   —   A 1_last A )/( A 1_prev A - A 1_last A )
 
   If the initiation entered the state NEXTA 1 , the DC offset contribution estimation for Uo 1  enters the state Uo 1 setB. In this state, the offset contribution Uo 1  is estimated tobe:
 
Ûo1=(Uoq —   A 1_prev B −Uoq —   A 1_last B )/( A 1_prev B=A 1_last B )
 
   In both cases, the DC offset contribution estimation for Uo 1  then returns to the state WAITUo 1 . 
     FIG. 10  is a finite state machine illustrating the estimation of Uo 3  by the gain and DC compensation control portion  74 . 
   As long as none of the gain factors A 1 , A 2  and A 3  is to be changed, an estimation is not required and a state WAITUo 3  is maintained. 
   As soon as the gain and DC compensation control portion  74  learns that one of the gain factors A 1 , A 2 , A 3  is to be adjusted, it determines whether the initiation of the offset estimation calculations changed thereupon to state RETURNA 2  or to state NEXTA 2 . 
   If the initiation entered the state RETURNA 2 , the DC offset contribution estimation for Uo 3  enters the state Uo 3 setA. In this state, the offset contribution Uo 3  is estimated to be:
 
Ûo3=(Uoq —   A 2_prev A −Uoq —   A 2_last A )/( A 2_prev A -1 /A 2_last A )
 
   If the initiation entered the state NEXTA 2 , the DC offset contribution estimation for Uo 3  enters the state Uo 3 setB. In this state, the offset contribution Uo 3  is estimated to be:
 
Ûo3=(Uoq —   A 2_prev B =Uoq —   A 2_last B )/(1 /A 2_prev B -1 /A 2_last B )
 
   In both cases, the DC offset contribution estimation for Uo 3  then returns to the state WAITUo 3 . 
     FIG. 11  is a finite state machine illustrating the estimation of Uo 2  by the gain and DC compensation control portion  74  and in addition the scaling of the compensation quantity Uoq. 
   As long as none of the gain factors A 1 , A 2  and A 3  is to be changed, a scaling is not required and a state WAITUoq is maintained. In this state, the compensation quantity Uoq resulting in the DC estimation portion  72  in the integration is used directly for the DC offset compensation. Moreover, a variable A 1 _old is set to the value of gain factor A 1  which is currently used and a variable A 2 _old is set to the value of gain factor A 2  which is currently used. Thus, the variable A 1 _old corresponds always to variable A 1 _lastA or A 1 _lastB, whichever was updated more recently, while the variable A 2 _old corresponds always to variable A 2 _lastA or A 2 _lastB, whichever was updated more recently. 
   As soon as the gain and DC compensation control portion  74  learns that one of the gain factors A 1 , A 2  and A 3  is to be adjusted, a state SCALEUoq is entered. Actually, the state SCALEUoq has to be entered only each time any of gain factors A 1  and A 2  are to be changed, as a change of gain factor A 3  does not have any effect on the offset compensation loop. 
   In the state SCALEUoq, first, a variable A 1 _new is set to the value of gain factor A 1  which is to be used next and a variable A 2 _new is set to the value of gain factor A 2  which is to be used next. 
   Then, the last DC offset contribution Uo 2  is estimated to be:
 
Ûo2=Uoq_last- A 1_new*Ûo1-1 /A 2_new*Ûô3
 
   Here, Uoq_last is equal to Uoq_A 1 _lastA, Uoq_A 1 _lastB, Uoq_A 2 _lastA or Uoq_A 2 _lastB, whichever was stored last. 
   It has to be noted that while the DC offset contributions Uo 1 , Uo 2 , Uo 3  can be estimated directly as proposed, an averaging over a plurality of preceding gain changes may be beneficial. 
   Based on all DC offset contribution estimates Ûo 1 , Ûo 2  and Ûo 3 , and in addition on the stored gain factor values, a scaling factor is then determined by the gain and DC compensation control portion as:
 
 S =( A 1_new*Ûo1+Ûo2+1 /A 2_new*Ûo3)/( A 1_old*Ûo1+Ûo2+1 /A 2_old*Ûo3)
 
   The determined scaling factor S is then provided by the gain and DC compensation control portion  74  to the DC estimation portion  72  for scaling the determined compensation quantity Uoq in accordance with the following equation:
 
Uoq(new)=Uoq(old)* S, 
 
where Uoq(old) is the determined compensation quantity and Uoq(new) the scaled compensation quantity.
 
   As the DC offset contribution estimates Ûo 1 , Ûo 2  and Ûo 3  can be obtained during the normal receiver AGC and DC compensation operation, accurate and up-to-date values are always available, which allows a tracking of changes in the DC offset contributions to avoid DC steps. 
   It has to be noted that in a practical implementation, the DC offset contribution Uo 3  within the compensation loop is sometimes minimal. In this case, Uoq values and gain factors have only to be stored before a respective change of gain factor A 1  for an estimation of the DC offset contributions Uo 1  and Uo 2 . The DC offset contribution Uo 3  can be neglected in this case in the determination of the scaling factor S. 
     FIGS. 12 to 15  present simulated results of a conventional DC offset compensation and of the DC offset compensation described with reference to  FIGS. 6 to 11 . The simulation results were obtained in the Matlab environment. 
     FIG. 12  presents from top to bottom a first diagram showing a gain sequence for gain factor A 1 , a second diagram showing a gain sequence for gain factor A 2 , a third diagram showing a gain sequence for gain factor A 3 , and a fourth diagram showing a gain sequence resulting for the entire receiving path by combining the gain factors A 1 *A 2 *A 3 . These gain sequences were used as basis for the simulations. 
   Each of  FIGS. 13 to 15  presents from top to bottom a first diagram with the waveform of the signal Uout 1 =(Uin+Uo 1 )*A 1  output by a first gain stage subject to gain. changes and baseband offset creation, corresponding to LNA and mixer  61 , a second diagram with the waveform of the signal Uout 2 =(Uout 1 +Uo 2 )*A 2  output by a first LPF stage  25 / 62 , a third diagram with the waveform of the signal Uout 3 =(Uout 2 +Uo 3 )*A 3  output by a second LPF stage  27 / 63 , and a fourth diagram with the waveform of the compensation quantity Uoq output by an integrator  52 / 72 . 
     FIG. 13  illustrates the time domain behavior of Uout 1 , Uout 2 , Uout 3  and the compensation quantity Uoq resulting in a conventional integration based DC compensation. It can be seen that each change in each of the gain factors A 1 , A 2  or A 3  in either direction introduces a DC step and a level shift in the signal Uout 3  that will settle to zero with a speed set by the integration time constant employed by the DC estimation portion  72 . 
     FIG. 14  illustrates the time domain behavior of Uout 1 , Uout 2 , Uout 3  and the compensation quantity Uoq of a integration based DC compensation in accordance with the invention. In the beginning, the estimates Ûo 1 , Ûo 2  and Ûo 3  do not have correct values yet, as preceding gain factors and Uoq values are not available for determining the estimates. Thus, no correct scaling can be made from the beginning. As a result, DC steps and an integrator settling will result for the first few gain steps, just as in the conventional DC offset compensation. However, when the gain factors have been changed at least once, the estimates Ûo 1 , Ûo 2  and Ûo 3  can be calculated and used in scaling the compensation quantity Uoq for a DC stepless operation. It can be seen that towards the end of  FIG. 14 , the algorithm has been able to find the estimates and no DC steps occur anymore in the signal Uout 3 . Thereby,  FIG. 14  also shows the capability of initializing the offset estimates for a further improved compensation. 
     FIG. 15  illustrates further the time domain behavior of Uout 1 , Uout 2 , Uout 3  and the compensation quantity Uoq of a integration based DC compensation in accordance with the invention. In this case, however, the DC offset contribution estimates Ûo 1 , Ûo 2  and Ûo 3  are initialized by suitable values. As a result, a completely DC stepless operation is achieved. 
   The presented scaling principle scales dominant state variables of the DC compensation system. In addition to the presented scaling, it is also possible to scale the state variables of the analogue base-band filter whenever a gain step in gain factors A 1 , A 2  or A 3  is executed. This is useful and avoids ringing in case the order of the analogue filter is high. In case of a low order filter, however, no ringing is expected and a scaling of the filter state variable is not necessary, as in the presented simulation example. 
   It has to be noted that the normalization with 1/(A 2 *A 3 ) is also suited by itself, that is without subsequent scaling of the compensation quantity Uoq, to desensitize the DC compensation loop and the AGC scheme from influences by DC offset contribution Uo 2 . When assuming, for example, that DC offset contribution Uo 2  is the only or a strongly dominant offset source, then no DC step and new settling would result after a gain change, as the compensation quantity Uoq does not have to be changed. If no normalization and no scaling is performed, a DC step would result at the output of the analog-to-digital converter  64  every time the gain of the signal path is changed, and the DC compensation loop would need to settle to a new compensation value, possibly resulting in a loss of signal or in a phase shift, etc. 
   While there have been shown and described and pointed out fundamental novel features of the invention as applied to a preferred embodiment thereof, it will be understood that various omissions and substitutions and changes in the form and details of the devices and methods described may be made by those skilled in the art without departing from the spirit of the invention. For example, it is expressly intended that all combinations of those elements and/or method steps which perform substantially the same function in substantially the same way to achieve the same results are within the scope of the invention. Moreover, it should be recognized that structures and/or elements and/or method steps shown and/or described in connection with any disclosed form or embodiment of the invention may be incorporated in any other disclosed or described or suggested form or embodiment as a general matter of design choice. It is the intention, therefore, to be limited only as indicated by the scope of the claims appended hereto.