Abstract:
A quadrature modulator with set-and-forget carrier leakage compensation. The quadrature modulator comprises an in-phase and a quadrature branch. In the in-phase and quadrature branches, real-time digital signals are converted to analog signals, the analog signals are filtered, and the filtered analog signals are modulated with a carrier signal and a ninety degrees phase shifted version of the carrier, respectively. The modulated in-phase and quadrature signals are added so as to form a quadrature amplitude modulated signal. Preferably upon powering up of the quadrature modulator, in the in-phase and quadrature branches, carrier leakage is measured. The measured carrier leakage is supplied to comparators, which toggle, when carrier leakage is minimal in the respective in-phase and quadrature branches. Upon powering up of the quadrature modulator, the state machine starts signal generators which inject compensation signals into the in-phase and quadrature branches, respectively, so that dc-offsets in the in-phase and quadrature branches are reduced, thereby reducing the carrier leakage in the in-phase and quadrature branches. When the comparators toggle, the state machine is commanded to stop controlling the corresponding signal generators of which output signals are then frozen.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a quadrature modulator. 
     The present invention further relates to a radio communication device comprising a quadrature modulator, and to a modulation method. 
     2. Description of Related Art 
     In the U.S. Pat. No. 5,012,208 a quadrature modulator is disclosed with adaptive suppression of carrier leakage in an in-phase and a quadrature branch of the quadrature modulator. By applying an appropriate dc-offset (direct current) to the in-phase and quadrature branches, such a carrier leakage can be substantially eliminated, when initially adjusting the quadrature modulation, and selecting matching components. It is described that many factors, such as temperature, frequency, load impedance, and carrier power variations may not be adequate to compensate for carrier leakage. In said U.S. Pat. No. 5,012,208 a complicated control loop is disclosed for adaptively adjusting the local oscillator&#39;s leak signal to the output of the rf-mixer, such a control loop having correlators and integrators in its feedback paths. At an output of an rf-power amplifier, which is coupled to the quadrature modulator, the control loop continuously measures and correlates the rf-power to be transmitted, and, in dependence thereto, continuously injects dc-offset compensation values at input side of the quadrature modulator. 
     In the U.S. Pat. No. 5,396,196 a similar type of carrier leakage compensation as in said U.S. Pat. No. 5,012,208 is disclosed. In addition thereto, the carrier leakage compensation control loop described therein needs a complicated Pseudo-Noise generator. 
     In the Japanese Abstract No. 07202961, in a transmitter with a quadrature modulator, a carrier leakage compensation circuit is disclosed. A dc-level detector measures dc-levels at a base band side of the quadrature modulator. On the basis of such measured base band dc-offset signals, an adjusting circuit adjusts dc-levels of the base band quadrature signals. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a quadrature modulator with a simple but robust carrier leakage compensation means. 
     It is another object of the invention to provide a quadrature modulator in which carrier leakage compensation is performed at predetermined instants. 
     It is still another object of the invention to provide a quadrature modulator in which possible dc-offsets or other disturbing effects generated by compensation means itself are eliminated. 
     It is yet another object of the invention to easily control application of carrier leakage compensation. 
     It is yet another object of the invention to eliminate effects of cross talk. 
     In accordance with the invention, a quadrature modulator is provided comprising: 
     an in-phase quadrature modulation branch comprising a first series arrangement of a first digital-to-analog converter, a first anti-aliasing filter, a first summing means, and a first mixer; 
     a quadrature modulation branch comprising a second series arrangement of a second digital-to-analog converter, a second anti-aliasing filter, a second summing means, and a second mixer; 
     a local oscillator means for providing a first local oscillator signal and a second local oscillator signal to said first and second mixers, respectively; 
     a third summing means coupled to respective outputs of said first and second mixers, said third summing means providing a quadrature modulated signal; 
     carrier leakage measurement means for measuring a first carrier leakage signal of said first local oscillator signal in said in-phase modulation branch, and a second carrier leakage signal of said second local oscillator signal in said quadrature branch; 
     a controllable signal generating means for generating a first monotonously increasing signal and a second monotonously increasing signal; and 
     holding means for holding values of said first and second monotonously increasing signals, said holding means being coupled to said first and second summing means so as to form feedback paths; 
     said carrier leakage measurement means adopting a first state in which said first and second carrier leakage signals are measured, and a second state in which said controllable signal generating means is controlled to stop generating said first and second monotonously increasing signals, said second state being adopted from said first state during measurement of said first and second carrier leakage signals. 
     The invention is based upon the insight that usually parameters influencing carrier leakage are slowly varying with time so that there is no need for continuous compensation. Based upon this insight, it was realized that a simple but robust carrier leakage compensation could be implemented being operative at predetermined points in time, such at switching on power, or, when used in a communications apparatus, possibly also at channel switching. When used in a communication device such as a broad band CDMA communications device, in which, in principle, no channel switching is needed, there is only a needed to apply carrier leakage compensation at power on of the communications device. In such a case, after power on, power to components used for carrier leakage compensation could even be switched off, thus achieving power savings in such a portable communications device. 
     Preferably, said carrier leakage detection means comprises a first series arrangement of a first synchronous detector and a first comparator, and a second series arrangement of a second synchronous detector and a second comparator, respective output signals of said first and second comparators controlling said controllable signal generating means to stop generating said first and second monotonously increasing signals. Herewith, carrier leakage is accurately measured, while at the same time a well-defined criterion is available for stopping carrier leakage compensation. 
     Preferably, said controllable signal generating means comprises a first counter, and a first state machine coupled between said carrier leakage measurement means, and a second counter, and a second state machine coupled between said carrier leakage measurement means, said first and second state machines feeding clock pulses to said first and second counters when said carrier leakage means is in said first state, and stopping to feed clock pulses to said counter when said carrier leakage means is in said second state. Herewith, a very simple means is obtained for digitally generating a controllable amount of dc-offset compensation, until the carrier leakage measurement means stop the dc-offset compensation. 
     For simply injecting an analog dc-compensation signal representing the digitally generated compensation, digital-to-analog converters are provided. Filters are provided for filtering double frequency carrier components that are introduced by synchronous detectors included in the measurement means. Before these synchronous detectors, amplifiers are provided to suppress dc-offsets introduced by the synchronous detectors, the synchronous detectors otherwise introducing additional carrier leakage. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 schematically shows a block diagram of a radio communication device. 
     FIG. 2 is a block diagram of a first embodiment of a quadrature modulator according to the present invention. 
     FIG. 3 is a block diagram of a second embodiment of a quadrature modulator according to the present invention. 
     FIG. 4 shows generator signals for use in a quadrature modulator according to the present invention. 
     FIG. 5 shows a state machine for use in a quadrature modulator according to the present invention. 
     FIG. 6 shows an electronic circuit diagram of a part of a quadrature modulator according to the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Throughout the figures, the same reference numerals are used for the same features. 
     FIG. 1 schematically shows a block diagram of a radio communication device  1 . The radio communication device  1  comprises at least a transmitter  2 , which is coupled to an antenna  3 . In case the radio communication device  1  is a transceiver, the radio communication device  1  further comprises a receiver  4  of which only a low-noise amplifier  5  is shown, and a duplexer  6 , or any other suitable device, for coupling the transmitter  2  and the receiver  4  to the antenna  3 . The transmitter  2  comprises a quadrature modulator  7 , and a power amplifier  8  coupled between the quadrature modulator  7  and the antenna  3 . Such transceiver architecture as such is well known in the art. The shown transceiver can be a cellular radio transceiver, in an FD/TDMA system, in a CDMA system, in a cordless telephony system, or any other suitable system. Input signals to the quadrature modulator  7  are real time digital signals I(n) and Q(n), to be supplied to an in-phase and a quadrature branch of the quadrature modulator, respectively, n being an integer indication bits, chips or symbols to be modulated. The quadrature modulator  7  comprises a power control input  9  via which selective parts of the quadrature modulator can be powered down, powering down of selective parts of a circuit as such being well-known in the art of electronic circuits. 
     FIG. 2 is a block diagram of a first embodiment of the quadrature modulator  7  according to the present invention. The quadrature modulator  7  comprises an in-phase modulation branch  10  and a quadrature modulation branch  11 . The in-phase modulation branch  10  comprises a first series arrangement of a first digital-to-analog converter  12 , a first anti-aliasing filter  13 , a first summing means  14 , and a first mixer  15 , and the quadrature modulation branch  11  comprises a second series arrangement of a second digital-to-analog converter  16 , a second anti-aliasing filter  17 , a second summing means  18 , and a second mixer  19 . The analog-to-digital converter  12  provides a reconstructed signal I(nT), T being a reconstruction period. The anti-aliasing filter  13  provides a filtered analog signal I(t). Similarly, in the quadrature branch  11 , the digital-to-analog converter  16  provides a reconstructed signal Q(nT), and the anti-aliasing filter  17  provides a filtered analog signal Q(t). The quadrature modulator  7  further comprises a local oscillator  20 , which is coupled to a differential input  21  of the first mixer  15 , and through a ninety degrees phase shifting device  22  to a differential input  23  of the second mixer  19 . The local oscillator  20  provides a carrier signal to the in-phase and quadrature mixers  15  and  19 . The quadrature modulator  7  further comprises summing means  24  for summing mixed quadrature modulator signals Im(t) and Qm(t). The quadrature modulator provides a quadrature amplitude modulated output signal s(t). In accordance with the present invention, the quadrature modulator  7  further comprises carrier leakage measurement means for measuring a first carrier leakage signal in the signal Im(t) and for measuring a second carrier leakage signal in the signal Qm(t), the carrier leakage measurement means comprising a first series arrangement  25 , and a second series arrangement  26 , the first series arrangement  25  being coupled to the in-phase branch  10  and the second series arrangement  26  being coupled to the quadrature branch  11 , in the example given to respective outputs of the mixers  15  and  16 . An output  27  of the first series arrangement  25  is coupled to an input  28  of a state machine  29 , and an output  30  of the second series arrangement  26  is coupled to an input  31  of the state machine  29 . The first series arrangement  25  is a series arrangement of a first amplifier  40 , a first synchronous detector  41 , a first filter  42 , and a first comparator  43 . The second series arrangement  26  is a series arrangement of a second amplifier  44 , a second synchronous detector  45 , a second filter  46 , and a second comparator  47 . Further inputs to the state machine  29  are a control signal ctl and a system clock signal clk. At output side, the state machine  29  is coupled to controllable signal generating means for generating a first monotonously increasing signal sig 1  and a second monotonously increasing signal sig 2 , to be subtracted from the signals filtered analog signals I(t) and Q(t), respectively. In more detail, the controllable signal generating means comprises a first counter  50  and a second counter  51 , the first and second counters  50  and  51  being controlled by the state machine  29 . The quadrature modulator  7  further comprises holding means for holding values of the signals sig 1  and sig 2 , the holding means comprising a third digital-to-analog converter  52  for holding the value of the signal sig 1 , and a fourth digital-to-analog converter  53  for holding the signal sig 2 . In the shown quadrature modulator  7 , the local oscillator  20  is tunable by a tuning control input  60 . As regards carrier leakage compensation, the quadrature modulator  7  of the present invention can be characterized as a set-and-forget quadrature modulator. In principle, the quadrature leakage is measured at powering up of the quadrature modulator  7  or radio communication device  1 , and the quadrature leakage compensation is done once. If need be, however, the compensation can also be done at other instants, when tuning from one channel to another, for instance. 
     In order not to introduce dc-offsets from the modulation branches  10  and  11 , digital input signals to the branches  10  and  11  are set to zero. Preferably, during carrier leakage compensation, the power amplifier  8  is switched off so as to prevent transmission of an unmodulated carrier. 
     The operation of the set-and-forget carrier leakage in the quadrature modulator  7  is as follows. Upon powering up of the quadrature modulator  7 , the first and second synchronous detectors  41  and  45  measure the carrier leakage in the in-phase and quadrature branches  10  and  11 , while at the same time the state machine  29  initiates the first and second counters  50  and  51  so that the monotonously increasing signals sig 1  and sig 2  are subtracted from the filtered analog signals I(t) and Q(t), respectively. Feedback paths are thus provided for carrier leakage reduction. When carrier leakage in the in-phase and quadrature branches  10  and  11  is minimal, the comparators  43  and  47  toggle, respectively, commanding the state machine  29  to stop the counters  50  and  51  so that the digital-to-analog converters  52  and  53  hold the current values of the counters  50  and  51 , respectively. 
     FIG. 3 is a block diagram of a second embodiment of the quadrature modulator  7  according to the present invention. In this very simple embodiment of a set-and-forget carrier offset compensation, only dc-offset compensation for the filters  13  and  17  is obtained. In this embodiment, the comparators  43  and  47  should exhibit a much lower dc-offset than the filters  13  and  17 . Instead of branching off in the in-phase and quadrature branches, the carrier leakage measurement means can branch off the output signal s(t). Then, a single control signal controls the state machine  29  to stop injection of compensation signals in both the in-phase and quadrature branches  10  and  11 . To save pin counts, the outputs  27  and  30  of the comparators  43  and  47  can be multiplexed. 
     FIG. 4 shows generator signals for use in the quadrature modulator  7  according to the present invention. Preferably, the controllable signal generating means is comprised of the counters  50  and  51 , a simple digital implementation of the present invention. In such an embodiment, the counters  50  and  51  are started by the state machine  29 , the compensation signals sig 1  and sig 2  being ramp signals rmp as shown in FIG.  4 . Other monotonously increasing compensation signals can be applied, such as a shown square root signal sqt. Such a square root signal exhibits a steeper slope when starting up the carrier leakage compensation. Herewith the overall compensation process is faster. Digital signal generators for generating square root signals as such are well known in the art. Instead of applying digital generators as described, controllable analog signal generators can be applied as well, although there is a considerable risk that the set-and-forget compensation signal runs away, i.e., is not stable in the long run. As a hold means, capacitive hold means could be used, alternatively. 
     FIG. 5 shows the state machine  29  for use in the quadrature modulator  7  according to the present invention. The state machine  29  comprises an input  70  for a start/stop control signal str of the system clock clk. To this end, both the start/stop control signal str and the system clock signal clk are input to respective AND-gates  71  and  72  for in-phase and quadrature branch injection control of the compensation signals. If the start/stop signal str has a logic “0” value, the AND-gates  71  and  72  block the system clock clk so that the counters  50  and  51  do not receive further clock pulses and thus preserve their current values. If the start/stop signal str has a logic “1” value, the system clock is passed to respective further AND-gates  73  and  74  of which outputs  75  and  76  are coupled to the counters  50  and  51 , respectively. Herewith, the counters  50  and  51  generate a ramp signal while being clocked. The state machine  29  further comprises a first negative edge triggered D-flip flop  77  for in-phase branch counter control, and a second negative edge triggered D-flip flop  78  for quadrature branch counter control. Output signals of the comparators  43  and  47  are supplied to clock inputs  79  and  80 , respectively, of the D-flip flops  77  and  78 . If the comparators  43  and  47  toggle, at minimum carrier leakage, negative edges are generated, clocking the D-flip flops so that the inverse of the logic “1” appears at the inverse Q-output, i.e., a logic “0” blocking the AND-gates  73  and  74 , as the case may be. Herewith, the system clock clk is blocked by the AND-gates  73  and  74 , respectively. In the example given, the in-phase and quadrature counter control is done independently. Upon powering up of the quadrature modulator  7 , a reset signal rst resets the D-flip flops  77  and  78  so that signal generation can be initiated by the system clock clk again. In the example, as described before, where carrier leakage is measured from the output signal s(t), system clock control for in-phase and quadrature branches is done simultaneously. Then, the state machine  29  is simplified to two AND-gates and a single D-flip flop. 
     FIG. 6 shows an electronic circuit diagram of a part of the quadrature modulator  7  according to the invention. Shown is the first series arrangement  25  of the carrier leakage measurement means, the series arrangement of the amplifier  40 , the synchronous detector  41 , the filter  42 , and the comparator  43 , as coupled to the mixer  15 , the mixer being coupled to the local oscillator  60 . For the second series arrangement  26 , similar electronic circuit means can be used. In the example given, all signals are differential signals. The mixer  15  is a balanced mixer comprising transistors Q 1 , Q 2 , Q 3 , and Q 4 , the signal I(t) being supplied to tail transistors Q 5  and Q 6  of the mixer  15 . In the mixer  15 , further shown are biasing resistors  80 ,  81 ,  82 , and  83 , and a tail current source  84 . At output side, the mixer  15  is coupled to the amplifier  40 , comprising transistors Q 7  and Q 8 , the transistors Q 7  and Q 8  having biasing resistors  90 ,  91 ,  92 , and  93 . The amplifier  40  suppresses the dc-offset generated by the mixer  41 . In the circuit shown, also carrier leakage being caused by capacitive coupling within the transistors Q 1 , Q 2 , Q 3 , and Q 4  is cancelled. At output side, the amplifier  40  is coupled to the mixer  42 , a synchronous detector comprised of the balanced transistors Q 9 , Q 10 , Q 11 , and Q 12 , with tail transistors Q 13  and Q 14 , and tail current source  93 , and further current sources  94  and  95  coupled to supply rail Vc. The filter  42  is embodied as a single capacitor. The filter  42  eliminates the double carrier frequency generated by the synchronous detector  41 . 
     In view of the foregoing it will be evident to a person skilled in the art that various modifications may be made within the spirit and the scope of the invention as hereinafter defined by the appended claims and that the invention is thus not limited to the examples provided. It is to be understood that the word “comprising” in the appended claims does not exclude the presence of other elements or steps than those listed in a claim.