Abstract:
The invention relates to a transmitting device ( 19 ) including a quadrature modulator ( 3 ) for carrying out the quadrature modulation of a complex input signal (I, Q) and including a power amplifier ( 9 ) connected in outgoing circuit to the quadrature modulator ( 3 ). A quadrature demodulator ( 19 ) for carrying out the quadrature demodulation of the output signal of the power amplifier ( 9 ) is provided for a feedback loop. A first differential amplifier ( 26 ) and a second differential amplifier ( 27 ) are connected in incoming circuit to the quadrature modulator ( 3 ). The input signal and the fed back quadrature modulated signal are supplied to the inputs of the differential amplifiers. When switching over from the transmit mode to a transmit-interruption mode, the output of the first and second differential amplifiers ( 26, 27 ) can be directly connected to the compensation input (−) of the first or second differential amplifier ( 26, 27 ) via a direct signal path ( 32, 33 ) while bypassing the quadrature modulator ( 3 ), the power amplifier ( 8 ) and the quadrature modulator ( 19 ).

Description:
BACKGROUND OF THE INVENTION 
   The invention relates to a transmitter with a quadrature modulator and a power amplifier, which is linearised by what is known as a Cartesian feedback with a quadrature modulator. 
   EP 0 706 259 A1 discloses a transmitter in which a baseband input signal is applied via two differential amplifiers to a quadrature modulator, which effects a quadrature modulation of the in-phase component and the quadrature-phase component of the complex input signal. The signal is applied to an up-converter, which brings the signal from the baseband up to the transmission frequency. Power amplification takes place in a downstream power amplifier. A feedback loop is provided in order to compensate for the non-linearity of this power amplifier, generally known as a Cartesian feedback. Located in this feedback loop, firstly, is a down-converter which converts the transmission signal uncoupled from the output of the power amplifier back down to the baseband. Located in the baseband is a quadrature demodulator, which separates the feedback signal into a feedback in-phase component and a feedback quadrature-phase component. The feedback in-phase component together with the in-phase component of the input signal is forwarded to a first differential amplifier connected upstream of the quadrature modulator. Likewise, the feedback quadrature-phase component together with the quadrature-phase component of the input signal is applied to a second differential amplifier. The non-linearities of the power amplifier are compensated through the feedback signal as a result. 
   In order to compensate the DC components of the quadrature modulator, document EP 0 706 259 A1 proposes operating a training sequence during which no input signal is applied to the transmitter. The output signal of the two differential amplifiers is integrated in a respective integrator and applied to a respective sample and hold circuit connected downstream of the integrator. During the training sequence, the sample and hold circuit is in the sampling state and applies a compensation signal to a negative feedback input of the co-operating differential amplifier such that the direct voltage components of the associated arm of the quadrature modulator are compensated. During normal transmission mode, the sample and hold circuit is in the hold state and applies the compensation level determined during the training run to the input of the respective differential amplifier. Document EP 0 706 259 A1 also proposes running another training sequence, during which switches provided on the output of the quadrature modulator are opened, to determine the phase shift for a phase shifter provided between a local oscillator and the quadrature modulator by detecting the output signal of the quadrature modulator in this state with two different input signals. 
   When using a transmitter which operates on the principle of Cartesian feedback in aeronautical radio communications, particularly with digital aeronautical radio communications operating in TDMA-simplex in accordance with the VDL standard (VHF digital link), a problem arises in that it is very difficult to switch rapidly between transmission mode and reception mode because on switching from transmission mode to reception mode the power amplifier and the local oscillator have to be completely disabled in order to prevent radiation in the receiver. However, the high-frequency feedback loop between the outputs and the compensation inputs of the differential amplifiers necessarily has to be broken in the process. When the power amplifier and the local oscillator are re-enabled as the operating mode is switched from reception to transmission, the feedback loop therefore has to be re-built, which leads to undesirable signal jumps during the switch from reception to transmission. Whilst transmission is interrupted, control of the feedback loop would be adjusted to the positive or negative control end. When re-enabled, the full transmission power would immediately be applied. Document EP 0 706 259 A1 does not suggest any means by which this problem can be eliminated. 
   SUMMARY OF THE INVENTION 
   Accordingly, the underlying objective of the invention is to propose a transmitter with a power amplifier, linearised on the principle of Cartesian feedback, which enables rapid switching from transmission to reception mode, and to specify a corresponding method for switching this transmitter from transmission mode into a transmission-interrupt mode or reception mode. 
   In terms of the transmitter, this objective is achieved by the characterising features of claim  1  and in terms of the method by the characterising features of claim  10  respectively, in conjunction with the known generic features. 
   The invention is based on the principle whereby, connected in parallel with the high-frequency signal path formed by the quadrature modulator, the power amplifier and the quadrature demodulator is another direct signal path, via which the output of the differential amplifier is connected to the feedback input when switching from transmission mode to reception mode, bypassing the quadrature modulator, the power amplifier and the quadrature demodulator. The output of the differential amplifier is therefore connected to its feedback input at all times—during transmission mode via the high-frequency signal path and during reception mode via the direct (DC) signal path. The switch from reception to transmission is preferably operated so that when switching from transmission mode to reception mode, the direct (DC) signal path is closed first of all, before the high-frequency signal path is opened. On switching from transmission mode into reception mode, the sequence is reversed accordingly. This avoids signal jumps when switching from transmission to reception. 
   Advantageously, two additional differential amplifiers are provided respectively in both the in-phase signal path and the quadrature-phase signal path, by means of which compensating voltages can be coupled into the signal paths in order to compensate both the DC offset of the quadrature modulator and the DC offset of the quadrature demodulator. As a result, in the disabled state when no signal is present on the I and Q input, the voltage value OV is obtained at the input and output of the differential amplifier enabling the DC signal path to be activated surge-free. 
   The quadrature modulator is compensated when the high-frequency signal path is closed. The quadrature demodulator, on the other hand, is compensated with the high-frequency signal path open and the DC signal path closed. Compensation is operated in such a way that the output voltage at the differential amplifiers used for compensation purposes is minimised. This can be accomplished with very little complexity in terms of measurement and at a high measuring rate. In addition, the quadrature demodulator can be trimmed whilst the high-frequency signal path is still closed, in which case the output power of the power amplifier when the input signal is disabled is used as the measured value, i.e. can be measured by means of a logarithmic detector. 
   A simplified example of an embodiment of the invention will be described in more detail below with reference to the drawings. Of the drawings: 

   
     BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS 
       FIG. 1  is a block diagram depicting an example of one embodiment of the transmitter proposed by the invention; 
       FIG. 2  is a detail from the transmitter illustrated in  FIG. 1 ; and 
       FIG. 3  is a schematic diagram of the output power of the transmitter as a function of time with a view to explaining a preferred compensating method. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  is a block diagram illustrating the operating principle of an example of a first embodiment of the transmitter proposed by the invention. 
   A digital signal processor (DSP)  2  generates a complex input signal for a quadrature modulator  3 , which consists of an in-phase mixer  4 , a quadrature-phase mixer  5  and a summer  6  as well as a phase shifter  7 . The complex input signal consists of an in-phase component I and a quadrature-phase component Q, the in-phase component I being forwarded to the in-phase mixer  4  and the quadrature-phase component Q to the quadrature-phase mixer  5 . The output signal of a local oscillator  8  is applied to the phase shifter  7  and the phase shifter  7  forwards this oscillator signal to the in-phase mixer  4  without any phase shift and to the quadrature-phase mixer  5  with a phase shift of 90°. 
   Connected downstream of the quadrature modulator  3  is a power amplifier  9 , which amplifies the power of the quadrature modulated signal to the transmission power of the transmitter  1  and forwards it via a circulator  10 , a power detector  11  and a transmit-receive switch  12  to an antenna  13 . In the embodiment illustrated as an example in  FIG. 1 , the digital signal processor  2  simultaneously serves as a control unit for switching from transmission to reception mode and controls the transmit-receive switch  12  so that the antenna is connected to the power amplifier  9  in transmission mode and to a receiver denoted by RX in reception mode. The circulator  10  connected to the terminating resistor  14  is used to prevent feedback of any reflected transmission power into the power amplifier  9 . 
   Located in the signal path between the power amplifier  9  and the antenna  13  is an output coupler  15  which couples the output signal of the power amplifier  9  into a feedback loop  16 . Located in the feedback loop  16  is a selector switch  17 , by means of which an input  18  of a quadrature demodulator  19  can be selectively connected to the output coupler  15  or a terminating resistor  20 . A logarithmic power detector  39  is disposed between the output coupler  15  and the selector switch  17 . The quadrature demodulator  19  consists of a signal splitter  21 , which splits the input signal evenly between an in-phase mixer  22  and a quadrature-phase mixer  23 . A phase shifter  24  is also provided, to which the output signal of the local oscillator  8  is forwarded via an adjustable phase shifter  25 . The phase shifter  24  operates in the same way as the phase shifter  7  and forwards a non-phase-shifted oscillator signal to the in-phase mixer  22  and an oscillator signal phase-shifted by 90° to the quadrature-phase mixer  23 , the oscillator signal having been previously phase-shifted overall by a phase angle φ by a phase shifter  25 . 
   A feedback in-phase component I′ is generated at the output of the in-phase mixer  22  and a feedback quadrature-phase component Q′ at the output of the quadrature-phase mixer  23 . The in-phase component I of the input signal is sent to the (+) input of a first differential amplifier  26  whilst the feedback in-phase component I′ is sent to the (−) input of the first differential amplifier  26 . In the same way, the quadrature-phase component Q of the input signal is applied to the (+) input of a second differential amplifier  27  whilst the feedback quadrature phase component Q′ is applied to the (−) input of the second differential amplifier  27 . As a result of this feedback arrangement, generally referred to as a Cartesian feedback, linearisation errors of the power amplifier  9  are compensated by the quadrature demodulator  19  and the differential amplifiers  26  and  27  disposed in the feedback loop  16 . However, care should be taken to ensure that the feedback signal I′, Q′ is forwarded to the differential amplifiers  26  and  27  with a phase shift of 0° relative to the input signal I, Q. The correct phase position is adjusted by the adjustable phase shifter  25 , whose phase angle φ can be varied via the digital signal processor by means of a control signal. 
   Since both the quadrature modulator  3  and the quadrature demodulator  19  have a DC offset, this DC offset needs to be compensated accordingly. 
   A third differential amplifier  28  is used for this purpose, disposed between the in-phase mixer  22  of the quadrature demodulator  19  and the first amplifier  26 . A fourth differential amplifier  29  is provided between the quadrature-phase mixer  23  of the quadrature demodulator  19  and the second differential amplifier  27 . Whilst the feedback in-phase component I′ is forwarded to the (+) input of the third differential amplifier  28 , a first compensating voltage V I1 , is applied to the (−) input of the third differential amplifier  28  so that the DC offset of the I′ component of the quadrature demodulator  19  is compensated at the output of the third differential amplifier  28 . Similarly, the feedback quadrature-phase component Q′ is applied to the (+) input of the fourth differential amplifier  29  whilst a fourth compensating voltage V Q1  is forwarded to its (−) input. 
   A fifth differential amplifier  30  is used to compensate the DC offset of the quadrature modulator  3 , the (+) input of which is forwarded to the output of the first differential amplifier  26  whilst a third compensating voltage V I2  is applied to its (−) input. A sixth differential amplifier  31  is also provided, the output of which is connected to the quadrature-phase mixer  5  of the quadrature modulator  3  and the (+) input of which is applied to the output of the second differential amplifier  27 . A fourth compensating voltage V Q2  is applied to the (−) input of the sixth differential amplifier  31 . The compensating voltages V I1 , V Q1 , V I2  and V Q2  are shown as controllable voltage sources in  FIG. 1  for the sake of clarity in the drawing but in practice, these compensating voltages may be internally generated in the digital signal processor  2 . 
   Switching rapidly between transmission mode and reception mode using a feedback loop  16  operated on the Cartesian feedback principle causes a problem in that the high-frequency signal path of the loop, consisting of the quadrature modulator  3 , the power amplifier  9 , the quadrature demodulator  19  and the differential amplifiers  26  and  27 , has to be interrupted when the switch is made from transmission mode to reception mode because the power amplifier  9  and the local oscillator  8  have to be disabled. When the power amplifier  9  and the local oscillator  8  are re-activated and the high-frequency signal path restored via the feedback loop  16 , a switching surge is produced because the voltages of the control system, in other words the output voltages of the two differential amplifiers  26 ,  27 , act in the positive or negative control sense when the high-frequency signal path is open. This leads to an unacceptable jump in power to the maximum possible transmission power of the power amplifier  9 . If, as is the case in applications using VDL digital aeronautical radio (VHF digital link), only a short switching time is available, the Cartesian feedback system cannot be used without taking specific measures. In a TDMA system (such as VDL, for example), the power of the adjacent channel must not be adversely affected by burst mode operation. The definition of the VDL standard theoretically permits the transmitter to be enabled and disabled free of interference. The feature proposed by the invention guarantees an ideal interference-free spectrum in the sampled range. 
   In order to get round this problem, the invention proposes that, in addition to the high-frequency signal path from the output of the differential amplifiers  26  and  27  to the (−) input of the differential amplifiers  26  and  27  via the quadrature modulator  3 , the power amplifier  9  and the quadrature demodulator  19 , two direct DC signal paths  32  and  33  be provided, which connect the output of the respective associated differential amplifier  26  or  27  directly to the (−) input of the respective differential amplifier  26  or  27 . In the embodiment illustrated as an example here, the direct DC signal paths  32  and  33  consist respectively of a controllable switch  34  or  35 , which may be provided in the form of field effect transistors, for example, and a resistor  36  or  37  connected in series. 
   The switch from transmission mode to reception mode as proposed by the invention is made in such a way that before the high-frequency signal path is opened, the switches  34  and  35  are firstly closed so that both the high-frequency signal path via the feedback loop  16  and the direct DC signal paths  32  and  33  are operating. Then, the switch  17  is activated by the digital signal processor  2  so that the input  18  of the quadrature demodulator  19  is no longer connected to the output coupler  15  but to the terminating resistor  20 , which means that the high-frequency signal path via the feedback loop  16  is broken. Since there is therefore no longer any input signal at the input of the quadrature demodulator  19 , the level at the (−) input of the first and second differential amplifiers  26  and  27  is defined by the feedback of the DC signal path  32  or  33  and the constant output voltage of the third and fourth differential amplifiers  28  and  29 . Even before the high-frequency signal path is opened by reversing the switch  17 , the current supply (bias) of the power amplifier  9  can be shut off. The transmit-receive switch  12  at the input of the antenna  13  can already be switched, once the I/Q input signal has been reduced (ramping), before the switches  34 ,  35  and  17  are activated and before the current supply of the power amplifier  9  is broken, as a result of which a good breaking isolation is immediately obtained. Reflections occurring at the transmit-receive switch  12  are forwarded via the circulator  10  to the terminating resistor  14 . Finally, the local oscillator  8  is switched off. 
   When switching to transmission mode, the sequence is operated in reverse order. 
   Firstly the local oscillator  8  is enabled and the current supply (bias) connected to the power amplifier  9 . The high-frequency signal path via the feedback loop  16  is then closed by switching the selector switch  17 . The switches  34  and  35  are then opened so that the DC signal paths  32  and  33  are broken again. The transmit-receive switch  12  is switched so that the output of the power amplifier  9  is connected to the antenna  13 . 
   The system of overlapping switching between DC signal path and high-frequency signal path as proposed by the invention ensures that no signal jumps occur during switching because the output of the first and second differential amplifier  26  or  27  is constantly connected to its (−) input either via the high-frequency signal path or via the DC signal path  32  or  33 . Consequently, there is always a defined signal level at the (−) input of the differential amplifiers  26  and  27 . 
     FIG. 2  is a more detailed circuit diagram illustrating the connections of the differential amplifiers  26 ,  27 ,  28 ,  29 ,  30  and  31 , only the signal path for the quadrature-phase component Q, in other words the differential amplifiers  29 ,  27  and  31 , being shown. An identical circuit is provided for the in-phase component I. 
   The input terminal  41  is connected to the output of the quadrature-phase mixer  23  of the quadrature demodulator  19  and internally with the (+) input of the differential amplifier  29 . Between the (+) input of the differential amplifier  29  and the circuit earth  42  is a resistor  43 . Another resistor  44  is disposed between the (−) input of the differential amplifier  29  and the circuit earth  42 , the compensating voltage V Q1  being forwarded to the (−) input of the differential amplifier  29  via a series resistor  45 . Between the output of the differential amplifier  29  and its (−) input is another resistor  46 . The output of the differential amplifier  29  is connected to the (−) input of the differential amplifier  27  via a series resistor  47 . 
   The quadrature-phase component Q of the complex input signal is applied to the (+) input of the differential amplifier  27  via a terminal  48 . Between the terminal  48  and the circuit earth  42  is another resistor  49 . A further resistor  50  is provided between the output of the differential amplifier  27  and the circuit earth  42 . Between the output of the differential amplifier  27  and the (−) input of the differential amplifier  27  is a serially-connected RC-element, consisting of the capacitor  51  and the series resistor  52 . Connected in parallel therewith is the DC signal path  33 , which consists of the controllable switch  35  and the series resistor  37 . Consequently, by closing the controllable switch  35 , a potential equalisation is produced between the output of the differential amplifier  27  and its (−) input. The measurement voltage V QM  lies on the output of the differential amplifier  27  at the measuring point  53 , the importance of which will be explained in more detail below. 
   The wiring of the differential amplifier  31  is identical to the wiring of the differential amplifier  29  and the layout of the resistors  54 – 57  corresponds to the layout of the resistors  44 – 47 . The correction value for the quadrature-phase mixer  5  of the quadrature modulator  3  can be taken from the output terminal  58 . A RC-element consisting of the capacitor  59  and the resistor  60  connected in parallel therewith is disposed at the output terminal  58 . The RC-element defines the bandwidth of the high-frequency signal path. 
   The potential at the (+) input of the differential amplifier  29  is U 1 , whilst the potential at the output of the differential amplifier  29  or at the (−) input of the differential amplifier  27  is U 1 −V Q1 . Similarly, the potential at the output of the differential amplifier  27  or at the (+) input of the differential amplifier  31  is U 2 +V Q2 , which means that a potential of U 2  is set at the output of the differential amplifier  31 . The variable compensating voltages V Q1  and V Q2  are set by an interactive compensating process so that the potential at the (−) input and at the output of the differential amplifier  27  is respectively zero, i.e. U 1 −V Q1 =0 and U 2 +V Q2 =0. Since the potential at the (−) input and output of the differential amplifier  27  is uniformly zero, there are no switching surges when the switch  35  is operated and the DC signal path  32  can be switched on and off surge-free. 
     FIG. 3  illustrates the method proposed by the invention as a means of switching between the transmission mode and a transmission-interrupt mode or reception mode. A compensating process proposed by the invention which may advantageously be used within the context of the invention will simultaneously be explained with reference to this schematic timing diagram.  FIG. 3  gives a logarithmic presentation of the output power P of the power amplifier  9  as a function of time t. 
   The VDL standard prescribes that at the start of the transmission period, a start signal should firstly be transmitted for a period of 3 data symbols, the complex input signal exclusively having an in-phase component I but no quadrature-phase component Q. Within this time period, denoted by  71 , the phase angle φ for the phase shifter  25  can therefore be measured. Since only an in-phase component I is transmitted during period  71 , the voltage at the measuring point  53  would have to be zero. The phase angle φ can therefore be selectively modified before the next transmission period (burst) so that the measurement voltage at the measuring point  53  is optimised to a smallest possible value. This phase angle φ is then maintained until the next transmission period and can be further optimised in the subsequent transmission period. 
   Data are transmitted between the instants t 2  and t 3 . At instant t 3 , the actual transmission process is terminated. In accordance with the VDL standard, situations arise in which a rapid switch has to be made between transmission mode and reception mode within a few 100 μs. This is indicated by line  75  in  FIG. 3 . In this case, the procedure is as described above: the transmit-receive switch  12  is switched and the DC signal paths  32  and  33  are firstly established by closing the switches  34  and  35 . The current supply (bias) of the power amplifier  9  is then switched off and the input  18  of the quadrature demodulator  19  switched from the output coupler  15  to the terminating resistor  20 . Finally, the local oscillator  8  is disabled. 
   In VDL operation, however, there are also situations which permit a slower switching between transmission mode and reception mode, where there are approximately 2.5 ms within which to effect automatic compensation. This automatic compensation process is described below. 
   The quadrature demodulator  19  is trimmed during the period  72  as part of an optional trimming process. This trimming may also be dispensed with if necessary. To this end, the input signal I/Q is firstly reduced to zero so that the power amplifier generates only a minimal residual power P 2 . The compensating voltages V I1 , and V Q1 , which compensate the DC offset of the quadrature demodulator  19 , are optimised so that a minimal residual power P 2  is detected at the logarithmic power detector  39 . Since there is no input signal I/Q, the ideal output power P 2  is zero and an existing output signal will essentially be determined from the DC offset of the quadrature demodulator  19 . 
   In the subsequent period  73 , the quadrature modulator  3  is adjusted in that the measurement voltage V IM  of the in-phase component is measured at the measuring point  61  and the measurement voltage V QM  of the quadrature-phase component at the measuring point  53  in  FIG. 1 . Again with this measurement, both the in-phase component I and the quadrature-phase component Q of the input signal generated by the digital signal processor  2  are zero so that the measured voltage V QM  is essentially derived from the DC offset of the quadrature modulator  3 . By adjusting the voltages V I2  and V Q2 , the measurement voltages V IM  and V QM  are minimised towards zero. As a result, the DC offset of the modulator  3  is compensated. 
   The measurement is taken during period  72  and  73  during which the high-frequency signal path still closed, i.e. the switches  34  and  35  are still open and the selector switch  17  connects the input  18  of the quadrature demodulator  19  to the output coupler  15 . Furthermore, the voltage supply (bias) for the power amplifier  9  is still connected. 
   At instant t 4 , the two switches  34  and  35  are firstly closed, after which the selector switch  17  is switched to the terminating resistor  20  so that the DC signal paths  32  and  33  are now active but not the high-frequency signal path. Before the selector switch  17  is operated, the current supply to the power amplifier  9  is switched off. 
   Since the input signal at the quadrature demodulator  19  is zero as a result and because the digital signal processor  2  continues to generate an input signal I,Q of zero, a measurement voltage V IM  and V QM  measured at the measuring points  53  and  61  is essentially generated as a result of the DC offset of the quadrature demodulator  19 . By shifting the compensating voltages V I1 , and V Q1  in the period  74 , this DC offset and hence the measurement voltage V IM  or V QM  can be minimised. The values of the compensating voltages V I1 , V Q1 , V I2 , V Q2  found as a result of this compensating procedure can be used for the next transmission period. 
   At instant t 5 , the level of the local oscillator  8  is additionally reduced in order to prevent radiation in the receiver. This further enhances isolation between the transmitter and the receiver. 
   The invention is not restricted to the embodiments illustrated as examples here. In particular, the compensating steps may also be run in a different sequence or individual compensating steps dispensed with.