Abstract:
A signal generator includes a baseband unit, generating a digital, and an I/Q modulator, modulating the digital baseband signal. A storing memory stores a time slot of said digital baseband signal and an optical display unit represents the digital baseband signal stored in said memory according to a predetermined representation mode.

Description:
FIELD OF THE INVENTION  
         [0001]    The invention relates to a signal generator for generating a digitally modulated high-frequency signal, for example a mobile-radio signal according to the GSM (Global System for Mobile Communications) Standard or a W-CDMA (wideband code division multiple access) standard.  
         BACKGROUND OF THE INVENTION  
         [0002]    As a rule, hitherto signal generators do not have a display of the baseband signal currently being generated. At best, a static display is available of the theoretically generated signal, but not of the signal actually being generated currently. The observation of the change in the signal as a function of time or a signal evaluation of the baseband signal actually being currently generated is not possible.  
         SUMMARY OF THE INVENTION  
         [0003]    There is therefore a need to provide a signal generator with which a display of the baseband signal currently being generated is possible.  
           [0004]    According to one embodiment of the invention, a recording memory is provided that records a time segment after the fashion of an instantaneous picture (snapshot) of the baseband signal. The baseband signal thus recorded can be optionally displayed after suitable signal conditioning in a predeterminable display mode, for example as a constellation diagram, spectrum, code-domain power display or as a CCDF (complementary cumulative distribution function) display. This provides a “pseudo real-time” display.  
           [0005]    The display device according to an embodiment of the invention enables the signal change to be observed that the baseband signal undergoes, for example, in a noise unit, a fading unit or a distortion unit. The signal conditioning in the baseband unit can also be checked by observing the signal, for example, upstream and downstream of a pulse-shaping filter or upstream and downstream of a sampling rate converter (resampler). To display the code-domain power of a CDMA signal, it is advantageous to route the signal to the recording memory at the symbol clock level upstream of the pulse-shaping filter since the maximum memory capacity is utilized under such conditions and no symbol clock regeneration is necessary.  
           [0006]    It is advantageous for a decimation unit to be available that decimates the data stream of the digital baseband by a predeterminable decimation factor. Consequently, the baseband signal can be observed, for example, at a decimated system clock level and the decimation factor can be optimally matched to the signal to be observed, for example the symbol length or slot length.  
           [0007]    There are applications in which the signal can be stored without triggering in the recording memory. For a number of applications, however, triggering is expedient, for example if the memory capacity is smaller than the desired observation time period or if the baseband signal is periodic, for example frame-periodic, and periodic segments of the signal are to be combined for a particular evaluation.  
           [0008]    Still other aspects, features, and advantages of the present invention are readily apparent from the following detailed description, simply by illustrating a number of particular embodiments and implementations, including the best mode contemplated for carrying out the present invention. The present invention is also capable of other and different embodiments, and its several details can be modified in various obvious respects, all without departing from the spirit and scope of the present invention. Accordingly, the drawing and description are to be regarded as illustrative in nature, and not as restrictive. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    The invention is described in greater detail below by reference to the drawing. In the drawing:  
         [0010]    [0010]FIG. 1 shows a block circuit diagram of an exemplary embodiment of the signal generator according to the invention;  
         [0011]    [0011]FIG. 2 shows an example of a constellation diagram of a QPSK signal displayed on the display device of the signal generator according to the invention;  
         [0012]    [0012]FIG. 3 shows an example of a constellation diagram of a W-CDMA signal displayed on the display device of the signal generator according to the invention; and  
         [0013]    [0013]FIG. 4 shows an example of a CCDF (complementary cumulative distribution function) display on the display device of the signal generator according to the invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0014]    [0014]FIG. 1 shows a block circuit diagram of the signal generator  1  according to the invention. The signal generator  1  serves to generate a digitally modulated high-frequency signal, for example a digitally modulated mobile-radio signal according to the GSM standard, the GSM-EDGE standard or a W-CDMA standard for third-generation mobile radio (UMTS).  
         [0015]    A baseband unit generates a baseband signal having an I (in-phase) component and a Q (quadrature phase) component. The digital baseband signal generated by the baseband unit  2  can be routed to an I/Q modulator  5  via a plurality of signal-modifying units, for example a noise unit  3  that exposes the baseband signal to noise, and a fading unit  4  that exposes the baseband signal to fading or distortion. Between the baseband unit  2  and the I/Q modulator  5 , a signal path is present for the I component and a signal path for the Q component, respectively. The I/Q modulator  5  modulates the digital baseband signal into an analog signal that is converted by the high-frequency unit  6  into the corresponding high-frequency band. The digitally modulated high-frequency signal is available at the output  7  of the signal generator  1 .  
         [0016]    The baseband unit  2  has a data source  8  that generates data symbols comprising an I component and a Q component at a symbol clock f sym . One pulse-shaping filter  9   a  and  9   b , respectively, is provided in each case for the I component and the Q component, respectively. The pulse-shaping filters  9   a  and  9   b  have, for example, a low-pass characteristic and transform the pulses at the output of the data source  8  into a pulse shape favourable for transmission. In one sampling rate converter (resampler) in each case for the I component and Q component  10   a  and  10   b , respectively, the sampling rate is increased to the system clock rate f sys . The system clock rate f sys  may, for example, be 80 MHz.  
         [0017]    An input  11   a  may be present for the I component of an externally suppliable signal and an input  11   b  may be present for the Q component of an externally suppliable analog signal. Said external analog signal undergoes in one low-pass,  12   a  and  12   b  respectively, in each case, a band limitation and is converted in each case into a digital I component and digital Q component, respectively, at analog/digital converters  13   a  and  13   b , respectively. Said external I component can be added to the I output component of the baseband unit  2  at an adder  14   a , while the external Q component can be added to the Q output component of the digital baseband unit  2  at an adder  14   b.    
         [0018]    According to the invention, a recording memory  15  is present that can record in each case a time segment of the digital baseband signal. For this purpose, an I input  16   a  of the recording memory  15  can be connected via a first switching device  17   a  and a Q input  16   b  of the recording memory  15  can be connected via a second switching device  17   b  having a plurality of positions in the signal path of the I component or the Q component. In the exemplary embodiment shown, a signal tap is situated at the output of the data source  8 , a signal tap is situated at the output of the pulse-shaping filters  9   a  or  9   b , respectively, a signal tap is situated at the inputs of the adder  14   a  or  14   b , respectively, a signal tap is situated at the output of the adder  14   a  or  14   b , respectively, a signal tap is situated at the output of the noise unit  3  and a further signal tap is situated at the output of the fading unit  4  and the input of the I/Q modulator  5 .  
         [0019]    Preferably, one or more decimation units  18   a  and  18   b , respectively, and  19   a  and  19   b , respectively, are situated at the input of the recording memory  15 . While the decimation units  18   a  and  18   b  decimate the data stream of the baseband signal by a decimation factor N, the data stream is decimated in each case by the decimation unit  19   a  and  19   b  by, for example, the decimation factor N:10, which is smaller by a factor of 10, the decimation factor N being freely selectable. The I input  16   a  can be connected either directly to the output of the first switching device  17   a  or to the output of one of the decimation units  18   a  and  19   a  via a third switching device  20   a . Correspondingly, the Q input  16   b  of the recording memory  15  can be connected either directly to an output of the second switching device  17   b  or to an output of one of the decimation units  18   b  and  19   b.    
         [0020]    The start of the time segment that the recording memory  15  records can be controlled by a trigger signal TS that is routed to a trigger input  21  of the recording memory  15 . The trigger signal TS is generated, for example, by a digital signal processor  22  that is connected via a control bus  23  also to the digital baseband unit  2 , the noise unit  3 , the fading unit  4 , the I/Q modulator  5 , the high-frequency unit  6  and the decimation units  18   a ,  19   a ,  18   b  and  19   b . The connection of the control bus  23  is designated by (*). The digital signal processor  22  controls the entire signal conditioning and is, consequently, capable of generating the trigger signal TS in time synchronism with the baseband signal at the corresponding signal tap.  
         [0021]    Furthermore, a display device  23  is present with which the digital baseband signal stored in the recording memory  15  can optionally be displayed in a predeterminable display mode after a suitable display conditioning in the digital signal processor  22 . The display device  23  is, for example, a viewing screen. The recording memory  15  according to the invention makes possible, in conjunction with the display device  23  according to the invention, a “pseudo real-time” signal display and a signal analysis in the signal generator  1 , for example, for displaying a complementary cumulative distribution function CCDF, a constellation diagram, the variation with time of the I signal and Q signal, the spectrum or a code-domain power display. With respect to the code-domain power display in general, but not in the case of a signal generator, reference is made to DE 100 56 258 A1 of the same applicant.  
         [0022]    [0022]FIG. 2 shows an example of a display of a constellation diagram for a QPSK signal. The four state points in the I/Q level and the transitions at the state change are evident.  
         [0023]    [0023]FIG. 3 shows an example of a plurality of superimposed W-CDMA signals in the constellation diagram.  
         [0024]    [0024]FIG. 4 shows a CCDF display. The “complementary cumulative distribution function” (CCDF) indicates the frequency W at which the contributions of the complex sampling values exceed a certain predetermined amount, for example the mean power. If the limit amount is interpreted as the radius of a circle around the origin, the radius is plotted to the right and the frequency of the sampling values that lie outside said circle is plotted upwards.  
         [0025]    The recording memory  15  makes possible, so to speak, a “snapshot” of the signal, that is to say a recording of the signal in a limited time segment. After the signal has been recorded in the recording memory  15 , the recording memory  15  in the exemplary embodiment is read out by the digital signal processor  22  and the next signal recording can be started immediately. The recorded signals are either displayed immediately on the visual display device  23  or, after analysis and preprogramming steps to be undertaken in the digital signal processor  22 , are displayed in appropriate graphical form on the display device  23 . The display of external signals routed via the inputs  11   a  and  11   b  is also possible. Consequently, it is possible to check, for example, whether the injected signal exhibits the expected behaviour or whether the measurement setup is correctly cabled. Since it is not an idealized, theoretically calculated signal that is displayed but the signal actually measured, all the effects of the baseband signal generation, for example the bit resolution of the digital components, transient processes and the effect of the analog band-limitation filters  12   a  and  12   b , can be recognized and an assessment of the signal quality is possible. If the output signal of the signal generator  1  is used as a stimulus for a test object, it is consequently already possible to investigate in the signal generator  1  the signal errors which the output signal of the signal generator  1  has. Consequently, the assessment of the signal modification by the test object is facilitated.  
         [0026]    Furthermore, the consequences of parameter changes in the setting of the signal generator  1 , for example changes in the type of modulation, the noise amplitude or the fading are immediately evident on the display device  23 . The monitoring of external interfaces is also possible. With the increasingly complex digital transmission systems, for example of the third generation mobile radio (UMTS), the signal display makes it possible to check the set parameters in “pseudo real time” and, consequently, for the first time makes the operation reasonably possible as complexity increases. For example, the consequence of limitation effects (clipping) or connecting up a second base station or mobile station on the code domain of a CDMA signal of the signal displayed on the display device  23  is possible and facilitates the selection of the parameters affected.  
         [0027]    Depending on the type of display and depending on whether the signal has been generated internally or externally, a signal tap at different points is expedient. A tap at the level of the symbol clock f sym  upstream of the pulse-shaping filters  9   a ,  9   b  is expedient, for example, for a code-domain power display since the maximum memory capacity is utilized and symbol clock regeneration is unnecessary. A tap at the level of the system clock f sys  makes possible a maximum time resolution of the internal and external signals. An optimum matching of the display to internal and external signals is possible at the level of a system clock f sys /N decimated by the decimation units  18   a ,  19   a ,  18   b ,  19   b  (for example reduction of the system clock from 80 MHz to a decimated system clock of 40, 20, 10 or 5 MHz). The decimation factor N may either be adjusted automatically, for example matched to the signal bandwidth, or can be set manually in order to observe and evaluate the signal in various enlargement steps (zoom steps).  
         [0028]    For particular types of display, it is necessary for the time segment captured by the recording memory  15  to have a certain length since the combination of medium and shorter signal segments is not easily possible. For the purpose of displaying spectra and time-dependent signal display, this minimum length depends on the desired resolution. For a code-domain power display, it is expedient to be able to record at least two symbols of maximum length since the determination of the channel power of a code channel requires at least one complete symbol. If the time segment captured by the recording memory  15  captures at least two symbols, it is certain that the complete symbol is contained in the time segment captured even in the case of an untriggered measurement.  
         [0029]    It is also possible to combine a plurality of signal segments from a plurality of time segments captured consecutively. In the case of some types of display, the combination of subsegments of the baseband signal is possible without triggering. This applies, for example, to the CCDF display shown in FIG. 4, an averaged spectrum or the constellation diagram shown in FIGS. 2 and 3.  
         [0030]    If the signal segment to be analysed is longer than the time segment capturable as a maximum by the recording memory  15 , a plurality of time segments can be seamlessly joined to one another by suitable choice of the trigger signal TS. This makes it possible to analyse longer signal segments even with limited memory capacity of the recording memory  15 . In the case of a frame-periodic W-CDMA signal subdivided into a plurality of timeslots, the individual timeslots of a frame can be recorded consecutively if the recording memory  15  cannot capture a complete frame because of its limited memory capacity. The complete frame can be seamlessly assembled from the individual subrecordings. This method can also be used to display an eye diagram at symbol level.  
         [0031]    While the present invention has been described in connection with a number of embodiments and implementations, the present invention is not so limited but covers various obvious modifications and equivalent arrangements, which fall within the purview of the appended claims.