Abstract:
A scanning antenna diversity system for motor vehicles having at least two antennas mounted in a multi-antenna installation, and connected to a switching logic circuit. The circuit has discrete switching positions, and a reversing position USmax. A receiver is coupled to logic circuitry for receiving a RF signal from one of switching positions or reversing position USmax. There is a diversity processor coupled to and controlled by receiver for producing a derived received signal from the RF signal, with diversity processor cyclically stepping the logic circuitry to another switching position during the presence of receiver interference. There is a maximum level indicator having its input coupled to the antennas, and having its output connected to the controllable logic circuitry for sensing the strongest actual levels of the antenna signals. Level indicator  39  continuously compares the received signals at antennas and activates the logic switching circuit in response to any interference.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a scanning antenna diversity system for vehicles having a multi-antenna installation. 
     2. The Prior Art 
     Circuit arrangements for antenna diversity systems are known from U.S. Pat. No. 4,752,968. With an antenna diversity receiving system for eliminating interference in frequency-modulated (FM) radio transmissions, a diversity processor is provided with a number of antenna signals, wherein a selected antenna signal is switched through to the receiver at each interval in time. From this received signal, a signal is derived in the intermediate frequency (IF) range of the receiver by conversion and coupled to the diversity processor for interference detection. When an interference is detected, switching signals are derived in the diversity processor for reversing to another antenna signal. Therefore, audio frequency interference caused by multichannel reception is largely avoided if adequate reception conditions are present. 
     However, in areas having poor reception conditions, U.S. Pat. No. 4,752,968 discloses circuit arrangements which have frequent reversing or switchover processes. These circuit arrangements have interference caused by such changeovers, particularly in areas where the signals are weak, and the reception quality in the motor vehicle is good. The audio quality is thus often impaired by these switchover interferences. 
     With frequency-modulated (FM) radio signals (for example ultra shortwave reception or television audio reception), reception interference can occur in connection with an antenna actually locked on to, the interference being caused by superimposing a plurality of partial waves with different amplitude, phase, and transmission time differences at their reception location. The level breakdowns caused by this superpositioning are correlated with frequency interference peaks, and cause signal distortions depending upon the modulation level in the audio frequency range. The minimum test time therefore corresponds with the interference detection time. If the interference detector detects interference or noise, the diversity processor induces further switching, and the further antenna signals and their linear combinations formed in an antenna matrix, if any, are all tested. If all available HF-signals show interferences, all HF-signals are continually and successively switched through to the receiver. Since this search process is repeated in rapid sequence, an audible interference signal in the form of a crackling noise is superimposed on the received signal on the low-frequency level, under special received conditions. 
     Therefore, measures are implemented in DE 440 03 612 in order to limit the frequency of further or continuing switching, in particularly, unfavorable receiver positions. This is accomplished, according to this patent, with the help of a stop device which prevents further switching when there is frequent interference indicated by the diversity processor, in a way that is adapted to the conditions of the reception. The interferences caused by reversing are in fact reduced by this measure. However, the reception interference indicated by the diversity processor naturally has an interference effect in the received signal. 
     SUMMARY OF THE INVENTION 
     Therefore, an object of the present invention is to provide for a scanning antenna diversity system, an apparatus which reduces further switching, and consequently the interferences and noise connected with this further switching. 
     In the invention, there is provided a scanning diversity antenna system for motor vehicles having a multi-antenna installation, with at least two antennas. The received voltages from the antenna signals are formed as linear combinations, and supplied to a controllable logic switching element. The received signal, which varies in terms of diversity, is switched through in different switching positions to a receiver. The receiver controls a diversity processor with a received signal derived from the varying received signals, the diversity processor cyclically switching the logic element to another switching position during reception interference. The received antenna signals are additionally supplied, to a maximum-level indicator, isolated from the signal path of the received signal, where the peak levels of the antenna signals, and thus the associated maximum-level switching position of the logic switching element are continuously determined. These determinations are carried out by a comparison of the received signal that is switched to the actual receiver, without any auxiliary modulation or the addition of any auxiliary signal. The peak level switching position is continually updated as an address signal at the output of the maximum-level indicator, and also supplied to the logic switching element. The logic switching element is designed so that in the cyclic through-switching of the received signals to the receiver, and while testing for lack of interference, a reversing position is available, in addition to the discrete switching positions of the logic switching element, for switching over to the maximum-level switching mode. The reversing position is used at least once during each cyclic pass through the switching positions. The cyclic stepping mode is interrupted, and the logic switching element continually switches the strongest signal received through to the receiver, with the help of the address signal, for the peak switching position. In the event that an interference is indicated in the diversity processor, the system reverses from the maximum-level switching mode back to the stepping mode. 
     The invention has the advantage that the received signal selected by the diversity processor is available at any time in the reverse position, and provides the strongest signal level due to pre-selection of the received signals so that there is low probability of interference. Therefore, the advantage is that the pre-selected received signal has an interference probability less than with the discrete signals, and thus there is a lower rate of interference detection of the diversity processor. On the other hand, in FM radio transmissions, extremely strong. interferences may occur, even with large received signals, such as during multichannel reception with large differences in transmission time, or due to interferences with an adjacent or even the same channel. Therefore, it is necessary that the selection, based on minimal interference, retains priority,and that the diversity processor also tests the maximum-level signals for freedom from interference, and, if need be, switches to a signal present in the discrete form. 
     Another advantage of the invention is that it is no longer necessary to invade the signal chain of the receiver connected to the loudspeakers to find the received signal with the highest level, since the available received signals are supplied to the maximum level indicator by way of the level comparator switch. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects and features of the present invention will become apparent from the following detailed description considered in connection with the accompanying drawings which disclose several embodiments of the inventions. It is to be understood, however, that the drawings are designed for the purposes of illustration only, and not as a definition of the limits of the invention. 
    
    
     In the drawings, wherein similar reference characters denote similar elements throughout the several views: 
     FIG. 1 a  shows a scanning antenna diversity system with a multi-antenna installation; 
     FIG. 1 b  shows a scanning antenna diversity system similar to FIG. 1 a , with the addition of a maximum-level switching mode; 
     FIG. 2 a  shows a scanning antenna diversity system with a level comparison tuner as the maximum level indicator; 
     FIG. 2 b  shows sample pulses of s level comparison for two switching position over a time interval; 
     FIG. 3 shows a scanning antenna diversity system wit h a simple logic element; 
     FIG. 4 shows a scanning antenna diversity system with a multi-diode switch; and 
     FIG. 5 is a graph which shows the received signal levels from two switching positions, over a travel distance. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 a  shows a scanning antenna diversity system with a multi-antenna installation  1 , with antennas A 1 , A 2 , and A 3  connected to a switch  2 , which in turn is connected to a receiver  3 , and a diversity processor  4  according to the prior art. Different received signals  5  are switched to the receiver  3  by means of the stepping signals or pulses  28 , by means of the cyclic stepping of switch  2 . 
     FIG. 1 b  shows an improved scanning antenna diversity system over the one shown in FIG. 1 a , in that the system is expanded by a maximum level switching mode. The switching mode is initiated in the reverse switching position, USmax with the help of a maximum-level indicator  39 , consisting of a level comparator switch  33  controlled by the reversing signal  12  from a level indicator  23  and the level comparator device  8 , address signals  19  are supplied to logic circuit  2  as the output signal of maximum level indicator  39 . For this purpose, antenna signals A 1 , A 2 , A 3  . . . etc. are supplied to the level comparator switch  33  in parallel at contacts V 1 , V 2  and V 3 . In the maximum-level switching mode, address signal  19  continually supplies the updated maximum-level switching position (Smax) for switching through a received signal  5  at the maximum level. When an interference occurs in reversing position USmax, and is indicated in diversity processor  4 , the system reverses from the maximum-level switching mode back to the cyclic stepping mode. However, as a rule, no cyclic stepping will take place in reversing position USmax since received signal  5  with the highest HF-level is switched through to receiver  3  while the car is driven. Logic circuit  2  represents almost any desired multi-antenna installation  1 , supplying at any point in time, a received signal  5  to receiver  3 . With different discretely available switching positions P 1 , P 2  . . . , different received signal  5  are available, as known from German Patent DE 44 03 612 and U.S. application Ser. No. 08/803,239. However, different received signal  5  as in FIG. 1 of German Patent DE 44 03 612, can be formed also from linear combinations of the antenna signals with the help of an antenna matrix  10  in this figure. The advantage of scanning antenna diversity systems of this type is the low cost in using receiver  3 , which does not require any multiple tuners for each antenna or similarly any costly accessories, or optional features, but allows at low cost, the formation of a great number of different received signals  5  with the help of different switching positions (P 1 , P 2  . . . ). A preferred diversity processor  4  is the type of processor that has an extremely rapid interference detector. In the past, a processor of type TEA  6101  was found to be extremely efficient for this purpose. This type of processor reacts to interferences with an indication time of about 30 microseconds, which is substantially determined by the intermediate frequency bandwidth of receiver  3 . 
     FIG. 2 a  shows a scanning antenna diversity system according to the invention, in which level indicator  23  of maximum-level indicator  39  is shown designed in the form of a level comparison tuner  37 , and electronic stepping switch  31  is controlled via a stepping address logic circuit  38  receiving stepping pulses  28  from diversity processor  4 . The maximum-level switching position (Smax) of level comparator device  8  is supplied to logic circuit  38  through line  19 . 
     FIG. 2 b  shows sample pulses in,the event of an s level comparison according to two switching positions within level comparison time interval  6  of about twice the time interval of sample time  16  after each time interval  7 . 
     FIG. 3 shows another embodiment of a scanning antenna a diversity system wherein logic element  2  consists of both an electronic stepping switch  31 , and an addressable electronic reversing switch  30 , whose inputs P 1 , P 2  . . . and S 1 , S 2  . . . , are connected respectively, in parallel to the RF signals of antennas A 1  . . . A 3 . During the pulse width of level comparison time  6  as shown in FIG. 2 b , the maximum-level switching position (Smax) was determined from the possible switching positions S 1  . . . S 3 , adjusted, in each case, for the duration of time interval  7 . The output signal of electronic reversing switch  30  was supplied to one of the inputs (in the example, P 4 =USmax) of electronic stepping switch  31 . The discrete signals of antennas A 1 , A 2  and A 3  are supplied to the other or remaining inputs (in the example: P 1  . . . P 3 ) of electronic stepping switch  31 . When an interference is detected in the intermediate-frequency (IF) signal  9  of superhet receiver  3 , and inputted to diversity processor  4 , steps electronic stepping switch  31  in each case, by one switching position. An antenna signal  50  is supplied to a level comparison RF tuner  37  from level comparison switch  33 , the latter being controlled by means of reversing signal  12 , and the maximum-level switching position (in the example: Smax=S 3 ) is continually determined in level comparator device  8 . Level comparator device  8  is connected via line  19  to reversing switch  30 . To reduce costs, the L.O. signal of oscillator  40  is used both in receiver  3  and level comparison RF tuner  37 . Tuner  37  has an input amplifies stage  36 , a mixer  35 , an IF filter  34  and an output detector  43 . 
     The level comparator device  8  in turn consists of a time interval clock or timer  11 , which generates the level comparison pulses  15  at periodically recurring time intervals  7 . The level comparison pulses  15  are supplied to a sample timer  20  via a logic circuit  13 . This sample timer  20  generates the sample and hold pulses  41 , which are supplied to a hold decider  17 . The sample timer  20  and hold decider  17  supply a binary signal  18  to logic circuit  13 , which transmits the maximum-level switching signal  19  (Smax) to antenna switch  30  to allow switch  30  to select the strongest received signal. 
     FIG. 4 shows a further embodiment of a scanning antenna diversity system with a logic element  2  consisting of a multi diode switch  32   a  and a stepping logic circuit  2   a , the logic element  2   a  representing electronic stepping switch  31 . Here, electronic changeover switch  33  consists of a changeover logic circuit  2   b  and a multi-diode switch  32   b . The maximum-level switching mode is initiated by the alternative activation of one of the diodes through changeover logic  2   b  by changeover signal  12  from logic circuit  13 . In this process, the address signal  19  from logic circuit  13  for the maximum-level switching position (Smax) is determined in the interval of level comparison time  6 , and the address of the maximum-level switching position (Smax) is updated in stepping logic circuit  2   a , in the address list (Smax, S 1 , S 2 , S 3 , or S 4 ) present in the stepping logic circuit  2   a  in response to address signal  19 . In this connection, Smax corresponds to one of the switching positions S 1 , S 2 , S 3  or S 4 . If an interference occurs, a stepping pulse  28  is generated during a time interval  7  on the output of diversity processor  4  in response to an IF signal  9 . This activates the stepping mode, and the address list is serviced in each case by one further position through activation of the associated diode. Logic circuit  13  can be advantageously designed so that diode switches  32   a  and  32   b  will not switch through the same antenna signal at the same time. 
     FIG. 5 shows the relative received signal level in dB from 2 switching positions and the higher receiver level over a relative displacement distance of travel of the vehicle based on wavelength. 
     The basic mode of operation of the present invention can be graphically explained with FIG. 5, by looking at the received signal  5  resulting for two different switching positions S 1  and S 2  over the driven distance, which is based on wavelength λ of the received frequency (f). This is shown for the case of an interference level that is notably below the median value of the two received signal  5 . Each of the two received signal  5  has the known level breakdowns caused by the multi-way reception. Without the circuits of the present invention, the diversity processor  4  would, in each case, reverse to the other switching position if the interference spacing were too small for received signal  5  within the range of the level drops of the signal. The consequence thereof is that about  5  reversals take place over the viewed distance of travel under the received conditions shown by way of example in FIG.  2 . Now, according to the invention, the greater of the two received signal  5  is additionally made available next to the discretely available switching positions S 1 , S 2  as a separate maximum-level switching position (Smax) after each of the cyclically occurring level comparison processes during the successive time interval  7 , with the help of the level comparison device in logic switching circuit  2 . In an interference situation as shown in FIG. 5, the greater received signal  5  from switching positions S 1 , S 2  will not trigger any activity in diversity processor  4  because, at each point in time, it is notably above the interference level. The change in the switching positions is shown at the bottom of FIG. 5, wherein the maximum-level switching position (Smax) is formed in the various zones of the distance traveled. The change itself takes place in each case by rapid reversals. In the cyclic test of the received signal  5  according to switching positions S 1 , S 2 , Smax during time intervals  7 , because of the reduced probability of an occurrence of an interference, diversity processor  4  will, as a rule, remain significantly longer with switching position Smax, than on switching positions S 1 , S 2 . This means that the reversing frequency is substantially reduced. 
     It is assumed in connection with the representation in FIG. 5 that the cyclically recurring time intervals  7  are selected by a number of orders of magnitude shorter than the time duration that the motor vehicle needs in order to travel a distance of half a wavelength of receiving frequency (f), so that time intervals  7  can not be recognized there in FIG.  5 . Advantages according to the invention can still be obtained with time intervals  7  where the motor vehicle moves ahead by ⅕ of the wavelength (⅕λ). However, it will be deemed desirable in practice that at least 10 time intervals will be encountered during the movement of the vehicle by half a wavelength (½λ). If v is the driving speed, the following can apply as the standard value for time interval 7=Ti then 
     
       
         Ti≦λ/(2*10 *v ). 
       
     
     The level comparison time 6=Tp , on the other hand, has to be set substantially smaller than Ti. With the negligibility of the pure reversing times given in practice, the cycle time Tz for the entire process results in 
     
       
           Tz =Ti. 
       
     
     And the cycle frequency in 
     
       
           fz =1 /Tz , i.e. approximately 1/Ti. 
       
     
     This means that the cycle frequency has to be selected greater than the standard value: 
     
       
           fz min=2*Vkmh*fMHz/100 in Hz. 
       
     
     In the ultra-short wave frequency range, with fMHz=100, the standard value for the minimal cycle frequency fzmin in Hz thus corresponds with twice the driving speed in km/h. 
     The invention can be advantageously and with low cost applied to a great number of available switching positions as well. Received signal  5  according to the maximum-level switching position (Smax), because of the continually changing level conditions on the antennas due to multi-way propagation, is obtained by preselection from the multitude of antenna signals constantly alternating with each other. The greater the number of available different switching positions with different received signal  5 , the smaller the drops or breakdowns in the curve of the received signal  5  at the maximum-level switching position (Smax). It is important in this connection, especially in reception areas with low to medium signal levels, that diversity processor  4 , which operates based on an interference criterion, is at no time blocked, but that the additionally provided maximum-level signal is nonetheless avoided by diversity processor  4  in the event an interference is detected. 
     The additional advantage obtained with the invention is that the average interference spacing of received signal  5  according to maximum level switching position (Smax) is notably enhanced as well. Due to pre-selection of the maximum-level switching position (Smax) from the available switching positions, the associated received signal  5  follows the break in signal level after a maximum, far less steeply and is, with high frequency replaced with good interference spacing by a more favorable switching position with the updated Smax. 
     A major advantage of the invention is that the maximum level switching position is found without adding auxiliary signals to received signal  5 , or without modulating this signal with an auxiliary signal. Due to the linear distortions occurring during the transmission over the radio path, these signals lead, in some radio receiver situations to undesirable receiver distortions caused by band spreading effects. 
     Accordingly, while only several embodiments of the present invention have been shown and described, it is obvious that many changes and modifications may be made thereunto without departing from the spirit and scope of the invention.