Abstract:
An antenna diversity receiver is disclosed. The diversity receiver has a multipath detector for controlling an antenna selective switching circuit. The switching circuit couples one of a plurality of antennas to a RF receiver input. The receiver allows suppression of adjacent channel interferences without unwanted side effects by employing a switching disabling circuit to automatically disabling the switching circuit when the signal delay between the occurrence of multipath distortions in the RF reception signal and the switching between antennas exceeds a predetermined critical signal delay value.

Description:
FIELD OF INVENTION 
   This invention relates to an antenna diversity receiver which has a selective switching circuit for coupling one of a plurality of antennas to a RF receiver input, as well as a multipath detector for controlling the selective switching circuit. 
   BACKGROUND OF INVENTION 
   The reception of a wanted RF broadcast transmitter signal may be disturbed or otherwise deteriorated by various phenomena, such as multipath reception and/or adjacent channel interferences. In general, multipath reception is caused by signal reflections at and/or against environmental physical obstacles such as mountains, trees, buildings, fences and the like. Due to such signal reflections a RF broadcast signal may arrive at a certain reception location through various different signal paths which result in different amplitude and phase conditions. The summation of these multipath signals at the antenna of the receiver results in unpredictable signal amplitude and/or phase distortions. These conditions often effectuate partial or complete cancellation of the useful RF reception signal. These signal cancellations, hereinafter also being referred to as signal dips, strongly depend on the RF carrier frequency of the received RF broadcasting signal and on the location of reception. 
   The signal dips severely deteriorate the wanted RF broadcasting signal and therefore also the overall signal reception quality. However, a relatively small shift in the antenna position may strongly improve signal reception quality. This solution is used in so-called antenna diversity receivers used with mobile FM receivers to avoid reception of multipath distorted RF signals. Such antenna diversity receivers are provided with two or more mutually spaced apart antennas coupled to a RF input of a receiver. Only the antenna having best local receiving conditions with respect to the other antenna(s) is actually connected to the RF receiver input. This antenna is hereinafter referred to as an actual antenna and is effective in the reception and supply of the wanted RF broadcasting signal to the receiver as long as the multipath distortion at the actual antenna remains smaller than a certain predetermined multipath threshold level. As soon as the received multipath distortion exceeds the predetermined multipath threshold level, a change of the RF signal supply to the receiver from the actual antenna to another antenna positioned at a location with better receiving conditions, is initiated. The receiver is thus continuously optimized for minimum multipath reception. 
   However, inherent to the antenna diversity feature are short interruptions in the RF signal supply to the receiver during the antenna change over or switching action. Due to the delay between the occurrence of an actual multipath caused signal dip and the detection thereof, the RF signal interruptions may be detected as being caused by multipath effects, and may initiate a subsequent false antenna switching action. The false antenna switching action may in its turn be detected as a multipath originated signal dip initiating a further false antenna switching action with an oscillating effect as a result. To reduce the risk of oscillating antenna switching actions, known antenna diversity receivers using Philips&#39; TEA 6101 model antenna diversity integrated circuit are provided with means to disable any switching action following a preceding switching action within a certain predetermined fixed time period. 
   This known measure however, is not effective in receivers with dynamic IF selectivity, such as receivers with adjacent channel suppression. As mentioned above, adjacent channel interferences are another important source of signal distortion and are usually caused by FM radio broadcast signals modulated on a carrier positioned in frequency adjacent to the carrier frequency of a wanted FM radio broadcast signal. Due to peak values in the FM modulation signal, these adjacent channel FM radio signals may temporarily exceed the allocated channel bandwidth breaking through into the frequency range of the wanted FM radio broadcast signal. Adjacent channel suppression receivers suppress such adjacent channel interferences by varying the bandwidth of the variable bandwidth intermediate frequency (IF) selector circuits dependent on the deviation of the adjacent channel signal within the frequency range of the wanted FM radio broadcast signal. The larger the deviation, the smaller the bandwidth of the variable bandwidth intermediate frequency (IF) selector circuit. The dynamically varying IF selectivity thus effectuates a suppression of the adjacent channel interferences. 
   There is thus a need for a receiver which combines the benefits of the antenna diversity feature with those of the dynamic IF selectivity feature while preventing unwanted effects from occurring. There is a further need for an improved performance antenna diversity receiver. 
   SUMMARY OF THE INVENTION 
   These needs may be addressed by the present invention which is may be embodied in an antenna diversity receiver having antenna selective switching means for coupling one of a plurality of antennas to a RF receiver input, and a multipath detector for controlling the antenna selective switching means. The receiver is characterized by switching disabling means for automatically disabling the switching means when the signal delay between the occurrence of multipath distortion in the RF reception signal and the activation of the switching means exceeds a predetermined critical signal delay value. The present invention is based on the recognition that in an antenna diversity receiver the delay between the actual occurrence of a multipath caused signal dip and its detection is determined by the receiver&#39;s IF selectivity. 
   In a receiver with dynamic IF selectivity, the IF bandwidth is variable, therewith causing the delay to vary as well such that the smaller the IF bandwidth the longer the delay and vice versa. However, the longer the delay, the greater the risk of oscillating false antenna switching actions. By introducing the delay as a new parameter for disabling antenna switching actions in accordance with the invention, any antenna switching action is disabled when and for the time the delay exceeds the predetermined critical value. This allows for a combination of the features of antenna diversity and dynamic IF selectivity, while preventing antenna switching actions from oscillating. 
   Preferably, the antenna diversity receiver is characterized by an adjacent channel detector coupled to a bandwidth variable intermediate frequency (IF) circuit. The predetermined critical signal delay value defines a critical bandwidth for the bandwidth variable IF circuit. The switching means is automatically disabled when the bandwidth of the bandwidth variable IF circuit is smaller than the critical bandwidth. This measure is based on the above mentioned correspondence between the IF bandwidth and the delay between the actual occurrence of a multipath caused signal dip and the detection of it. The bandwidth of the bandwidth variable intermediate frequency (IF) circuit is accurately reflected in the output signal of the adjacent channel detector. This allows for a simple implementation of the invention. 
   A further preferred embodiment of the antenna diversity receiver is characterized by a threshold circuit coupled between an output of the adjacent channel detector and a control input of the switching disabling means. The threshold circuit compares the output signal of the adjacent channel detector with a threshold value corresponding to the critical bandwidth and supplies a switching disabling control signal to the switching disabling means when the output signal of the adjacent channel detector effectuates a bandwidth smaller than the critical bandwidth. 
   With a simple adjustment of the threshold voltage, the critical bandwidth can be set at a value preventing the antenna switching actions from oscillating and providing an effective adjacent channel suppression on the other hand. An antenna diversity receiver, which is provided with a fixed timer circuit introducing a fixed switching disabling period following each antenna switching action in accordance with the invention is preferably characterized by the bandwidth variable intermediate frequency (IF) circuit effecting a signal delay at the critical bandwidth corresponding to the fixed switching disabling period. 
   Such a fixed timer circuit is included in the Philips&#39; TEA 6101 antenna diversity integrated circuit and may well be combined with the antenna switching disabling functionality initiated by the above switching disabling control signal. The use of the fixed switching disabling period as a reference for the determination of the critical bandwidth avoids the occurrence of antenna switching oscillations throughout the complete bandwidth control range of the bandwidth variable intermediate frequency (IF) circuit. 
   In practice the fixed switching disabling period following each antenna switching action in the Philips&#39; IC TEA 6101 is 20 usec., defining the critical bandwidth to be substantially within the range between 40 and 50 Khz. 
   Another preferred embodiment of antenna diversity receiver provides for a simple combination of both the adjacent channel suppression feature with the antenna diversity feature is characterized in providing the adjacent channel detector with a multiplex input coupled to an output of the demodulator for detecting adjacent channel reception at the occurrence of both an amplitude variation in the IF signal level as well as distortion components in the demodulator output signal. 
   In yet another preferred embodiment, the plurality of antennas includes an antenna for receiving radio broadcast RF signals as well as an antenna for receiving telecommunication RF signals. 
   It is to be understood that both the foregoing general description and the following detailed description are not limiting but are intended to provide further explanation of the invention claimed. The accompanying drawings, which are incorporated in and constitute part of this specification, are included to illustrate and provide a further understanding of the method and system of the invention. Together with the description, the drawings serve to explain the principles of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
       FIG. 1  shows an antenna diversity FM receiver according to one embodiment of the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   While the present invention is capable of embodiment in various forms, there is shown in the drawings and will hereinafter be described a presently preferred embodiment with the understanding that the present disclosure is to be considered as an exemplification of the invention, and is not intended to limit the invention to the specific embodiment illustrated. 
     FIG. 1  shows an antenna diversity FM receiver having first to fourth antennas  2 ,  4 ,  6  and  8  coupled through an antenna selective switching circuit  10  to a RF receiver input circuit  12 . The RF receiver input circuit  12  is followed by a mixer device  14  which is supplied with a tunable local oscillator signal from an oscillator circuit  16 , a variable bandwidth intermediate frequency (IF) selector circuit  18  (selective IF circuit), a IF amplifier  20 , an FM demodulator  22  and a baseband modulation signal processor  24 . The baseband modulation signal processor  24  is coupled to stereo left and stereo right signal reproducing circuits  26  and  28 . 
   The signal processing in the antenna diversity receiver described so far is known. A RF broadcasting signal is received at one of the first through fourth antennas  2 ,  4 ,  6  and  8 . In the embodiment shown, the third antenna  6  is the actual antenna. The third antenna  6  is supplied via the antenna selective switching circuit  10  and a RF receiver input terminal to the RF receiver input circuit  12  for broadband selection and amplification of the RF broadcasting signal. The output signal of the RF receiver input circuit  12  is then mixed in the mixer device  14  with the tunable local oscillator signal for a first demodulation of a wanted RF broadcast signal into an intermediate frequency (IF) signal. This IF signal is filtered in the selective IF circuit  18 , subsequently amplified by the IF amplifier  20  and demodulated by the FM demodulator  22  into a baseband modulation signal which is a stereo multiplex (MPX) signal. The MPX signal is further processed in the processor  24  to obtain stereo left and stereo right signals to be converted into acoustic signals in the stereo left and stereo right signal reproducing circuits  26  and  28 . 
   The antenna diversity receiver also includes a multipath detector  30  for detecting multipath distortion in the received RF signal. The detection of multipath distortion is based on two criteria: the occurrence of a (fast) amplitude dip in the level of the IF signal and the occurrence of distortion components occurring within the frequency range of the baseband modulation signal above the frequency spectrum of the MPX signal. The first and second input terminals of the multipath detector  30  are therefore coupled respectively to an output of the selective IF circuit  18  and to an output of the FM demodulator  22 . If a IF signal dip is detected to occur simultaneously with distortion components within the frequency range of the baseband modulation signal, then the multipath detector  30  supplies a switching control signal to a switching control signal input terminal  31  of an antenna switching control device  32 . This causes the antenna selective switching circuit  10  to change over reception from the antenna  6  which is the actual antenna up to this switching action, to another antenna, having better receiving conditions than the antenna  6 . In this example, the antenna  8  is selected (the connection to the RF receiver input circuit  12  is not shown). 
   The next multipath caused signal dip detected will initiate a subsequent antenna switching action as described above. In receivers using Philips&#39; TEA 6101 antenna diversity integrated circuit the antenna switching control device  32  has a disabling circuit  33  which prevents any antenna switching action from being followed by a subsequent antenna switching action within a fixed time period of 20 usec. These time periods are referred to as fixed switching immunity periods. The antenna switching disabling circuit  33  has a timer circuit (not shown) using a counter, which is set to start counting monotonously up or down at the occurrence of each switching control signal and stops counting after the fixed time period of 20 usec. This provides immunity from switching control signals during counting and prevents switching actions during these fixed switching immunity periods from occurring. 
   The antenna diversity receiver is provided with an adjacent channel detector  38  functioning as bandwidth control which has an input coupled to an output of the FM demodulator  22 . The channel detector  38  also has an output coupled to a bandwidth control input of the selective IF circuit  18 . This output varies the bandwidth dependent on adjacent channel interferences, such that the bandwidth of the selective IF circuit  18  is smaller, the more the adjacent channel signal is overlapping or trespassing the frequency area of the wanted RF signal. The interferences caused by such adjacent channel signals are thereby reduced. For a more detailed description of the functioning of the adjacent channel feature in suppressing adjacent channel interferences, reference is made to U.S. Pat. No. 4,907,293 hereby incorporated by reference. 
   In addition to the fixed switching immunity periods the receiver is also provided with an immunity for switching control signals from the multipath detector  30  when and for the time, the bandwidth of the selective IF circuit  18  is smaller than a certain critical bandwidth value. To avoid switching actions from oscillating at any bandwidth of the selective IF circuit  18  within its entire bandwidth variation range, the length of the fixed switching immunity periods following each antenna switching action provided by the timer circuit is used to determine the critical bandwidth value, such that the bandwidth variable intermediate frequency (IF) circuit will effect a signal delay at the critical bandwidth corresponding to the fixed switching disabling period. At a length of the fixed switching immunity periods of 20 usec. the critical bandwidth value, which is referred to as the predetermined threshold value of the IF selectivity bandwidth, will be substantially within the range between 40 and 50 KHz. 
   Therefore the receiver has a threshold circuit  36  coupled between an output of the adjacent channel detector  38  and a control input  35  of the antenna switching disabling circuit  33  for comparing the output signal of the adjacent channel detector  38  with a threshold value corresponding to the critical bandwidth value and for supplying a switching disabling control signal to the antenna switching disabling circuit  33 , when the output signal of the adjacent channel detector  38  effectuates a bandwidth smaller than the critical bandwidth value. The threshold value is set with a well chosen predetermined threshold voltage Vth supplied from a threshold voltage input terminal  34  to the threshold circuit  36 . 
   It will be apparent to those skilled in the art that various modifications and variations can be made in the method and system of the present invention without departing from the spirit or scope of the invention. For example, the plurality of antennas may include an antenna for receiving radio broadcast RF signals as well as an antenna for receiving telecommunication RF signals. The present invention is not limited by the foregoing descriptions but is intended to cover all modifications and variations that come within the scope of the spirit of the invention and the claims that follow.