Abstract:
Method and system for tuning a tunable antenna uses a comparison between a signal response at two different tuning frequencies to determine how or if the tuning needs to be further adjusted. With the approach, the method and system arrive at a frequency shift that is centered about the desired antenna frequency, which point there is no net change in the signal response. In a further aspect, a frequency to which a tuned antenna is tuned is shifted to verify whether the antenna is still in tune. Modifications to minimize disturbance of the output received signal that would otherwise be caused by the frequency shifting are contemplated.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a method and system for iteratively tuning an antenna, particularly a narrow-band antenna. A particular example of the present invention relates to a method and system of automatically and dynamically tuning such an antenna. 
       BACKGROUND OF THE INVENTION 
       [0002]    To receive terrestrial television transmissions, especially digital video broadcasting (such as DVB-T and DVB-H), in a portable or mobile environment, a compact omni-directional wideband antenna covering the UHF TV band (470-860 MHz) is desirable. Reference herein to “wideband” is meant to refer to a situation in which a desired tuning range of the antenna is many times the bandwidth of the antenna. Such an antenna is difficult to implement, especially if an antenna having minimal gain is desired. 
         [0003]    It is conventionally considered easier to construct a narrowband antenna having, for example, a dipole, loop, patch, or meander structure. Such antennas can be tuned either electrically (using variable reactance) or mechanically (by varying the length of the elements) to cover the desired band. U.S. Pat. No. 6,606,069, for example, describes an electrically tuned antenna. 
         [0004]    However, narrowband antennas must accurately track a desired signal frequency in order to avoid loss of gain. This can be difficult because a variety of factors can easily or rapidly cause detuning (such as the presence of a person, metallic objects, and the like in proximity to the antenna). 
         [0005]    Conventional transceiver systems (such as short-wave marine radios) having a tunable antenna often use transmit mode tuning. That is, the  5  antenna is tuned according to a signal being transmitted. More specifically, the antenna is tuned to achieve a maximum return loss (or minimum voltage standing wave ratio (“VSWR”)). In the receive mode, the antenna keeps the same tuning as in the transmit mode. JP 60 097 728 generally describes a non-automatic system of this type. 
         [0006]    If, however, the antenna becomes detuned in receive mode with respect to a received signal, the tuning cannot be readjusted to compensate without going into transmit mode. It will be appreciated that this issue becomes significant in many applications (such as transporting digital information (data, video, etc)) because the received signal is continuous and there is no feasible period when the receiver can be disconnected to allow retuning in the transmit mode. In particular, television receiver radiation emissions are strictly regulated such that applying even a very small signal into the antenna for transmit mode tuning is unlikely to be permissible. 
       SUMMARY OF THE INVENTION 
       [0007]    In view of the foregoing, the present invention provides a method and system of for dynamically tuning an antenna as described in the claims appended hereto. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    Embodiments of the invention will now be more fully described, by example, with reference to the drawings, in which: 
           [0009]      FIG. 1  illustrates conventional digital video broadcast tuner architecture (particularly, a DVB-H standard architecture); 
           [0010]      FIG. 2  generally illustrates the frequency response of a tunable narrowband antenna that can be used in the tuner architecture illustrated in  FIG. 1 ; 
           [0011]      FIG. 3  generally illustrates the relationship between shifting the frequency to which an antenna is tuned and the resultant response; 
           [0012]      FIG. 4  is a general block diagram of an embodiment of the present invention, given by way of example; 
           [0013]      FIG. 5  is an embodiment of the present invention using analog storage, given by way of example; 
           [0014]      FIG. 6  is an embodiment of the present invention using a frequency shift, given by way of example; 
           [0015]      FIGS. 7   a - 7   d  illustrate applications of the invention to various signal types; 
           [0016]      FIG. 8  illustrates an example of a signal of interest affected by an adjacent signal on the spectrum; 
           [0017]      FIG. 9  illustrates an embodiment of the present invention, given by way of example, that obtains response level detection from the use of automatic gain control; and 
           [0018]      FIG. 10  illustrates spurious modulation of the output signal during detuning. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0019]    Some preferred embodiments of the present invention are described hereinbelow. 
         [0020]      FIG. 1  illustrates a general block diagram of a digital video broadcast (DVB) tuner. (such as a DVB-H tuner). In general DVB tuner  10  includes an antenna  12  providing a signal to a pre-amplifier stage  14 , such as a gain-controlled RF pre-amplifier. The pre-amplifier  14  and a local oscillator  16  both have outputs connected to a mixer  18 . Local oscillator  16  is tunable to a given frequency for receiving a desired signal. The combined output from mixer  18  may be filtered by filter  20  and amplified by a variable gain amplifier  22  before being received by DVB decoder  24 . Typically, the gains of pre-amplifier  14  and variable gain amplifier  22  are controlled by the DVB decoder  24 , especially to provide automatic gain control. Thereafter, the DVB decoder  24  outputs the desired data stream  26 . 
         [0021]      FIG. 2  generally illustrates a tuning response profile for a tunable antenna, as a relationship between frequency and gain. As can be seen, a given signal has a corresponding frequency to which the antenna can be tuned to obtain a maximal gain response, and this gain response falls off to either side in a standard curve profile. That is, as the extent to which the antenna is mis-tuned increases, the signal quality deteriorates as illustrated by the curve. For the sake of visual clarity, only a few signal response curves are shown in  FIG. 2 . 
         [0022]      FIG. 3  generally illustrates the manner in which an antenna can be tuned so as to converge on a desired frequency  30 . As in  FIG. 2 ,  FIG. 3  illustrates a signal response curve  32  corresponding to a predetermined desired frequency  30 . The vertical axis corresponds to any known response parameter that is a maximum when the antenna is tuned to frequency  30 , such as gain. 
         [0023]      FIG. 3  illustrates several pairs of frequency shifts S 1 , S 2 ; S 1 ′, S 2 ′; S 1 ″, S 2 ″; and S 1 ′″, S 2 ′″. 
         [0024]    Each frequency shift corresponds to a change in the signal response. For example, the shift from S 1  to S 2  corresponds to the signal response. change indicated at A; S 1 ′ to S 2 ′ at A′; S 1 ″ to S 2 ″ at A″; and S 1 ′″ to S 2 ′″ at A′″. 
         [0025]    In general, a given frequency shift corresponds to a change in signal response of a certain magnitude and sense. For example, the shift from S 1  to S 2  corresponds to a positive change A (that is, an increase in signal response). It will be apparent that the magnitude of A corresponds, for a given signal response curve  32 , to the extent of the frequency shift from S 1  to S 2 . Also, the sense of A can change. For example, a shift from S 2  to S 1  would correspond to a negative change A in the signal response. 
         [0026]    These relationships can be used as follows to iteratively arrive at a maximum of the signal response curve, so as to tune an antenna to a desired frequency. 
         [0027]    For a given frequency  30  and signal response curve  32 , the tuning of an antenna is considered with respect to a starting frequency S 1 , as shown in  FIG. 3 . 
         [0028]    The tuning of the antenna is shifted by an arbitrary amount to S 2 , as discussed above. This results in a corresponding change in signal response indicated by A in  FIG. 3 . It will be clear to one of skill in the art that the change in signal response depends at least on the shape of the signal response curve and the extent of the frequency shift  34  between S 1  and S 2 . 
         [0029]    It will be appreciated that a positive change in signal response generally indicates that the frequency shift is moving towards the frequency at which the signal response is maximal (i.e., towards the desired frequency  30 ). An exception would be if the frequency is shifted past frequency  30  but to a frequency at which the signal response is still higher than the initial frequency. However, the iterative use of frequency shifts as explained herein addresses this issue, as will be seen below. 
         [0030]    The next frequency shift  36  is between S 1 ′ and S 2 ′, as seen in  FIG. 3 . S 1 ′ is a frequency that is somewhat lower than S 2  in the prior frequency shift, but always higher than S 1 , so as to obtain, in effect, an overlap of frequencies. The shift from S 1 ′ to S 2 ′ corresponds to a change in signal response A′. 
         [0031]    The extent of the overlap (i.e., the locating of a new first frequency for a subsequent frequency shift) may depend on, among other things, whether a change of sense occurs in the change of signal response in the prior frequency shift and/or the magnitude of the change of signal response in the prior frequency shift. 
         [0032]    A third frequency shift  38  from S 1 ″ to S 2 ″ starts at a frequency somewhat lower than S 2 ′. This frequency shift corresponds with a negative change in signal response A″. It will be also noted that the rate of change (or mathematical slope) in the signal response curve  32  over the frequencies between S 1 ″ and S 2 ″ is relatively smaller than with respect to the frequency shift  34  from S 1  to S 2  or the frequency shift  36  from S 1 ′ to S 2 ′. Accordingly, the magnitude of A″ is correspondingly smaller than A or A′. 
         [0033]    Because A″ is negative for the first time, it can be distinguished that the sweep has passed over frequency  30  to the frequency range to the right thereof, as seen in  FIG. 3 . This is a factor in the selection of the S 1 ′″ to S 2 ′″ sweep relative to S 1 ″ to S 2 ″. Note in particular in  FIG. 3  that the magnitude of S 1 ′″ to S 2 ′″ is selected to be smaller than S 1 ″ to S 2 ″, and that the former does not overlap the latter, but is instead within the boundaries of the latter. 
         [0034]    S 1 ′″ to S 2 ′″ corresponds to a change A′″ of zero, indicating no relative difference in response. Here, S 1 ′″ and S 2 ′″ are centered on frequency  30 , and the frequency shift  40  crosses over beyond frequency  30  to the other side of signal response curve  32 . As can be seen in the figure, the signal response at S 1 ′″ is the same as that at S 2 ′″ (even though the response at S 2 ′″ is with respect to the downward sloping side of the signal response curve  32 ). As a result the change in signal response at A′″ becomes zero, which, as mentioned above, indicates that the frequency shift is centered on the desired frequency  30 . 
         [0035]    If frequency shifts are periodically repeated between S 1 ′″ and S 2 ′″ (suggested by the “return”  42  to S 1 ′″ from S 2 ′″ in  FIG. 3 ) (or, alternatively, any other frequency shift centered on frequency  30 ), the change in signal response should remain zero as long as the tuning remains stable and/or unaffected by external factors, as discussed below. If a change in signal response is detected, the above-described iterative tuning process can be started again. 
         [0036]    The number of frequency shifts before “locating” frequency  30  in  FIG. 3  is strictly by way of example. The relative magnitudes of the frequency shifts are also strictly by way of example. Specific mention is made of the fact that the frequency shifts are not necessarily equal in magnitude. In addition, the iterative process is completely feasible from the opposite side of frequency  30  (that is, to the right of frequency  30  as seen in  FIG. 3 ), so as to use a plurality of negative or decreasing frequency shifts while comparing changes in signal response. 
         [0037]      FIG. 4  is a block diagram illustrating an embodiment of the present invention by way of example. 
         [0038]    In general, a tunable antenna  40  is shifted between frequencies F 1  and F 2 . At each frequency F 1  , F 2 , the resultant signal response is measured by a level detector  42  and stored at storage  44 ,  46  respectively. The values stored in  44 ,  46  are compared by a comparator  48 . The antenna  40  is then retuned according to an output from comparator  48 . 
         [0039]    More particularly with respect to  FIG. 4 , the tuning of antenna  40  is controlled by a combination of signals: an antenna tuning signal  50  generated by an antenna tuning signal generator  52  (optionally controlled by an external control signal  54 ); an antenna tuning modulation signal  56  corresponding to a respective one of frequencies F 1  and F 2 ; and an antenna tuning correction signal  58  corresponding to the output from comparator  48 . This combination of signals  50 ,  56 ,  58  is combined at adder  60  to provide an applied antenna tuning signal  62  that controls the tuning of antenna  40 . 
         [0040]    The antenna tuning modulation signal  56  is switched between F 1  and F 2  by, for example, a switch  64 . It is noted that F 1  and F 2  most generally indicate two frequencies different from one another. 
         [0041]    Once tuned to a certain frequency, the antenna  40  outputs a received signal  66 , which may optionally undergoing one or more forms of signal processing  68  not germane to the present invention. 
         [0042]    The output signal  66  is analyzed by level detector  42  to determine, for example, the level of the signal  66  itself. Alternatively, the level of some conventional signal parameter corresponding to and directly affected by the signal  66  may be determined. The conventional signal parameter may be, for example and without limitation, a bit error rate, a packet error rate, a signal to noise ratio, and the like. Determining a signal level or a parameter level in this manner is deemed to be known to a person skilled in the art and does not itself form part of the present invention. 
         [0043]    The output of level detector  42  is selectively provided to one of storage  44  and storage  46 , according to, for example, a position of switch  72 . The setting of switch  72  depends at least in part on the setting of switch  64 , so that, for example, an output corresponding to frequency F 1  (based on the setting of switch  64 ) is stored in storage  44  (according to the setting of switch  72 ), whereas the output corresponding to frequency F 2  would be stored in storage  46 . 
         [0044]    Accordingly, it is desirable to synchronize switches  64  and  72  by providing a synchronizing means  70  that may, for example, operate in correspondence with an external synchronization control signal  76 . In addition, sample and hold techniques may be used to permit storage of the signal response (or parameter response) values before comparison. 
         [0045]      FIG. 5  is a block diagram illustrating an example implementation of an embodiment of the present invention that is somewhat similar to the arrangement shown in  FIG. 4 . 
         [0046]    Several aspects of the arrangement illustrated in  FIG. 5  are sufficiently similar to those of  FIG. 4  that the like features are merely identified here without a repeated detailed description. 
         [0047]    A tunable antenna  80  is tuned in accordance with a received applied antenna tuning signal  82 . The applied antenna tuning signal  82  is an electrical combination of an approximate antenna tuning signal  84 , an antenna tuning modulation signal  86 , and an antenna tuning correction signal  88 , combined by an adder  90 . The approximate antenna tuning signal  84  is generated by an antenna tuning signal generator  92  in accordance with an applied control signal  94 . 
         [0048]    In this arrangement, generally, a received signal  96  is output from antenna  80  (with or without optional conventional signal processing  98 ) for output as the desired signal. A signal level or a level of a parameter corresponding to and affected by the signal (such as, for example and without limitation, a bit error rate, a packet error rate, and a signal to noise ratio) is detected by level detector  100  using conventional electrical methods. Respective values of the detected levels S 1 , S 2  are stored, in an analog implementation, by capacitors C 1  and C 2  by opening and closing corresponding switches  102 ,  104 , respectively, in a known fashion at appropriate occasions. 
         [0049]    A comparator  106  (which can be, for example, a differential amplifier) compares the respective stored samples and the comparison result becomes the antenna tuning correction signal  88 . 
         [0050]    In a particular embodiment of the present invention, an antenna (which may be correctly tuned as desired) is periodically and intentionally “detuned” in accordance with the foregoing description. This is done to verify the tuning of the antenna. That is, an antenna (particularly, but not necessarily only, a mobile antenna) can become detuned by external factors, such as proximity to conductive objects. Therefore, in order to verify that the antenna is correctly tuned, a frequency shift, centered on a desired frequency (such as frequency  30  in  FIG. 3 ), is applied. If a net change in signal response occurs, it indicates that the frequency range is no longer centered on a desired frequency and that the antenna is therefore “out of tune.” For example, see frequency shift  42  in  FIG. 3 , as discussed above. 
         [0051]    Theoretically, these self-induced frequency shifts can undesirably disturb the output signal. This problem can be reduced by making the transition between the ends of the frequency shift continuous (or otherwise gradual) instead of discrete (as with discrete switching between F 1  and F 2  using switch  64 , as seen in  FIG. 4 ). 
         [0052]    To address this issue, example embodiments of the present invention as illustrated in  FIGS. 5 and 6  may include, for example, a conventional ramp generator  108  for generating a continuous alternating antenna tuning modulation signal  86 . The waveform of the antenna tuning modulation signal  86  is not particularly critical to the invention, but, for example, a triangular waveform is technically simple to generate. 
         [0053]    The operation of switches  102 ,  104  is preferably synchronized with that of ramp generator  108  using a synchronization unit  110  (that may be controlled by an external control signal  112 ). 
         [0054]    In an analog TV application, it may be desirable to synchronize the modulation signal transition and level sampling periods with the picture field frequency. In this way, the level sampling can take place during the vertical blanking period of the image so that the above-mentioned disturbances to the picture and/or sound are invisible and/or inaudible. 
         [0055]    Also, by making the transition period long (e.g., several field periods) the risk of disturbing the sound signal is reduced. As the shifting of the antenna because of external factors is slow, it is possible (particularly in implementations using digital storage or software) to carry out a correction cycle only periodically (instead of continuously). This further reduces the possibility of disturbing the signal when it is carried out at a non-critical time (such as during a program change). 
         [0056]    Disturbing the output signal as described here may only be an annoyance at worst, depending on the situation, such that the aforementioned solutions can be reasonably considered useful but optional to the present invention. 
         [0057]    Taking into account  FIG. 10  with respect to  FIGS. 3 and 4  as discussed above, there will be an additional amplitude modulation of the antenna output signal level  66  as seen in  FIG. 10 , even with a gradual transition. The amount of amplitude modulation of the antenna output will depend on the frequency shift applied to the antenna. Reducing the magnitude of the frequency shift could reduce the amplitude modulation added to the signal, but it could also reduce the sensitivity and precision of the tuning system. 
         [0058]    The signal processing in a DVB tuner architecture often includes an automatic gain control (AGC), as was shown in  FIG. 1 . 
         [0059]    As seen in more detail in  FIG. 9 , one or more variable gain amplifiers  120 ,  122  are placed in the signal path. The signal level at the output of the system is measured by level detector  124  and an error signal  126  (with respect to a reference voltage V ref ) is applied to the variable gain amplifier  120  to maintain the level of the signal constant. The invention can use the error signal  126  as a measure corresponding to the signal level. If the AGC system is able to react fast enough, it can remove the amplitude modulation caused by the shifting of the antenna center frequency with the tuning modulation signal of the sort illustrated at the bottom-most part of  FIG. 10 . 
         [0060]    In many applications, the signal received by the antenna is not a simple carrier and its bandwidth may be only slightly narrower than that of the antenna. Accurate centering of the antenna therefore becomes very important. 
         [0061]    For example, an analog TV signal consists of a picture carrier and sound carrier whose power is about a tenth that of the picture carrier. Depending on the transmission standard, the sound carrier can be 4.5, 5.5, 6.0, or 6.5 MHz higher in frequency than the picture carrier. If the level detector only measures the level of the picture carrier, the antenna center frequency will be positioned on the picture carrier as shown in  FIG. 7A . However, if the total power is measured (i.e., taking into account both the picture and sound carriers), the system will tend to center the antenna response nearer to the center of the complete signal, as shown in  FIG. 7B . 
         [0062]    Perfect centering can only be achieved when the two carriers have the same amplitude ( FIG. 7C ). In the case of a complex signal with a Gaussian or rectangular spectrum, measuring the total power will also achieve correct alignment, as seen, for example, in  FIG. 7D . 
         [0063]    It is also possible to shift the centering of the antenna. This can allow the antenna response to be centered on an asymmetrical signal, such as that seen in  FIG. 8 . 
         [0064]    The simplest method of introducing such shifting is to add an offset  113  at the input of the comparator  106 , as seen in  FIG. 6  (which is in all other respects identical to  FIG. 5  such that the other features therein are therefore likewise numbered). 
         [0065]    In the above description only the signal level has been considered. When there is a strong disturbing signal close to the desired signal, the tuning of the antenna can be disturbed. For example, when a digital TV signal is adjacent to an analog TV signal that may be 30 or 40 dB stronger than the desired digital signal, the digital signal can be disturbed. In this case it may be advantageous to use information other than the signal level, such as bit error rate (BER), packet error rate (PER), or signal to noise ratio (C/N), which all would give more accurate information about the received signal than the signal level. Most generally, in a digital TV decoder, this information is often available and can be used to tune the antenna such that the effect of the adjacent disturbing signal is minimized. 
         [0066]    As seen in  FIG. 8 , the optimum antenna tuning may not occur in this case when the antenna response is centered on the desired signal. The frequency response of the antenna can provide additional “filtering” of the disturbing signal. In spite of some distortion to the desired signal, the best BER, for example, can occur with the antenna center frequency shifted away from the disturbing signal. The maximum of the parameter being considered is always sought, whether it is signal level, PER, BER, etc. This information may be used in place of the signal level or in addition to the signal level to optimize the tuning of the antenna. 
         [0067]    Although the present invention has been described above with reference to certain particular preferred embodiments, it is to be understood that the invention is not limited by reference to the specific details of those preferred embodiments. More specifically, the person skilled in the art will readily appreciate that modifications and developments can be made in the preferred embodiments without departing from the scope of the invention as defined in the accompanying claims.