Abstract:
A method and system are provided for improving the isolation characteristics of a combiner. In a system comprising a digital and analog radio signal combiner and a digital transmitter, an analog transmitter, an antenna, and a reject load coupled to the combiner, adjusting an impedance matching device coupled between the reject load and combiner so as to reduce the energy transmitted from the digital transmitter to the analog transmitter through the combiner.

Description:
FIELD 
     This invention relates generally to transmission of analog and digital radio signals. 
     BACKGROUND 
     Traditional terrestrial commercial radio broadcasters transmit an analog signal. However, use of a digital signal allows broadcasters to transmit more data in the same amount of bandwidth as an analog signal. Such digital audio broadcasting is therefore capable of providing listeners with a much higher audio quality and various additional information typically in the form of scrolling text on the radio receiver&#39;s display, such as traffic and weather reports, song titles and artist information, sports scores, and the like. 
     Among the standards proposed for providing digital radio service, the In-Band On-Channel (“IBOC”) transmission method has gained acceptance for its ability to allow simultaneous transmission of analog and digital signals within the same channel. By using IBOC, a radio broadcaster avoids the need for additional frequency allocations. Accordingly, a listener with a digital radio receiver may tune to customary frequencies to receive digital broadcasts. The National Radio Systems Committee (“NRSC”) promulgates an IBOC digital radio broadcasting standard, presently NRSC-5, that sets out the requirements for a system for broadcasting digital audio and ancillary digital data signals over AM and FM broadcast channels that may contain analog signals. 
     Broadcasters using IBOC digital radio broadcasting (commonly termed “HD Radio”) transmit digital subcarrier signals in sidebands along with analog signals, or hybrid transmissions. FM IBOC, for example, creates a set of upper and lower sidebands on each side of the analog carrier frequency. Those hybrid transmissions, including any noise and spuriously generated signals such as phase noise and intermodulation products, generally must fit within a spectral mask, for example, the spectral mask specified by the NRSC for hybrid transmissions.  FIG. 1  illustrates one such spectral mask  1  for an FM hybrid transmission within which the analog carrier frequency  2  and the upper and lower digital sidebands  3  of the hybrid transmission appear. 
     Broadcasters rely on a number of different methods for HD Radio implementation. For FM HD Radio, for example, one such method, known as “High-Level” Combined HD Radio, involves separately creating and amplifying analog and digital signals, combining those signals and feeding them to a common antenna, such as in the system illustrated in  FIG. 2 . In  FIG. 2 , a combiner  10  (also known as an injector) may receive at the analog RF input port  11  an analog signal from an analog FM transmitter  12  and receive at the digital RF input port  13  a digital signal from a digital FM transmitter  14 , combine the analog and digital signals and feed the hybrid signal to an antenna  16  through the antenna output port  15 . The process of combining analog and digital signals generally results in substantial RF energy loss, which is dissipated to a reject load  18  through the reject load output port  17 . For a 10 dB combiner, for example, such losses are normally at about 0.5 dB between the analog RF input port  11  and the antenna output port  15 , and at about 10 dB between the digital RF input port  13  and the antenna output port  15 . That is, the reject load  18  typically dissipates 90% of the digital signal and about 10% of the analog signal. 
     The combiner  10  may also isolate the analog and digital transmitters  12  and  14  to avoid the generation of spurious or intermodulation products. Typical combiners generally provide such isolation in the range of 30 to 45 dB. For example, according to the iBiquity Digital Corporation, an organization responsible for developing the IBOC standards promulgated by the NRSC, the minimum recommended isolation is 36 dB. 
     It has been found that a tube-type analog FM transmitter generally requires substantially more (at least 10 dB) isolation from a digital FM transmitter than solid-state analog FM transmitter. Approximately 30 dB of isolation from a tube-type analog FM transmitter into a solid-state digital FM transmitter, or from a tube-type digital FM transmitter into a solid-state analog FM transmitter is generally sufficient to avoid the generation of spurious or intermodulation products. 
     Typical combiners have been found to generally perform according to their manufacturers&#39; specifications when all four ports, i.e., the analog RF input port, the digital RF input port, the antenna output port and the reject load output port, are terminated with precision 50Ω test loads. In that situation, the isolation characteristics of the combiner typically meet iBiquity&#39;s recommended 36 dB of isolation. However, isolation between the analog and digital RF input ports typically degrades significantly when the precision loads at the antenna output port and reject load output port are replaced with a typical FM broadcast antenna and reject load, respectively, due to the actual load impedance of the antenna and combiner imperfections. Such degradation typically reduces the isolation to below the recommended 36 dB. It has been found, for example, that the isolation from a digital solid-state FM transmitter into a tube-type analog FM transmitter is typically degraded to the point that intermodulation products produced by the analog transmitter fall outside the limits of the standard spectral mask of  FIG. 1 , as is illustrated in  FIG. 3 . In the spectral plot of  FIG. 3 , the intermodulation products  20  appear above and below the digital carriers  21  outside the spectral mask  22 . Despite such degradation, though, the measured voltage standing wave ratio (“VSWR”) of the FM antenna over a 400 kHz bandwidth is typically less than the maximum 1.2:1 ratio recommended by iBiquity, and the measured VSWR of the reject loads is typically less than the maximum 1.1:1 to 1.15:1 ratio range recommended by iBiquity. Ideally, the characteristic impedance of the antenna is equal to the characteristic impedance of the reject load (typically at about 50Ω), and the VSWR may be used to indicate how well matched those impedances are. 
     It is therefore desired to improve the isolation from a digital FM transmitter into an analog FM transmitter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a spectral mask for an FM channel and spectral representation of an IBOC hybrid FM transmission. 
         FIG. 2  illustrates a typical high-level combined HD Radio system. 
         FIG. 3  illustrates a spectral plot against a spectral mask, the spectral plot showing intermodulation products resulting from poor isolation of digital and analog radio signal transmitters. 
         FIG. 4  illustrates one embodiment of a system utilizing an impedance-matching device to improve the isolation characteristics of a combiner. 
         FIG. 5  illustrates adjustment of the impedance matching device of the embodiment of  FIG. 4  by measuring the isolation between the digital RF input port and the analog RF input port of a combiner with a spectrum analyzer. 
     
    
    
     DETAILED DESCRIPTION 
     Isolation may be improved by adjusting the reject load impedance to a value that, through the combiner, “balances” the load impedance presented by the FM antenna. That is, the reject load may be adjusted so as to maximize the combiner isolation.  FIG. 4  illustrates one embodiment of an FM HD Radio transmission system configured for adjustment for maximum isolation. In the embodiment of  FIG. 4 , the system uses a typical 10 dB combiner  30  having a digital RF input port  33 , an analog RF input port  31 , a reject load output port  37  and an antenna output port  35 . A tube-type analog transmitter  32  is coupled to the analog RF input port  31  through a first directional coupler  40 , a solid-state digital transmitter  34  is coupled to the digital RF input port  33 , a reject load  39  is coupled to the reject load output port  37  through an impedance matching device  38 , and an antenna  36  is attached to the antenna output port  35 . A spectrum analyzer  42  may be coupled to the reflected port  41  of the first directional coupler  40 . An attenuator pad  42  may be used if further attenuation is needed for the spectrum analyzer  43 . Those skilled in the art will appreciate that the term “coupled” includes both direct and indirect connections. 
     The impedance matching device  38  may be any device, such as a double-stub tuner, used to field tune an FM antenna, i.e., reduce the VSWR of an FM antenna. The impedance matching device  38  may be finely adjustable. Such devices typically may have considerable range, and may be suitable for changing the impedance of a reject load  39  when placed between the reject load output port  37  of the combiner  30  and the input of the reject load  39 . 
     In the embodiment of  FIG. 4 , to determine the maximum feasible isolation of the combiner  30 , the amplitude of the energy from the digital transmitter  34  into the analog transmitter  32  through the combiner  30  may be monitored via the spectrum analyzer  43  while the impedance matching device  38  is adjusted to minimize the amount of digital energy entering the analog transmitter  32 . That method generally may compensate for any impact that the source impedances of the analog transmitter  32  or digital transmitters  34  have on the performance of the combiner  30 . Once the optimal impedance at the reject load output port  37  of the combiner  30  has been determined, the impedance matching device  38  could be replaced with an insulator such as a steatite “doughnut” or metal sleeve of appropriate size placed on the inner conductor of the reject transmission line  44 . In that way it may be possible to improve the isolation from the digital transmitter  34  in the analog transmitter  32  by approximately 10 dB. 
     Alternatively, as shown in  FIG. 5 , adjustment of the impedance matching device  38  of  FIG. 4  may be assisted by measuring the isolation between the digital RF input port  33  and the analog RF input port  31  of the combiner  30  with a spectrum analyzer  43  equipped with a tracking generator. As shown in  FIG. 5 , the RF input port  44  of the spectrum analyzer  43  may be coupled to the analog RF input port  31  of the combiner  30 , and the tracking generator output port  45  of the spectrum analyzer  43  may be coupled to the digital RF input port  33  of the combiner  30 . Adapters (not shown) may be used as required to couple the spectrum analyzer  43  to the analog RF input port  31  and digital RF port  33  of the combiner. 
     In another alternative, a network analyzer (not shown) may be used instead of the spectrum analyzer  43 . The network analyzer may be similarly coupled to the combiner  30 : the transmission/RF input port of the network analyzer may be coupled to the analog RF input port  31  of the combiner  30 , and the reflection/RF output port of the network analyzer may be coupled to the digital RF input port  33  of the combiner  30 . Adapters (not shown) may be used as required to couple the network analyzer to the analog RF input port  31  and digital RF port  33  of the combiner. Preferably, the network analyzer is set for s-parameter testing. 
     With reference again to  FIG. 4 , using the foregoing, the load impedance required to “balance” the combiner  30  to provide maximum isolation may not necessarily improve the impedance match at the reject load output port  37  of the combiner  30 . As a result, the VSWR seen by the digital transmitter  34  may also increase since the load impedance presented to the digital transmitter  34  is primarily determined by the reject load  39  impedance (again, in a high-level combined system using a 10 dB combiner, approximately 90% of the energy from the digital transmitter is dissipated in the reject load). Those skilled in the art will appreciate that care may be taken to insure that the VSWR at the digital transmitter input port  33  of the combiner  30  is low enough that it does not cause the digital transmitter  34  to “fold back.” 
     Those skilled in the art will also appreciate that the disclosed invention is not limited to use with “High-Level” Combined HD Radio systems. Similar isolation problems may also exist when using combiners with other coupling ratios, such as with the 3 dB, 4.33 dB and 6 dB combiners typically used in “Mid-Level” or “Split-Level” Combined FM HD Radio Systems. The methods described herein may also be used to achieve higher isolation between the digital and analog transmitters in those systems as well. Those skilled in the art will further appreciate that various terms used herein generally carry their ordinary and customary meaning, for example, as may be determined from the McGraw-Hill Dictionary of Scientific and Technical Terms, Sixth Edition.