Figure 1. Directional filter
Increasing the IBOC digital sidebands from -20dBc to -10dBc presents more challenges for broadcasters wanting to transmit from one antenna. Currently, at -20dBc, transmitting from one antenna requires digital and analog signals to be combined one of several ways: low-level or common amplification, high-level (couple digital power to transmission line at 10dB), or mid-level, which uses a combination of common amplification and hybrid coupling to minimize FM and digital losses. Common amplification is the most efficient and cost effective way to combine FM and digital sidebands; however, with increased digital sidebands it becomes difficult to use common amplification in a linear mode at powers above 20kW due to peak voltages. Operating the common amp transmitter in a linear mode is required to minimize intermodulation products generated by digital carrier amplification. High-level combining at -10dB is not practical from a cost standpoint due to the increase in size of digital transmitters and the excessive waste.
At the 2009 NAB Show, Myat introduced a patent-pending technique for combining FM and digital sidebands for transmission to a single antenna. The system uses filters to combine the digital sidebands operating in MP1 or MP3 more efficiently than high- and mid-level techniques, and, is ideal for the proposed 10dB digital power increase.
The low-loss IBOC combining system utilizes the typical directional filter circuit seen in Figure 1 and consists of two quadrature hybrids, two identically tuned filters and a reject load. A signal at the narrow band input port is split in quadrature (90-degree phase difference) and passes through the bandpass filters. There it is recombined at the output hybrid and delivered to the output terminal due to the phase relationship of signal at its input terminals. Signals at the broadband input will again be split in quadrature, reflect off the bandpass filter and be delivered back to the hybrid where they are recombined to the output terminal. The transmission response from each input to the output is shown in Figure 1 and color coded. Note the transmission response for each input is similar to the S-parameters of the bandpass filters (i.e. the narrow input is the filter's transmission response, S21, while the broadband input is the filter's reflected response, S11 or S22) and makes analysis of the directional filter simple.
There is nothing new about the directional filter layout in Figure 1 other than the type of filtering used to perform the combining function of analog and digital signals for IBOC. Directional filters have been used for years to combine multiple FM stations to one antenna while providing adequate isolation between transmitters. Isolation between inputs is provided by the system directional properties and filter function. Properly functioning hybrids and identical filter S-parameters are key to achieving a directional system — in particular the isolation from narrow input to the broadband input (typical isolation -40dB). The isolation from the broadband input to narrow band input gets additional help from the filter's rejection, which for classic FM channel combiners is typically around -30dB. This system does not require lossy unreliable circulators for isolation.
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