IBOC Combining Using Allpass Filter Phase Shift
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Solution Overview
Problem
Combining digital and analog signals in-band on-channel (IBOC) with high efficiency, flat group delay, low input VSWR, and high transmitter to transmitter isolation is challenging, especially for frequencies close together, as existing methods like 10 dB hybrid couplers or bandpass filters are inefficient and produce significant bandwidth separation.
Innovation Solution
The use of allpass filters with resonant cavity modules tuned to the digital sidebands and phase shifting technology, where two allpass filter modules are used to provide the same group delay and notch depth at different frequencies, connected in a constant impedance circuit with 90-degree hybrids to split and recombine signals effectively, reducing insertion loss and maintaining high power efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If 10 dB hybrid coupler or bandpass filter is used to combine digital and analog signals, then combining is achieved, but efficiency is low and bandwidth separation is significant
Solution Approach 1:
The patent transforms the traditional bandpass filter approach into an allpass filter approach by changing the filter type parameter. The allpass filter is tuned to the digital sideband frequency rather than passing a bandwidth, fundamentally changing how the combining is achieved. This parameter change enables high efficiency combining with minimal bandwidth separation.
Solution Approach 2:
The patent replaces the mechanical bandpass filtering system with an allpass filter system that uses phase shifting and resonant cavity tuning. This substitution eliminates the need for broad bandwidth filters and achieves combining through phase manipulation at specific frequencies, improving efficiency while reducing bandwidth separation.
2Reliability
If resonant cavities are tuned to analog carrier frequency, then group delay compensation is achieved, but digital sideband phase shifting is insufficient
Solution Approach 1:
Instead of tuning the resonant cavities to the analog carrier frequency as in traditional group delay compensation, the patent inverts the approach by tuning the cavities to the digital sideband frequency. This inversion ensures that the phase shifting is most accurate where needed (at the sidebands) rather than at the carrier frequency.
Solution Approach 2:
The patent applies local quality by making the resonant cavities specifically tuned to provide precise phase shifting at the digital sideband frequencies rather than providing uniform compensation across the entire bandwidth. Each allpass filter module is optimized for its specific sideband frequency, achieving high precision where required.
3Manufacturing precision
If two allpass filter modules are used tuned to different sideband frequencies, then phase shifting of both sidebands is achieved, but insertion loss notch is created at resonant frequencies
Solution Approach 1:
The patent accepts partial action by allowing the allpass filter modules to operate at −10 dB relative to the analog carrier level. The modules provide sufficient phase shifting precision at the sidebands while the insertion loss notch is managed by operating at appropriate power levels. The digital transmitter operates at this reduced level specifically to accommodate the filter characteristics.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach achieves approximately 10 dB reduction in out-of-band spurious emissions, improves mask compliance, and maintains high efficiency by minimizing heat-related losses, while ensuring excellent input VSWR and group delay performance.
Implementation Method 1
The heart of this approach lies with use of resonant filter cavities. Allpass filters provide a phase shift of 180 degrees at resonant frequency and a tapering phase shift on the adjacent frequencies.
Implementation Method 2
The 90 degree hybrid coupler is capable of maintaining excellent input VSWR while passing a high percentage of the power to the output port when the cavities in each allpass filter module are properly matched to one another.
Data Source
AI summary
A phase shift is defined as a point in frequency at which the phase is changed from 0 degrees to 180 degrees. A device is provided for combining analog and digital in-band-on-channel (IBOC) signals to feed a common antenna utilizing phase shifting allpass filter modules to provide a 180 degree phase shift to specific IBOC channels within a constant impedance dual-hybrid circuit. The IBOC Allpass combiner includes one input 90 degree 3 dB quadrature hybrid coupler, one output 90 degree 3 dB quadrature hybrid coupler, a load resistor, and two phase shifting allpass filter modules. Each phase shifting allpass filter module is comprised of a two coaxial cavity resonators coupled to a 90 degree 3 dB quadrature hybrid coupler. Components and modules are coupled using mating transmission lines. The four coaxial cavity resonators are used as devices to produce two distinct phase shifts at isolated upper and lower IBOC side band frequencies. The circuit is designed for one center analog frequency and two sideband IBOC OFDM carrier frequencies, such that all frequencies will combine in phase at the common antenna input with minimal loss and minimal group delay. Out of phase and spurious emissions are ported to the load resistor.