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Shared resources modulator-demodulator circuits for use with quadrature amplitude modulated signals
| Details |
Inventors: Koslov, Joshua Lawrence; Lane, Frank Anton; Scarpa, Carl G.;
Assignee: Hitachi America, Ltd. (Tarrytown, NY)
Primary Examiner: Chin; Wellington
Assistant Examiner: Tran; Congvan
Attorney, Agent or Firm: Michaelson & Wallace, Michaelson; Peter L.
A common transceiver circuit for use as either a modulator or demodulator and that is implemented through a shared resource approach. This approach is particularly, though not exclusively, suited for with quadrature amplitude modulated (QAM) or vestigial sideband (VSB) signals. Specifically, a QAM transceiver circuit (400), through strategically located multiplexing stages, physically re-uses both a complex Nyquist filter (310, 320) and an equalizer (140) for demodulation and modulation. Additionally, tap coefficients of the complex Nyquist filter are set such that a center frequency of an otherwise baseband Nyquist filter is translated upward to a symbol rate in order to eliminate a separate complex mixer (250, 260). Similarly, a VSB transceiver circuit (700), also through strategically located multiplexing stages, physically re-uses a complex vestigial Nyquist filter (610), a complex mixer (620) and an equalizer (785) during demodulation and modulation. The VSB transceiver also selects a particular configuration of a common complex Hilbert transform circuit (720) for use during either demodulation or modulation. In either transceiver, the same equalizer selectively provides both channel equalization, during de-modulation, and (sin x)/x compensation, during modulation, through use of differing corresponding sets of tap coefficients. |
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DETAILED DESCRIPTION We have advantageously overcome the deficiencies associated with use of separate modulator-demodulator circuits as conventionally taught in the art through our inventive transceiver circuits, one for QAM and another for VSB, that share, i.
e.
, physically re-use, various functional stages between both demodulation and modulation.
In that regard, through strategically located multiplexing stages, our inventive QAM transceiver circuit physically re-uses both a complex Nyquist filter and an equalizer for both demodulation and modulation.
Specifically, multiplexors, all controlled through a common mode signal, are positioned at a complex input to the complex Nyquist filter, at an input of the equalizer, and between an in-phase output of the equalizer and an in-phase output lead from the transceiver.
The mode signal is set to one of two states to cause the transceiver to de-modulate an input QAM signal applied as input to the transceiver and hence provide a stream of complex decisions on the transceiver output leads.
The mode signal is set to its other state in order to cause the transceiver to modulate a stream of complex symbols applied as input to the transceiver and hence provide a QAM modulated signal on one of the transceiver output leads.
Additionally, tap coefficients of the complex Nyquist filter are set such that a center frequency of an otherwise baseband Nyquist filter is translated upward to a symbol rate in order to eliminate a separate complex mixer from the transceiver that would otherwise exist in a conventional QAM modulator.
Similarly, also through strategically located multiplexing stages, our inventive VSB transceiver circuit physically shares a complex vestigial Nyquist filter, a complex mixer and an equalizer between demodulation and modulation.
Specifically, multiplexors, all controlled through the same mode signal, are positioned at a complex input to a complex Hilbert transform circuit, at one complex input to the complex mixer, at a complex input to the complex vestigial Nyquist filter, at a complex input to an equalizer, and between an output of the equalizer and an output lead of the transceiver
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Audio Signal Processing 
