| 摘要 |
Methods and apparatus for converting a relatively low frequency signal, e.g., a 1.5 MHz signal, to a high frequency signal, e.g., a 30-100 MHz signal, in the digital domain without the need for a digital mixer operating at the high frequency are described. The high frequency represents, e.g., the ultimate digital to analog conversion frequency. In accordance with the present invention an interpolation technique is used to convert the low rate digital signal to a high rate signal and to shift the carrier to a desired frequency. This is accomplished, by first positioning the information signal, e.g., the digital waveform to be modulated on a carrier at a relatively low rate using a digital mixer operating at a fraction of the ultimate digital to analog conversion frequency. The relatively low rate signal generated by the mixing operation is then converted to a high rate signal by one or more interpolator stages. An adjustable passband filter circuit is included in each interpolation stage. In one embodiment, the adjustable passband filter circuits are implemented using a fixed frequency filter preceded and followed by frequency shifting circuits which are used to upshift and then downshift the signal being filtered. By controlling the frequency shifting adjustable filter circuits are achieved via the use of fixed filters. One feature of the present invention is directed to a control circuit which is response to an H bit frequency control word representing a desired output carrier frequency.
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