Variable Phase Interpolation
The receiver in a digital communication system usually knows the nominal symbol rate used by the transmitter. Since the receiver is at a distance from the transmitter, has slightly different components, and is at a different temperature, the locally generated symbol clock in the receiver will differ in phase and slightly in frequency from the transmitter's clock. Therefore, the receiver must synchronize its symbol clock to the clock in the signal received from the transmitter. It must do this just using information derived from the received signal. Some codecs designed for modem front ends have built-in hardware capability for changing their sampling phase by small increments as directed by commands from the DSP they are connected to. The clock tone generator discussed in Chapter 11 can be used to determine the needed phase increments. The codec for the TMS320C6713 DSK does not have this capability and runs with a fixed phase. In this chapter, we will see how to implement the phase shifting in the DSP by a variable phase interpolator. First, a continuously variable phase shifter will be presented. Then a phase shifter using fine quantized steps will be discussed.
KeywordsImpulse Response Variable Phase Hanning Window Tracking Loop Symbol Rate
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