A report describes a non-standard direct digital synthesizer (DDS) implementation that can be used as part of a coherent transponder so as to allow any rational turnaround ratio to be exactly achieved and maintained while the received frequency varies. (A coherent transponder is a receiver-transmitter in which the transmitted carrier is locked to a pre-determined multiple of the received carrier’s frequency and phase. That multiple is called the turnaround ratio.)
The report also describes a general
model for coherent transponders that
are partly digital. A partially digital
transponder is one in which analog signal
processing is used to convert the signals
between high frequencies at which
they are radiated and relatively low frequencies
at which they are converted to
or from digital form, with most of the
complex processing performed digitally.
There is a variety of possible architectures
for such a transponder, and different
ones can be selected by choosing different
parameter values in the general
Such a transponder uses a DDS to create a low-frequency quasi-sinusoidal signal that tracks the received carrier’s phase, and another DDS to generate an IF or near-baseband version of the transmitted carrier. With conventional DDS implementations, a given turnaround ratio can be achieved only approximately, and the error varies slightly as the received frequency changes. The nonconventional implementation employed here allows any rational turnaround ratio to be exactly maintained.
This work was done by Larry R. D’Addario of Caltech for NASA’s Jet Propulsion Laboratory. NPO-47460
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