RADIXON WR-G35DDC EXCALIBUR ULTRA, Руководство пользователя

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WiNRADiO G35DDC User’s Manual
109
The preceding spectrum display is that of the WiNRADiO G313 receiver, a
typical representative of this type of SDR receivers, belonging to a class of
receivers also referred to as
SDR Tier Two or Software-Reconfigurable
Radio . In SDR Tier Two radios, software is used to control and redefine a
variety of modulation techniques, spectrum bandwidth, and other essential
parameters and functionality of a radio receiver.
DDC-Based Receivers – The Excalibur Ultra
The arrival of the digital down-convertor (DDC) has moved the hardware-
software dividing line of
SDR Tier Two category receivers closer to the
antenna, and introduced additional benefits.
Advances in digital electronics, especially the introduction of fast ADCs and
field-programmable gate arrays (FPGA) made it possible to eliminate
hardware-based heterodyne processes and introduce the concept of digital
down-conversion, where the local oscillator and a mixer can be replaced with a
single software-reconfigurable hardware component.
Modern ADCs can achieve a sampling rate of 100 MHz and beyond. The
Nyquist theorem says that the maximum frequency that can be sampled
needs to be smaller than half of the sampling rate (see
www.wikipedia.org/wiki/Nyquist_frequency ). As the top end of the HF
spectrum is nominally only 30 MHz, we have the potential to sample and
process the entire HF spectrum at once.
However, one ADC does not yet make an SDR, and more software processes
are involved. Because of the very high amount of data in a real time bandwidth
of 32 MHz, it is necessary to reduce this to a more manageable bandwidth for
further processing by the computer. The most common process for such a
reduction is digital down-conversion (DDC) which typically relies on a special
hardware component known as a field programmable gate array (FPGA).