WiNRADiO G35DDC HF Receiver
20
The DDC may be a new concept for newcomers to software-defined radio, so
it perhaps deserves a little more explanation:
In conventional superheterodyne receivers, the incoming signal is first
converted to a fixed
intermediate frequency
(
IF
) by mixing it with a variable
frequency signal produced by an internal local oscillator, and then further
processed by amplification, filtration and demodulation.
In the particular class of software-defined receivers that the WiNRADiO
Excalibur Ultra
represents, no such local oscillator and mixer exist. The entire
shortwave spectrum is digitized as a whole, and out of the digitized spectrum
data, a smaller chunk is selected by a process called
decimation
and down-
converted digitally, using a hardware component called a
Field-
Programmable Gate Array
(
FPGA
).
The particular FPGA used in the
Excalibur
Ultra
contains 10,320 logic elements, 46 memory blocks, 423,936 bits of RAM, 23
multipliers, 2 phase-lock loops and 192 programmable inputs or outputs.
The DDC is in fact a functional equivalent of the mixer, local oscillator and filter
- but rather than outputting an analog intermediate frequency signal, its output
is in a digital form. This digitally down-converted signal then arrives in the
computer via the PCI Express interface where it is filtered and demodulated
entirely in software, using purely computational digital signal processing
methods – there are no coils or crystals in the filters, and no diodes or
capacitors in the demodulator.
In the WiNRADiO
Excalibur Ultra
receiver, the chunk of the input spectra that
is processed by the DDC can be recorded and played back by the PC, making
it possible to “re-receive” signals located anywhere within that chunk. The
maximum width of the DDC chunk (called the
DDC bandwidth
) is 32 MHz.
The larger the DDC bandwidth, the more CPU processing power is required by
the computer. On slower computers, using the maximum DDC bandwidth may
not be possible as the computer may then become sluggish and the
application may “freeze”.
The chunk of the input signal spectrum seen and processed by the DDC is
shown at top-left; this is called the
DDC spectrum
. Its bandwidth is
determined by the
DDC BW
control at the top of the DDC spectrum.
Summary of Contents for WR-G35DDC EXCALIBUR ULTRA
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