WiNRADiO G33DDC User’s Manual
111
The process of reducing real time data and thus the usable bandwidth is
known as
decimation
, and in the
Excalibur Pro
this first decimation results in
a DDC bandwidth of 4 MHz. This 4 MHz bandwidth effectively allows the user
to place a 4 MHz wide window anywhere within the ADC bandwidth (up to
almost 50 MHz in the
Excalibur Pro
).
It is interesting (but not recommended to the faint-hearted) to note the
decimation description at:
www.wikipedia.org/wiki/Decimation_(Roman_army)
In fact, it is easy to imagine this decimation process to be similar to the mixing
and IF stages of a conventional superheterodyne receiver, with each filtering
stage progressively refining the output of the process.
The
Excalibur Pro
has two stages of decimation: firstly, from the HF spectrum
to the DDC spectrum, and secondly, from the DDC to the demodulator. The
first decimation is conducted within the FPGA, physically located in the
receiver module, and the second decimation takes place inside the PC in the
demodulator software.
One of the great practical limitations of SDR receivers is the ability to send the
data produced by the device into the computer. There are a number of
transport mechanisms, such as Firewire, Ethernet and USB. However, none
are perfect, and as a compromise between user convenience and flexibility,
USB 2.0 remains the favored interface of choice. USB 2.0 has a design limit of
480 Mbit/s, although this is reduced to a practical limit by protocol overhead
and other housekeeping to approximately 350 Mbit/s.
The output of the DDC is sent to the PC via USB 2.0, and in order to be able to
accept these large amounts of data, it is essential that the PC’s USB hardware
and software system is operating to its full potential.
It is important to note that the practical limit of 350 Mbit/s applies to a controller
that may drive more than one USB port. All devices connected to it may share
the bandwidth and none may achieve the theoretical 350 Mbit/s.
As the whole DDC spectrum is sent to the PC for processing, it is necessary to
use a ‘modern’ PC, typically with at least a dual core CPU. With slower CPUs,
it may not be possible to achieve the entire potential of the
Excalibur Pro
, in
particular the ability of processing the entire 4 MHz wide DDC bandwidth.
