WiNRADiO G33DDC HF Receiver
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Once the DDC data is efficiently transferred to the PC, it is then possible to
process the data by filtering and demodulation. Using the
Excalibur Pro
software running on a modern PC, it is possible to simultaneously demodulate
three signals, providing three
virtual receivers
. Each virtual receiver can be
placed at any frequency in the DDC spectrum, and each controlled, in respect
of mode and bandwidth setting, filtering, recording, etc., completely
independently.
The
Excalibur Pro
is provided with comprehensive recording facilities. In
addition to audio, the entire DDC spectrum can be recorded, up to the full 4
MHz bandwidth. However, this does have a high demand on the PC hardware.
For example, a 4 MHz wide DDC stream, with a low resolution, needs to
transfer data to the hard disk at approximately one GB per minute. Clearly, in
order to maintain this transfer rate, not only must the hard disk be relatively
large, but also the data transport inside the PC must operate at its optimum.
For example, with desktop PCs it is relatively easy to fit internal SATA disk
drives. However, with external (USB interfaced) disk drives, it is important to
remember that:
If the external hard drive is USB-interfaced and connected via a USB hub
shared with the Excalibur Pro, the total USB data transfer speed has to
be shared, thus reducing the performance of both devices.
So far we have discussed the clever digital ‘bits’ of SDRs, but we should not
ignore the RF issues associated with direct sampling, such as the effect of the
ADC performance.
Firstly, the dynamic range: With the
Excalibur Pro
, any signals ranging from
(almost) DC to (almost) 50 MHz will be digitally sampled and processed.
However, in order to provide sufficient sensitivity, it is practically necessary to
insert a degree of amplification before the ADC. This amplification can have a
detrimental effect if not correctly managed.
Traditional superheterodyne receivers often include many tuned circuits
designed to reduce various spurious ghost signals (birdies), often generated
by the RF and IF mixing processes involved. While with a direct-sampling
receiver such as the
Excalibur Pro
these birdies and ghosts are entirely
eliminated, another factor comes into play:
The ADC has a maximum input level, beyond which the sampler becomes
unable to operate correctly. With the
Excalibur Pro
, this is approximately 0
dBm. If the ADC is presented with a signal greater than this, the ADC is
