Practice shows there is only a small chance that frequency components of various sources overlap each other. The digital signal processing is based on this assumption (see also Tutorials FFT).
This assumption also constitutes the major limitation, however. With the combination of many overlapping frequency components, whereby the contribution of the interfering source in these components is stronger than that of the desired source, the results described below are not feasible. In these cases, we can always fall back to the nulling properties of the loop antennas and to analog or digital (DSP) noise cancelling. In the DSP, we can also increase the frequency resolution from 30 Hz to, for example, 7.5 Hz. This gives progressively better results, especially for CW. The extra delay in the processing, however, is very noticeable. A 30 Hz net frequency resolution seems a good compromise.
FT1000D hardware and Effective directivity by DSP
Despite the fact that the digital signal processing has not been carried out on the intermediate frequency level, it turns out to be very effective. The need for processing on the intermediate frequency level will only become noticeable if the interfering signal is much stronger than the desired signal. The desired signal will then be modulated too intensely by the AGC. It is than better to control the AGC manually.
This last limitation can be solved by using for instance the two phase coherent K2 receivers.
This is the way to go. We now have the antenna signals down converted while maintaining the amplitude and phase relationship. This makes much more processing possible in the DSP.
Last update: June 2, 2007