Ecran de visualisation et filtre audio avec AFCWhen using a normal SSB receiver at VHF/UHF frequencies the builtin noise blanker of the SSB radio usually works well. Problems with strong signals within the passband used by the noise blanker are as much of a problem as on more crowded bands.The noise blanker within linrad can be used to improve in case the analog blanker fails. To use the linrad blanker, the system has to be calibrated by use of a pulse generator. In normal operation with a good receiver having a reasonably flat frequency response calibration is not required As an illustration to the practical use of linrad for a single polarised antenna with a conventional SSB receiver fig. 1 shows the reception of the UNKN422.WAV file played to a pentium 60MHz computer with a SB16 (Vibra16C) audio board. The sampling speed is 5000Hz and the cpu usage is less than 15%. The parameters are set for high performance in copying really difficult signals. The time delay is 8.5 seconds from input to audio in the headphones, but nothing prevents the transmitter to start while the last part of the received signal is still being processed. The delay is caused by the narrow bandwidth used in the waterfall display and by the long averaging time used for the AFC. Further the 8.4 seconds delay contains an extra delay caused by the high resolution used in the baseband graph. For receiving there is no reason to use more than 0.5 seconds delay in the baseband filter and less is usually adequate. The baseband filter delays by 2.7 seconds with the settings shown here. |
The baseband graph of fig. 2 uses very high resolution to demonstrate
that the carrier of the signal passing the baseband filter is really
narrow which is proving that the AFC process has been sucessful in
reducing the bandwidth of the original signal for which the carrier swept
over a 15Hz range.
Setup parametersThe figures 2 to 5 below show the parameters that were used to produce fig.1. |
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