SM 5 BSZ - Processing SSB bandwidth with a Pentium 60MHz.
(Mar 21 2001)

Processing with conventional parameters

The Linux pc radio package can be used as an add on to a conventional radio for conventional filtering. The graphs of this were recorded with a uncalibrated system. Calibration improves as shown elsewhere but this page is intended to show what one gets directly.

Fig.1. shows the screen with a small size for the baseband fft. Here the second fft is disabled. The total processing delay is about 0.5 seconds which is what one needs to make the steep skirts of the filter shown in the baseband graph. With the system set up like this one can use filter bandwidths down to about 15Hz. The processing uses 13% of the total available time for the Pentium 60MHz.

Fig 1.
Near real time processing. A small baseband graph allows a small N (power of two) The number in the upper right corner, 7, indicates a fft size of 128. The two peaks in the baseband are two CW stations on 7MHz. They are separated by about 100Hz and it is easy to filter out one or the other.

In fig.2 the baseband graph is increased and it is easily seen that the CW signal is about 50Hz wide while the spurs (overtones of 50 Hz) are very sharp. The reason for the relatively wide CW signal is the extremely poor frequency stability of the TS520. This radio can not be used to demonstrate finding of weak signals, keying sidebands etc.

Fig 2.
Higher resolution and longer processing delay compared to figure 1. The processing does not use as much as 41.5% of the available time, the timing information in the left lower corner is updated very often, this uses a substantial fraction of the time.

The Pentium 60MHz is fast enough to run high performance processing for SSB bandwidth. With second fft enabled, noise blanker included and a resolution of about 0.5Hz only about one third of the processing power is used.

Fig 3.
Full processing with second fft and noise blanker. Note that the spurs on the left side of the waterfall graph show how the system actually works. The poor stability of the TS520 makes the shortwave signals far too wide and smears out their keying sidebands.