Linrad for single polarisation system using a conventional SSB receiver.
(Aug 31 2001)
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Spectrum display and audio filter with AFC

When 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 not 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) soundcard. The sampling speed is 5000Hz and the cpu usage is less than 15%. The UNKN422.WAV file was played from another computer running a standard Windows application.

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.

Fig. 1. A CW signal in SSB bandwidth received by Linrad on an uncalibrated system with a Pentium 60MHz. Note that the time delay (8.41 seconds) reflects the special choice of the baseband filter settings (green dB scale). Use the arrowed boxes at the upper left and right corner to place only about 10 points across the desired filter bandwidth and at the same time reduce the delay by more than 2 seconds.
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 parameters

The figures 2 to 5 below show the parameters that were used to produce fig.1.

Fig. 2. Parameters for the first fft. Note that the low value of First FFT amplitude is because a defective SB16 board was used. The line input was not working so the microphone input was used. Loosing 50dB of dynamic range makes no difference on a signal like this one because there is no narrowband signal that is comparable to the noise floor power.

Fig. 3. The 4096 point fft with about 1Hz bandwidth uses only about 5% of tha available processing power. The remaining processing is not as well optimised for speed. In typical Linrad applications with more bandwidth these parts contribute with a negliable processor load. It is only because of the small initial bandwidth that the main fft is not the dominating processor load in fig. 1.

Fig. 4. AFC is enabled.

Fig. 5. The frequency used to convert from about 570Hz to the baseband is based on the average spectrum over a 10 second interval. The averaging goes 6.5 seconds back in time and 3.5 seconds forward causing an extra delay of 3.5 seconds that is part of the 8.41 seconds delay shown in figure 1.

Fig. 6. The baseband sampling speed is 2500Hz/32 = 78Hz.
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