SM 5 BSZ - Interference on 137kHz.
(July 27 2003)
The data file used for all images on this pags

On low bands the noise floor is not constant in time like the galactic or thermal noise on VHF and UHF frequencies. There are many different kinds of impulse noise, QRN. Figure 1 shows a typical section of the recording when processed with a small FFT size without the wideband noise blanker. Fig. 2 shows the same sequence processed with the noise blanker enabled. The FFT size for these images was 2048 and since a sine squared window was used, the waterfall lines arrive at a rate of 93.75Hz.

Fig 1. Waterfall diagram at a line frequency of 93.75 Hz without noise blanker.

Fig 2. Waterfall diagram at a line frequency of 93.75 Hz with the noise blanker enabled. Compare to fig. 1.

A comparison of figs 1 and 2 shows that the pulses that repeat at about 1 Hz are removed very efficiently. These very strong pulses come from a local source, probably a farmers electrical fence.

It is also clear from these figures that there are noise bursts of long duration for which the wideband blanker is not the right tool.

Besides QRN, the recording also contains interference from the strong AM broadcast signal at 82 kHz on the frequency scale corresponding to 132.000 kHz at the antenna. Fig. 3 shows clearly how the modulation from this station is present as a double sideband signal at 48kHz. The mechanism is AM detection due to inadequate dynamic range of the hardware used. The DC voltage corresponding to the carrier itself can not reach the A/D converter because it is connected through capacitors. There is another strong broadcast station at 73 kHz on the scale, 123 kHz at the antenna. The difference frequency, 9kHz shows up at 57kHz on the scale. One can see how the 57kHz carrier goes up and down following the QSB of the two AM stations generating it.

Fig 3. The direct conversion hardware is not really adequate. This screen dump shows some of the intermodulation problems. See text.

The modulated signal at 58.600 is surrounded by the sidebands of the 132 kHz broadcast station. The sidebands are broadened by the modulation of the carrier so the whole feature is some sort of intermodulation product. The difference frequency is 23.4 kHz, I have no idea whether some computer screen or something else produced such a frequency during the recording. It could also be a third order intermodulation product from a signal at 155.4 kHz. The lower sideband of this intermodulation spur is the dominating interference on the 137 kHz QRSS signals.

Fig. 4 shows more clearly what the interference situation is at the 137 kHz anateur band. The broadcast station, sending music, happens to put out a few fairly pure tones now and then. That makes it fairly easy to see where the interference belongs.

The hardware I have used is inadequate for an evaluation of the true noise floor at 137 kHz. The graphs on this page demonstrate the difficulties one may encounter when using wideband direct conversion receivers.

The recording contains several signals that can be copied despite the high interference level and should be a good enough test signal for the inclusion of a qrss mode in Linrad.

Fig 4. Here some of the intermodulation is better visible. The amateur signals are at 57.8 kHz on the scale.