LF: DLF spurii reduced

[MarkusVester@aol.com]

Hi group,

since the beginning of Dec. 2002, the daytime 153kHz BC emissions were again accompanied by the infamous spurious sidebands centered around 136kHz up to -57dBc, and a number of amateur stations in South Germany have reported increased noise levels in QSOs. After another telephone call, the system engineers in Donebach could apparently fix the spurious sidebands yesterday, and it has been relatively quiet since 13:27 UT. 

The problem seems to be a design flaw in the 250kW AEG PANTEL transmitters, as it is present in both of their two units. It is caused by an unidentified parasitic oscillation in the 54-kHz swich-mode modulator drive circuitry. It was found empirically that chilling a certain CD4011 IC largely suppressed the sidebands, and the ad-hoc solution is opening up the modulator compartment and placing the affected circuit board on an extender board. - On the other hand, this condition is not very well reproducible, and the problem tends to reappear especially at high room temperatures in summer.

73 de Markus, DF6NM