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Leif Asbrink leif at sm5bsz.comHi Stefan, > Rtl-sdrs are quite cheap, but for now a user has no benefit of having > multiple sdrs in its system. Hmmm, with several rtlsdr units one can monitor several different frequency bands. I am sure there are users who find that beneficial:-) > That is why I'm searching a way to correlate the signals of the sdrs > without hardware modification. I think everyone of you has seen the noise from > the power source. Has anyone tried to build a filter to use that noise > to calculate the delay between multiple dongles? Surely it is possible to synchronize two or more dongles in software. They just have to receive the same signals to a sufficient amount. One would have to apply a fractional resampler to all but one rtlsdr to compensate for different sampling clocks. Those resamplers have to be adaptive to accomodate different frequency drifts. This means one has to find the time shift for best correlation, and establish how that time drifts to update the resampling ratio for the time shift to stay constant. There will be many interesting complications if one just uses the normal antenna signals. It would however be fairly easy to inject pulses at perhaps 30 Hz and use them for the synchronization. Those pulses would be known accurately so they can be subtracted from the data stream to not have any adverse effect on the desired signals. > I mean, they get the same noise. I don't know if it is sufficient, > but it is also a good spot, to put some artificial noise in, as it can be easily > accessed and does not interfere with rf circuitry when it is switched off. Synchronization can not be switched off, stability is not good enough for that - but the artificial noise is known so we can subtract it:-) > Is that an useful approach? > Do you think this could work? Did I miss something? Surely it can work. I do not think it will be trivial to get it running however, but you should be able to make a multi-channel receiver that could combine phase-coherent signals from several antennas for drastically improved S/N. 73 Leif > > > regards, > > Steve >