Double bandwidth monitoring with RTL-SDR in GNU Radio

The maximum usable and stable bandwidth of an RTL-SDR is about 2.4 MHz. In order to get larger bandwidths it is possible to combine two or more dongles, although doing so comes with a big limitation – since the clocks and signal phases between separate dongles would not be synchronised, it would be impossible to decode a wideband signal this way. However, combining dongles for larger bandwidths is still useful for visualizing the spectrum through an FFT plot, or perhaps for decoding various separate narrowband signals. Although creating a wide band FFT plot with multiple dongles is fairly simple, we haven’t seen much software do this before.

To  get a 4.4 MHz FFT display two RTL-SDR dongles are used at their max. band width of 2.4 MHz, then resampled to 4.8 MHz, then the first signal shifted down by 1MHz, the other one 1 MHz up, added together, the combined signal divided by 2 and finally fed into a FFT plot. The script can be used to get a combined 4.4 MHz spectrum visualization without a center dip from roll off, or a 4.8 MHz spectrum with rolloff.

Notice, that this isn’t any useful to decode or demodulate any signals, because of phase and clock misalignment, but it can be very handy, if you want to view a larger spectrum than 2.4 MHz and don’t want to spend money in a more expensive SDR, that can be used at higher sampling rates.

HackRF band width in comparison at4.8 MHz:

4.8_hackrf-700x360

Mirror: 4.4 MHz Script for GNU Radio

Mirror: 4.8 MHz Script for GNU Radio