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I'm using a SDR (specifically the RTL-SDR) to analyse the spectrum and i noticed a phenomena that does not seem logical.

The signal I want to capture is centred on 868.3 MHz and the central frequency of the SDR is centred on 869.3 MHz with a bandwidth of 2 MSamples. What I observed is that the 2 halves of the signal are displayed on both sides of the receiving window instead of just the half between 868.3MHz and 868.3MHz - 0.125MHZ ( as my signal bandwidth equals 125 KHz ) .

What do we call this phenomena and what is the explanation behind it ?

The first image is when the central frequency of the SDR is 868.3MHz Frequency display of the signal shown witout alteration

The second image is when the central frequency ofthe SDR is 869.3MHz Frequency display of the signal divided in two halves

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That's aliasing in its most classic form.

The anti-alias filter in your receiver isn't perfect, so that frequencies slightly outside of what you want to sample appear on the other end.

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