RTL-SDR performs a two-step downconversion: - At the hardware level, it downconverts BW of [(center_freq - 1/2*sample_freq), (center_freq + 1/2*sample_freq)] to [(1.5 MHz- 1/2*sample_freq), (1.5 MHz+ 1/2*sample_freq)]. This 1.5MHz is determined. This means that the signal you tapped out from the yellow wire is centered at 1.5 MHz. The message signal will be centered at 1.5 MHz, 1st null at 1.5 MHz + fysm, and the noise signal will be centered at 1.5 MHz + fint.
- At the software level, the signal coming out from the RTL-SDR Source block is downconverted to 0 Hz centered.
- Keep in mind the magic number 1.5 MHz when you are designing the cutoff.
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