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I have a rtlsdr dongle which apparently has a max sample rate of about 2.4 MHz. I'm wondering how it's possible to capture signals at say 900 MHz with this device. Doesn't the sample rate need to be 2x900 or 1.8 GHz to capture without aliasing?

Sorry, I'm so new to this stuff!

Thanks!

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The RTL-SDR, as most software-defined radios, has an analog front-end that includes a tunable mixer. This mixer can down-convert the frequency band you're interested in to baseband, where it is sampled.

For example, let's say that you want to receive a signal centered around 900 MHz and with bandwidth $2B$ -- that is, the band covers the range from $900-B$ to $900+B$ MHz. You tune the mixer to 900 MHz and (after filtering) you end up with a signal centered around DC, covering the band from $-B$ to $B$ MHz. In general, this is a quadrature signal, that can be sampled at rate $2B$ complex samples per second.

As you see, with this setup, the sampling rate of the device only limits the bandwidth it can capture, without regard for the value of the carrier frequency (within the limits of the filters and mixers, of course).

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