Having a conceptional issue.

If a SDR dongle has a 2MHz signal bandwidth, but that can be anywhere within the range of 50-2000 MHz, why is the bandwidth only 2 MHz.

The clock must be double the highest frequency, whether this is 2000 MHz or a lower baseband signal after analog down conversion of that 2000 MHz. So why can't the bandwidth be higher than 2MHz.

Someone said the bandwidth 2Mhz is related to the clock, but I am thinking it's likely more related to the tunable filter size, and that is 2MHz because wider than 2MHz maybe too difficult on those dongle chip boards?

Thanks for any help or explanation you can give


SDR is not applying a lowpass (baseband) sampling on its RF input, instead it effectively employs a band-pass signal sampling.

According to Shannon-Nyquist baseband sampling criteria, the bandwidth of the signal to be sampled should be less than half the sampling frequency. That's what you are talking about.

However for narrowband modulated signals bandbass sampling is employed for reducing the sampling frequency down to channel bandwidth (utilzing I/Q double sampling).

This is typical case in communication systems where a number of narrowband channels are stacked into a total range of frequencies, and only one of them is to be demodulated.

For a more practical answer, the RF input could already be downmixed into intermediate frequencies before actually being sampled using bandpass sampling.


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