# Bandwidth vs sample rate in SDR

In software defined radio (SDR) I often see sample rate and bandwidth interchanged or maybe I'm misunderstanding... If an sdr can sample a 2 MHz wide slice of rf spectrum does that imply it would be sampling at 2 million samples per second (2M S/s)? And if so what about bandwidth - is the implied bandwidth 2MHz? The particular sdr I am thinking of is the rtl2832u.

I'd like to understand if sample rate and bandwidth are tied together and if not how are they defined in terms of this or other sdr.

## 2 Answers

They are related, but not directly. The sample rate must be at least 2× the bandwidth in order to satisfy the Niquist criterion with respect to aliasing. However, since "real world" filters are not ideal brick-wall filters, you need to allow a higher margin on the sample rate — or accept a lower usable bandwidth.

Some texts might use "sample rate" as the definition of the individual sample rate of two separate sampling channels (often called I and Q), which could lead you to believe they're equivalent. But the combined sample rate of the two channels together is what makes the system meet the Nyquist criterion.

• -1 : IQ sampling does not require the sample rate to be 2X or greater than the bandwidth. That's because complex or quadrature samples can represent both positive and "negative" frequency spectra, unlike single channel ADCs. – hotpaw2 Feb 5 '17 at 6:28
• @hotpaw2 it's true that the sample rate isn't over 2x the bandwidth, but the samples per unit of time still must be - you just take two at a time half as frequently. That may ease implementation somewhat at the edge of what a given technology can handle, in the sense that may be easier than having two ADCs take turns on the same signal. – Chris Stratton Feb 5 '17 at 6:38
• Agreed, but the answer says "sample rate" (related to the sampling clock), not data rate (related to bits per second). – hotpaw2 Feb 5 '17 at 6:41

The rtl2832 does quadrature (IQ) sampling, thus the bandwidth is equal to the sample rate setting (as per its documented command parameters)

Quadrature sampling produces twice as much data (2 8-bit components) per IQ sample, which is partially a rational of why its bandwidth is twice that of a regular single channel Nyquist sampler using the same sample rate oscillator.