What is the Difference Between Sample Rate and I/Q Rate?

Issue Details

How can I adjust the I/Q rate if RF devices have a fixed sample rate?  Why is the sample rate fixed on the USRP?

Solution

RF devices often have a fixed ADC and DAC sample rate.  The sample rate is fixed on the USRP to avoid aliasing. This limits the IQ rate as well, or else you may see additional aliasing effects.  Each Rx hardware path has two converters (ADC) so it can sample I and Q data.

IQ rate also affects bandwidth of a device.  Due to filter roll off, it is good practice to set your IQ rate and bandwidth to not exceed the following limitation:
 
Filter roll off will causes data on the ends of the specified bandwidth to appear distorted.  Because of this, following the above equation is good programming practice.

For additional information on aliasing, bandwidth, and sampling, refer to the following white paper:
Acquiring an Analog Signal: Bandwidth, Nyquist Sampling Theorem, and Aliasing

It is also worthwhile to point out how setting your IQ rate can affect your program.  Data is saved onboard the USRP, and then a fetch is used to send this data from the USRP to the host.  If data is not managed appropriately, you can run into an underflow or overflow error.  An underflow error occurs if there is not data available on the USRP memory to send to the host, but the host tries to fetch data.  An overflow error occurs if data is not sent to the host quickly enough to keep up with data being stored on USRP memory.  This causes memory to be overwritten.