Interpolation Warning with USRP N310: Spurs In The Signal

Issue Details

When transmitting a signal using a USRP N310, there are spurs in the transmitted signal's spectrum. During transmission, the following warning message appears in the terminal output: 

WARNING] [0/DUC#0] The requested interpolation is odd; the user should expect passband CIC rolloff.
Select an even interpolation to ensure that a halfband filter is enabled.

Solution

While spurs may appear for different reasons, the first step to eliminate them when the above warning is present is to choose an appropriate sample rate.

On the N310, the digital interpolation chain includes halfband filters that are only enabled when the interpolation factor (Master Clock Rate ÷ Sample Rate) is an even integer.

Supported master clock rates for N310 are 122.88 MHz, 125 MHz (default) and 153.6 MHz, so make sure to use a sample rate that has an even interpolation factor with the chosen Master Clock Rate.

The desired Master Clock Rate can be chosen by adding the "master_clock_rate=<desired MCR>" in the arguments that are passed to USRP.

Example: --args "addr=192.168.10.2,master_clock_rate=122.88e6"