Revised: February 5, 2020

A common situation is a flow graph that was developed in one version of GRC, say GRC 3.6, will show flow graph error and not load on GRC 3.7, or a flow graph developed on the latest GRC version will not load on an earlier version of GRC 3.7.  This situation can be expected from the GRC flow graphs presented as SDR Projects: SDR Projects and DSP Projects: DSP Projects The solution to this problem is often quite simple and will be described here.

The basis of the problem are changes in the algorithms of the DSP blocks.  In the case of a change in the version library DSP block, the DSP block or blocks that will not load cause the red ‘flow graph error’ indicator to illuminate and the blocks are displayed in the work space with a skeletal appearance, different in appearance from the other blocks.  All that is necessary to fix the problem is to replace that block, say a filter block, with an equivalent filter block from the library that you are using and enter the relevant filter parameters.  One other loading problem, relates to changes in the parameters unique to the USRP Source block (receiver block).  The USRP Sink block will appear normally in the work space, the red ‘flow graph error’ button will not be illuminated, but the flow graph will not load.  One solution is to substitute the USRP Source block with the equivalent block from the library.  Another approach is to edit the USRP Source parameters.  Do follow this approach, open the ‘Properties’ window of the USRP Source, open the ‘FE Corrections’ tab.and change the parameters to those depicted below:

USRP Source Parameter FE Corrections


GNU Radio is in a state of constant development, read “change”.  A massive change occurred at the time when GRC transitioned from version 3.6 to version 3.7.  There are still DSP algorithm revisions in the GRC 3.7 era.  Flow graphs developed under GRC 3.6 or early GRC 3.7 will not run under later versions of GRC 3.7.