Hier Blocks and Parameters

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This tutorial describes how to create a hierarchical block, or Hier block, in GRC. The previous tutorial, Creating Your First Block, demonstrates how to create your first GNU Radio block using the Embedded Python Block.

Creating the Initial Flowgraph

A hier block is used to encapsulate and simplify multiple GNU Radio blocks into a single block. The example hier block will be a frequency shifter block which multiplies a Signal Source against an input signal.

The first step is creating the flowgraph. Drag and drop the following blocks into the workspace:

  1. Signal Source
  2. Multiply
  3. Noise Source
  4. Low Pass Filter
  5. Throttle
  6. QT GUI Frequency Sink
  7. QT GUI Range

Connect the blocks:

StartingFlowgraphHierBlock.png

Update the QT GUI Range properties:

  • Id: frequency
  • Default Value: 0
  • Start: -samp_rate/2
  • Stop: samp_rate/2

Update the Low Pass Filter properties:

  • Cutoff Freq (Hz): samp_rate/4
  • Transition Width (Hz): samp_rate/8

Create The Hier Block

Click and drag in the workspace window to select the Signal Source and Multiply blocks, as well as the connection between them:

ClickAndDragSelect.png


Right-click on the highlighted blocks and select More > Create Hier:

ClickCreateHier.png


A flowgraph will be created in a new GRC tab:

NewHierBlock.png


Double-click the Options block and edit the properties. Update the properties:

  • Id: FrequencyShifter
  • Title: Frequency Shifter Block
  • Generate Options: Hier Block

OptionsSelectHierBlock.png

The remaining properties will then change, showing the Category:

ShowGRCHierBlocksCategory.png

The Category is where the block can be found in the block library on the right hand of GRC. The hier block will be located under GRC Hier Blocks, instead of Core where the rest of the GNU Radio blocks are located.

Save the flowgraph.

Variables vs Parameters

A variable is different than a parameter in GNU Radio. A parameter creates an interface for the hier block to accept a value from an external source, where as a variable can only be accessed from within a hier block.

For example, the samp_rate variable can only be accessed from within the hier block:

HierBlockWithVariable.png


The samp_rate needs to be a parameter that way it can be passed in from another block in the larger flowgraph. Delete the samp_rate variable and add a Parameter block into the GRC workspace:

AddParameterToHierBlock.png


Edit the Parameter properties:

  • Id: samp_rate
  • Label: Sample Rate
  • Type: Float

EditParameterProerties.png


Add a second Parameter:

  • Id: frequency
  • Label: Frequency
  • Type: float

EditFrequencyParameterProperties.png


Add the frequency parameter to the Signal Source Frequency property. The flowgraph should now look like:

HierBlockSampRateFrequencyParameters.png


Input and Output Ports

A pad is used to specify input and output ports on a hier block. Add a Pad Source and Pad Sink to the flowgraph to act as the in and out ports:

AddPadSourcePadSink.png


Generate the Hier Block Code

Click Generate the flow graph to create the hier block source code:

ClickGenerateFlowgraph.png


A Python .py file and YAML .yml file will be created in your home directory:

/home/$USER/.grc_gnuradio/

HierBlockPyYml.png


GRC needs to update it's internal memory of the blocks it has access to before the Frequency Shifter block can be used in a flowgraph. Click the Reload Blocks button:

ClickReloadBlocks.png


You'll notice that there is a new category GRC Hier Blocks in the block library below Core, and the Frequency Shifter Block can be used in flowgraphs:

GRCHierBlocksUpdated.png

CoreCategoryOnly.png

Using the Hier Block

The hier block can now be used in a flowgraph. Return the starting flowgraph and delete the Signal Source and Multiply blocks:

DeleteSignalSourceMultiplyBlock.png


Add the Frequency Shifter Block to the workspace and connect it to the rest of the flowgraph:

ConnectFrequencyShifterBlock.png


Edit the Frequency Shifter Block properties by adding the samp_rate and frequency variables:

EditFrequencyShifterProperties.png


Running the flowgraph will bring up the QT GUI Frequency Sink window with the QT QUI Range slider:

HierBlockFreqSink.png


Dragging the frequency slider will pass the value through the Frequency Shifter Block parameter causing the signal's center frequency to be modified:

HierBlockFrequencyShift.png

Deleting a Hier Block

A hier block can be cleared from GRC memory by removing the files from /home/$USER/.grc_gnuradio. In a terminal, move to the .grc_gnuradio directory:

cd /home/$USER/.grc_gnuradio

Then remove the files. Warning! The rm cannot be undone!

rm FrequencyShifter.py FrequencyShifter.py.block.yml

DeleteHierBlockCommand.png


Click the Reload Blocks button to update GRC's memory of blocks, clearing it of the Frequency Shifter Block:

ClickReloadBlocks.png


You'll notice that the GRC Hier Blocks category is gone and only the Core blocks remain:

CoreCategoryOnly.png