GRC Examples

Below you will find a few software defined radio examples implemented in the GNU Radio Companion. They are very simple yet functional examples I created while I was learning GNU Radio and SDR – I hope they can be useful to others too, who are learning GNU Radio and/or software defined radios.

These GRC scripts are not compatible with current GNU Radio. If you have GNU Radio 3.7 or later, use the script to convert my GRC files to be compatible.

At the time of creation the only viable hardware option was the USRP series from Ettus Research. Therefore, the examples use UHD sources and sinks, but there is nothing wrong with changing these to e.g. gr-osmosdr blocks. You will, however, need to also update the sample rates throughout the flow graphs according to the device you are using.

You will find that in many of the examples tuning is done using a frequency translating FIR filter rather than changing the USRP frequency. This is because on some daughterboards (e.g. WBX) tuning is not fast enough for the user moving a GUI slider. If you are only using a daughterboard that is fast enough you can conveniently change the flow graph to tune the USRP directly and remove the frequency translating filter.

If you are looking for theoretical material for learning digital signal processing and software defined radios see Suggested Reading on the GNU Radio website. If you are an absolute newbie to SDR (like I was when I started) and haven’t even got a clue of what quadrature signals are, I suggest you start with Quadrature Signals: Complex, But Not Complicated and the excellent tutorials at

Most recently (2014), Michael Ossmann has started his educational series called Software Defined Radio with HackRF which can be used for learning DSP and SDR.

The examples are hosted on Github, see csete/gnuradio-grc-examples or create a local clone using

  git clone

Table of Contents

  1. Narrow Band FM receiver
  2. AM Receiver
  3. SSB/CW Receiver
  4. All-in-one AM/FM/SSB/CW Receiver
  5. SSB Transmitter
  6. Recording IQ data using dynamic file names
  7. NOAA APT decoder

Narrow Band FM Receiver

This is a simple narrow band FM receiver with two NBFM demodulators to allow switching between 5kHz and 2.5kHz deviation:

  • Enter USRP frequency in a text field
  • Tune within the passband using frequency translating filter (fast tuning)
  • Variable filter width and shape
  • Squelch
  • Supports 5kHz and 2.5kHz deviation
  • Download the GRC file

The video below shows the narrow band FM receiver in action receiving the AO-51 amateur radio satellite using the USRP, WBX and an Arrow antenna.

FM receiver flow graph

AM Receiver

This is a simple AM receiver featuring a variable low pass filter and automatic gain control with fixed rate and levels:

  • Enter USRP frequency in a text field
  • Tune within the passband using frequency translating filter (fast tuning)
  • Variable filter width and shape
  • Automatic gain control
  • Download the GRC file

AM receiver flowgraph

SSB/CW Receiver

This is a simple SSB/CW receiver featuring a tunable band pass filter and automatic gain control with fast/medium/slow rate:

  • Enter USRP frequency in a text field
  • Tune within the passband using frequency translating filter (fast tuning)
  • Variable filter width, offset and shape
  • Sideband is selected using the filter
  • Automatic gain control (fast/medium/slow)
  • Download the GRC file

SSB/CW receiver implemented in GNU Radio Companion

All-in-one AM/FM/SSB/CW Receiver


SSB Transmitter

Bandpass filtering

This SSB transmitter is based on the simple principle of AM modulation, where the carrier is modulated by the audio signal. Of course, carrier does not have the same meaning in a software radio so we just modulate a complex sine wave wave with frequency 0Hz.

The output from the modulator is an AM signal and we select the desired side band using a complex band pass filter.

  • Enter USRP frequency in a text field
  • Tune within +/- 25kHz of the USRP frequency
  • Variable filter width
  • Level controls for audio, carrier, signal and USRP gain
  • Download the GRC file for LSB and USB.
Simple USB transmitter implemented in the GNU Radio Companion using a complex band pass filter.

Recording IQ data using dynamic file names

Recording IQ data in GNU Radio companion is very easy, we simply add a filesink to the flow graph and connect it to the data we want to record. To prevent previously recorded files to be deleted when we restart the flow graph we can use the current date and time to generate a unique filename every time the GNU Radio flow graphs is started. The usage of the Python datetime module for this purpose is explained in Dynamic File Names in GNU Radio Companion.

The flow graph shown below implements a simple WFM receiver for NOAA APT downlink and a simple IQ data recorder using this datetime trick.

WFM receiver with IQ data recorder

To play back the recorded IQ data we simply replace the USRP source in the receiver with a file source and a throttle block, see more in Playback of the Recorded IQ Data.

Flowgraph for playing back the recorded IQ data.

Example GRC files for recording and playback are also included in the GIT repository – they are called xyz_rec.grc and xyz_play.grc.

NOAA APT decoder

A simple prototype for decoding Automatic Picture Transmission (APT) from the NOAA polar orbiting weather satellites is presented in this post. Instead of WAV file input you can connect it directly to the output of a WFM receiver like the one presented in the previous section. Be sure to use the correct sample rates in the band pass filter and the first resampler! The sample rate for the decoder must be 9.6 ksps which should be clear if you read the description of how the decoder works.

Simple APT decoder

Sorry there is no GRC file, but it should be very easy to reproduce from the screenshot.

Author: Alexandru Csete

Embedded software engineer in the satcom industry during the day. Radio amateur and SDR hacker during the night.

  • hawkingyy

    missing so many blocks now. Could you update these grc files?

  • Dhruv Desai

    Can you provide some data on designing a universal analog receiver using GNURadio

  • Mike

    Clicking on screenshots to (hopefully) view a larger, actually readable size results in a 404.


  • James Bromley

    Still getting the ‘404 file not found’ on the screenshots.