11. Analysis of the Recorded Signal in GNU Radio¶

Purpose of the section¶

To become familiar with GNU Radio as an environment for visually assembling SDR chains and to use it for analysis of a recorded IQ signal.

1. Why GNU Radio is needed¶

GNU Radio is useful because it allows the student to quickly build a working SDR chain from ready-made blocks and visually observe the signal path.

For an educational course this is especially valuable because the student gets another view of the same experiment:

without writing a large amount of code;
in an intuitive block-based form;
with quick structural changes during analysis.

2. Task of the section¶

Within the first block the student should:

open a recorded IQ file;
feed it into a GNU Radio flowgraph;
display the signal spectrum;
optionally display the time-domain waveform;
compare the picture with HDSDR, MATLAB, and Python.

3. Minimum flowgraph structure¶

The simplest scheme may include:

File Source
Throttle
QT GUI Frequency Sink
QT GUI Time Sink

If the file already contains complex samples in a suitable format, this is enough for the first demonstration.

"Suitable" means cf32: a File Source of type complex reads 32-bit float I/Q pairs. A course .ci16 file needs a File Source of type short followed by Interleaved Short To Complex; a raw rtl_sdr file is unsigned bytes and needs the 127.5 offset removed before the conversion; an HDSDR WAV file goes through Wav File Source with two outputs joined by Float To Complex. A wrong source type gives the same confident, wrong spectrum as on page 09.

4. What must be configured¶

It is necessary to set correctly:

file data type;
complex signal format;
sampling frequency;
FFT size;
display window length.

If these parameters are wrong, the spectrum will be interpreted incorrectly.

5. What the student should see¶

As a result of building the flowgraph, the student should observe:

a stable spectral peak of the test tone;
the time-domain structure of the signal;
agreement with the results obtained in other environments.

6. Why GNU Radio is useful in this course¶

GNU Radio complements the other tools well:

HDSDR: fast observation of a real signal;
MATLAB / Simulink: modeling and system-level analysis;
Python: automation;
C++: high-performance implementation;
GNU Radio: quick assembly and visual debugging of SDR chains.

Thus the student gets used immediately to the fact that the same signal can be studied with different engineering tools.

7. Minimum practical task¶

It is enough to assemble a flowgraph that:

reads an IQ file;
shows the spectrum;
shows the time-domain waveform;
allows display parameters to be changed;
confirms the presence of the test tone.

8. What can be extended later¶

In later stages the student can use GNU Radio for:

digital frequency translation;
filtering;
decoding;
analysis of multiple signals;
playback and modeling of SDR chains.

So already in the first block GNU Radio acts as a convenient educational platform that can later be expanded.

9. What to include in the report¶

It is recommended to include:

a screenshot of the flowgraph;
the spectrum display window;
the time-domain display window;
sampling-frequency settings;
a short comparison with other tools.

10. Conclusions¶

After completing this stage, the student should:

be able to assemble a simple flowgraph for recording analysis;
understand the role of GNU Radio in SDR development;
see how the same signal is analyzed in several engineering environments.