Lab 2.3 — I/Q Interpretation and Mirrored Spectrum¶

Goal¶

Demonstrate the difference between correct complex I/Q capture, swapped I/Q channels, and a real-valued capture that produces mirrored spectra.

Why this matters¶

Complex baseband separates positive and negative frequencies. If the I/Q order is wrong, the spectrum can be mirrored. If only a real-valued channel is used, positive and negative frequency content becomes symmetric and direction information is lost.

Experiment¶

The script builds a complex tone at 120 kHz and compares three cases:

correct complex I/Q;
I/Q swapped by exchanging the real and imaginary parts;
real-valued capture that keeps only the I channel.

The lab measures:

peak position for the correct complex signal;
mirrored peak for the swapped-I/Q case;
positive and negative mirror peaks for the real-valued capture.

Run¶

From the repository root:

python blocks/block_02_signals_and_sampling/python/iq_visualization.py

Or run the representative lab pack:

python tools/run_all_labs.py

Expected artifacts¶

Artifact	Meaning
`docs/assets/lab23_iq_components_time.png`	I/Q components in the time domain
`docs/assets/lab23_iq_interpretation_spectra.png`	correct, swapped and mirrored spectral views
`docs/assets/lab23_iq_metrics.json`	peak locations and mirroring checks

Interpretation checks¶

The correct complex I/Q case should show the main tone near +120 kHz.
The swapped-I/Q case should mirror the tone to the negative side of the spectrum.
The real-valued capture should show matching positive and negative peaks around +/-120 kHz.
The metrics JSON should confirm a small error for the correct case and near-symmetric mirror peaks for the real-valued case.

Report checklist¶

[ ] Explain why complex baseband can distinguish spectral direction.
[ ] Attach the time-domain I/Q plot and the spectral comparison plot.
[ ] Quote the correct, swapped and mirrored peak locations.
[ ] Explain what acquisition or parsing mistake produces an I/Q swap in practice.
[ ] State how the same issue would appear in a real SDR recording workflow.