GX LII-2 – Experimental Test Bench for Bode and Loop Gain Measurements
In progress
by giux 6 articles last on September 6, 2026 0 comments
The GX LII-2 is an experimental project dedicated to measuring the frequency response and loop gain of analog circuits. The system uses an injection transformer and A/B oscilloscope acquisitions to derive magnitude and phase through numerical processing in GNU Octave.
Development includes characterization of the measurement setup, compensation for differences between acquisition channels, comparison with LTspice, and analysis of THD, harmonics, residuals, clipping, and gain and phase crossover frequencies. Tests performed with different resistor dividers also make it possible to study the influence of DUT impedance and to build a progressively more automated, repeatable measurement procedure.
Future developments:
The project will evolve toward an increasingly automated procedure, with coordinated instrument acquisition, test-bench calibration, and systematic characterization of the effect of DUT impedance. In the longer term, it could provide the basis for a dedicated instrument for automatic magnitude, phase, and loop gain measurements.
Documents and handouts
- Measurement of loop gain in feedback systems April 1975 e (Middlebrook was a professor of Electrical Engineering at Caltech; the paper was received on 23 September 1974.)
Project log 6
- Aug 11, 2026 Self-Built Injection Transformer: First Loop Gain Measurements and Calibration Test Bench Self-built injection transformer for Bode and loop-gain measurements, with calibration bench, GNU Octave processing, and LTspice comparison.
- Aug 13, 2026 GX LII-2 – Experimental Characterization of the Measurement System GX LII-2 measurement-system characterization: injection amplitude, THD, A/B orientation, DUT impedance, and LTspice comparison.
- Aug 13, 2026 GX LII-2 – Lab Log: Bode Measurements and GNU Octave Processing GX LII-2 lab log: Bode measurements, A/B compensation, harmonic analysis, clipping diagnostics, and GNU Octave automation.
- Aug 17, 2026 GX LII-2 – Lab Log: Bode Measurement Campaign on Resistive Dividers GX LII-2 resistive-divider campaign: Bode measurements, A/B compensation, injection transfer, and FFT analysis in GNU Octave.
- Aug 18, 2026 GX LII-2 – Lab Log: Bode Measurements with Reactive Loads and Fixture Limitations GX LII-2 reactive-load Bode tests reveal fixture limits, repeatable phase rotation, and impedance-dependent measurement effects.
- Sep 6, 2026 How an IR Remote Control Works: Carrier, MARK, SPACE and NEC Protocol How an IR remote control works, explained from real oscilloscope measurements: carrier, MARK, SPACE, header, bit timing and NEC identification.