CO₂ Laser Tube Current: Analog Bandwidth Target

Along the same lines as the grayscale bars, a grayscale sine wave pattern allows direct bandwidth measurement:

Sine bars - 10 cycles
Sine bars – 10 cycles

The sine wave pattern comes from a totally cargo-culted Imagemagic invocation:

magick -size 100x100 gradient: -rotate 90 -function sinusoid 10.0,-90 'Sine bars - 10 cycles.png'

The pattern gets plunked into the same white/black frame as before, using GIMP because it’s easy.

Importing the resulting PNG image into LightBurn allows configuring the laser parameters. Each sine wave is 1 mm (ten whole pixels!) wide, so engraving at 250 mm/s covers one cycle every 4 ms for a 250 Hz signal:

Tube Current - analog - 10 sine - 250mm-s - 10 ma-div
Tube Current – analog – 10 sine – 250mm-s – 10 ma-div

Changing the engraving speed will change the test signal frequency, although the laser can’t get much beyond 500 mm/s.

The sine wave pattern goes from 0% to 100%, but at 250 Hz the controller output doesn’t reach those extremes, suggesting the output filter rolloff is lower than the 200 Hz inferred from the 1.5 ms risetime and falltime values.

Because the power supply output current isn’t matching the controller voltage excursion and its waveform is much rounder, its bandwidth is even lower.

The more I measure, the more puzzling it gets …