CO₂ Laser Tube Current: Controller Bandwidth Measurement

Use the sine-bar bandwidth pattern:

Sine bars - 10 cycles
Sine bars – 10 cycles

Engrave it in grayscale mode as a negative image with 0.1 mm line spacing:

LightBurn - bandwidth test pattern setup
LightBurn – bandwidth test pattern setup

Monitor the Ruida KT332N controller’s analog laser power control output:

Tube Current - analog bandwidth - 10 sine - 25mm-s - beam off - 254dpi
Tube Current – analog bandwidth – 10 sine – 25mm-s – beam off – 254dpi

The traces:

  • 1 X axis DIR, low = left-to-right (yellow)
  • 2 L-ON laser enable, low active (magenta)
  • L-AN analog voltage (cyan)

The scope triggers when the top two traces go low during a left-to-right scan with the laser beam active. The trigger point lies far off-screen to the left, with the delay set to pull the interesting part of the scan into view.

Although both the controller’s L-AN output and the laser’s IN input specify a signal range of 0 V to 5 V, the analog output voltage never goes below 0.4 V, but (as will seen later) that produces 0 mA from the laser power supply.

Set the X cursors to the top and bottom of the sine wave and read off the 4.36 V peak-to-peak value.

Set the Y cursors to matching points on successive cycles and read off ΔT=33.44 ms. Because each cycle is 1 mm wide, the scan speed is set to 25 mm/s and traveling 1 mm should require 40 ms, puzzle over that number and the related fact that 1/ΔT=29.91 Hz. This seems to happen only for speeds under 50-ish mm/s, for which I have no explanation.

Repeat the exercise at various speeds up through 500 mm/s:

Tube Current - analog bandwidth - 10 sine - 500mm-s - beam off - 254dpi
Tube Current – analog bandwidth – 10 sine – 500mm-s – beam off – 254dpi

The analog output voltage has dropped to 1.56 Vpp.

The average voltage increases from 2.66 V at 25 (or is it 33?) Hz to 2.78 at 500 Hz, which is reasonably close to the same value.

The signal’s -3dB point would be at √½ × 4.36 Vpp = 3.1 Vpp, which happens at 200 mm/s = 200 Hz:

Tube Current - analog bandwidth - 10 sine - 200mm-s - beam off - 254dpi
Tube Current – analog bandwidth – 10 sine – 200mm-s – beam off – 254dpi

Which is eerily close to the “around 200 Hz” bandwidth figured from the risetime measurements.

All of the analog output measurements as a slide show:

  • Tube Current - analog bandwidth - 10 sine - 25mm-s - beam off - 254dpi
  • Tube Current - analog bandwidth - 10 sine - 50mm-s - beam off - 254dpi
  • Tube Current - analog bandwidth - 10 sine - 100mm-s - beam off - 254dpi
  • Tube Current - analog bandwidth - 10 sine - 200mm-s - beam off - 254dpi
  • Tube Current - analog bandwidth - 10 sine - 300mm-s - beam off - 254dpi
  • Tube Current - analog bandwidth - 10 sine - 400mm-s - beam off - 254dpi
  • Tube Current - analog bandwidth - 10 sine - 500mm-s - beam off - 254dpi

One might now wonder whether there’s any bandwidth difference between the analog and PWM signals as measured in the laser tube current.

Data! We need more data!