CO₂ Laser Tube Current: PWM Bandwidth

As with the analog bandwidth measurements, start with the same sine-bar bandwidth target:

Sine bars - 10 cycles
Sine bars – 10 cycles

Return the laser power supply’s IN terminal (and the purple wire to the oscilloscope) to the Ruida KT332N controller’s PWM output:

Ruida KT332 - PWM laser control wiring
Ruida KT332 – PWM laser control wiring

Engraving the pattern in grayscale mode at 254 dpi produces 0.1 mm pixels and makes each bar 1 mm wide:

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

Engraving at 50 mm/s = 50 Hz lets the laser current once again hit full scale:

Tube Current - PWM bandwidth - 10 sine - 50mm-s - 10ma-div - 254dpi
Tube Current – PWM bandwidth – 10 sine – 50mm-s – 10ma-div – 254dpi

The traces:

  • 1 X axis DIR, low = left-to-right (yellow)
  • L-ON laser enable, low active (magenta)
  • PWM digital signal (cyan)
  • 4 tube current – 10 mA/div (green)

The PWM signal runs at 20 kHz and presents itself as a rather blurred trace, but you can see both the general tendency and the discrete steps between the vertical gray bars. As far as I can tell, the signal never reaches 0% or 100%, most likely to prevent the PWM filters from saturating in either condition.

The tube current drops from 23.8 mA to 13.8 mA, just over the half-power level of 12 mA, at 200 Hz:

Tube Current - PWM bandwidth - 10 sine - 200mm-s - 10ma-div - 254dpi
Tube Current – PWM bandwidth – 10 sine – 200mm-s – 10ma-div – 254dpi

So the PWM bandwidth is a little over 200 Hz, slightly higher than the analog bandwidth of a little under 200 Hz.

All of the measurements as a slide show:

  • Tube Current - PWM bandwidth - 10 sine - 25mm-s - 10ma-div - 254dpi
  • Tube Current - PWM bandwidth - 10 sine - 50mm-s - 10ma-div - 254dpi
  • Tube Current - PWM bandwidth - 10 sine - 100mm-s - 10ma-div - 254dpi
  • Tube Current - PWM bandwidth - 10 sine - 200mm-s - 10ma-div - 254dpi
  • Tube Current - PWM bandwidth - 10 sine - 300mm-s - 10ma-div - 254dpi
  • Tube Current - PWM bandwidth - 10 sine - 400mm-s - 10ma-div - 254dpi
  • Tube Current - PWM bandwidth - 10 sine - 500mm-s - 10ma-div - 254dpi

Now, with all the measurements in hand, maybe I can reach some sort of conclusion.