Photo Engraving Resolutions

It works pretty much like you’d think:

That’s from a sine-wave test target with 1 mm cycles:

Sine bars - 10 cycles

The target image is inverted to burn the white bars (invisible on a white screen background) on either side at maximum power.

But grayscale mode comes with another gotcha: the power supply uses a 200-ish Hz lowpass filter to demodulate the PWM into the tube current value. A crisp “dot” image requires fairly steep sides, so you want at least the third or maybe fifth harmonic of the signal to be under the filter cutoff, which means the dots can’t require more than 200 / 5 = 40 Hz.

Two adjacent 0.1 mm dots (one on, one off) means a full “cycle” spans 0.2 mm. At 40 Hz, the scanning speed would be (0.2 mm / cycle) × 40 cycle / sec = 8 mm/s, which isn’t much to write home about and probably puts an unreasonably low limit on the power level. The dots will become less crisp at higher speeds as the filter rolls off the edges.

The power supply can accept either analog or PWM signals on the same input pin, but the controller’s analog output comes from a similar lowpass filter and the two filters roll the tube current off at 100-ish Hz.

More details on my blog:

The linkies on that post go to the measurements behind the graph.

Of most interest will be the PWM bandwidth: