As you imply, the 200Hz PWM waveform doesn't make sense unless at full brightness. I wonder if the scope lost trigger or something as you changed brightness during the 200Hz sweep (I do see that the frequency and time domain measurements change to "?" during the sweep, for what that is worth). The 200Hz waveform should look more like your 600Hz waveform; the average voltage needs to drop for the LEDs to dim so you gotta get back to a square wave. If the dirty one was the higher frequency, then I'd suspect the scope maybe wasn't catching the waveform (ie. too low bandwidth or too low of a sampling frequency), but that isn't he case here. (Maybe use a good 'ol analog scope?).
--On Wednesday, January 3, 2024 9:50 AM -0800 "Levi C. Maaia, K6LCM" k6lcm@maaia.com wrote:
Happy New Year!
For my December birthday this year, I received a cool LED "neon" light. It is quite large so I have it hanging above my sofa in my shack. The attached photo of it is a little weird looking. The blue doesn't look like that in real life. I'll have to get another photo at night.
Anyway, I know that LED drivers can be a source of RF noise, especially cheap ones that use PWM to dim the light from LEDs. The one that came with this piece was visibly "dirty" to the point that I swear that I could see the flicker at certain brightness settings. I put my portable scope on the DC LED dimmer/driver output and it was very ugly.
I decided to do some research and see what I could learn about PWM and LEDs. Turns out that the frequency at which these drivers pulse the light varies from model to model with some pulsing the light on and off at as slow a frequency as ~200 Hz. Apparently that was enough for be to perceive flicker. I found another unit that was more that three times faster (~600Hz) and swapped it in. The flicker went away and the picture on the scope looks a lot cleaner now.
In the attached video I show the "clean" output, then the "dirty" output. I can't figure out how the dirty output manages to even dim the light. It barely budges off of 12 volts while the clean driver has nice, neat square pulses.
I didn't notice any noise on the ham bands with either, which is good. I have other LED drivers around the house that are noisier on the RF side. Maybe I'll try swapping those out.
Here is another more detailed video I did a while back about tracking down RF noise around the house. My kitchen LED uplighting is a continual source of RF noise. https://www.youtube.com/watch?v=keQmb2Maqes
LCM
Levi C. Maaia, K6LCM
www.levimaaia.com
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