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There are many stations in the low part of the AM band that make it hard to spot the harmonics from the LED fixture. Looking further I found good signals at 1120 and 1040. The difference is 80 kH. This means I should see signals at 560, 640, 720, 800, 880, 960, 1040, and 1120. Other than 640 most of the other frequencies are on a strong radio station or close. Now it appears my original 680 should be 640. This means the oscillator in the LED ficture is about 80 kHz, and somewhat spread in its spectrum to reduce AM radio interference.
What can you expect from a dimmer? It should have 120 Hz fundamental with some low pass filtering at the input, thus in the AM band fairly uniform noise across the whole band. This is going to be a difficult noise signal to minimize.
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