Did you measure the resonant frequency of these things?  Sounds like
they are little antennas...

-Adam

Bob Axtell wrote:

> Mike Harrison wrote:
>
>> On Thu, 8 Jul 2004 07:07:55 -0700, you wrote:
>>
>>
>>> A word of caution:
>>>
>>> I ran into a peculiar situation with these super high brightness
>>> devices: they radiate an inordinate amount of RF when switched.
>>> My approach was to PWM the LEDs to control brightness, but it failed.
>>>
>>> Every LED radiates a tiny pulse when switching from non-emission to the
>>> emission states. But these big ones have such a huge wavefront that
>>> in a
>>> radio application once (2 years ago) I was unable to get the energy
>>> squashed enough for the radio to work. Simply had to abandon the
>>> design.
>>> The radiation was detectable by simple sniffer well into the 900Mhz
>>> band, highest freq my sniffer would capture.
>>
>>
>>
>> Is this due to the current spike or something else ? Why wouldn't a
>> series inductor to control the
>> risetime fix this?
>
>
> I tried RF chokes of many values in series with the LED leads, tried
> improving the ground plane, caps, caps in combination with RF chokes,
> etc.
>
> Yes, it worked pretty well with a mu-metal shield surrounding the LED on
> every side except where the light came out. But it was too costly to
> manufacture.
>
> These noisy ones were made by HP.
>
> --Bob
>
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