Thanks Bob - that's invaluable advice.

Cheers,
Zik

On 9/4/06, Bob Axtell <engineer@neomailbox.com> wrote:
> Maybe I can help...
>
> Zik Saleeba wrote:
> > Apologies about the incorrect topic - reposting in [EE]...
> >
> > On 9/4/06, Zik Saleeba <zik@zikzak.net> wrote:
> >
> >> I'm making a control system for an autonomous plane. Every gram counts
> >> so I'm planning on using a high frequency step-down switching
> >> regulator to provide power. My worry is that EMI from the LT1940
> >> switcher may interfere with radio communications.
> If you design the switcher according to the recommendations of the chip
> maker, you will not radiate
> heavily enough to cause interference. Most important- Use a  multilayer
> PCB of which at least one
> layer is GND. Next, use a shielded inductor, and make sure the inactive
> (VCC) end of the inductor is
> the shield end: most assembly houses are not aware that inductors are
> sometimes "polarized".
>
> I have noticed that the National Semi 50Khz "Simple Switchers" seem to
> generate less EMI than the
> higher frequency ones. Just a place to start.
>
> >>  The plane has two
> >> independent radio systems - one for two-way telemetry and one to allow
> >> override of its control system when bugs happen. So I don't want EMI
> >> from the switcher to screw up either of these systems.
> >>
> What isn't clear to me is how I can tell how much of a problem I have
> >> with EMI. What can I do to measure the amount of EMI or the effect
> >> it's having on the radios? Preferably without crashing the plane in
> >> the process, that is.
> >>
> There are people all over the www that sell "broadband sniffers"; these
> are normally used to pick up room
> "bugs", but they also work well as general EMI locators. I paid less
> than $100 for mine.
>
> >> And if there are problems with EMI what would the lowest weight
> >> solutions be? Would putting foil around the power supply and some
> >> small ferrites on the power lines be likely to do the job?
> >>
> Be careful with ferrites, sometimes they ADD EMI if not carefully used.
> Usually, the ferrite needs
> to be used with caps on both ends to GND.
>
> Copper foils DO help, if you need it. But I don't think you will. Just
> layout the switcher in as tiny a
> footprint as possible; the smaller you make it, the quieter it will be.
>
> --Bob
> >> Thanks,
> >> Zik
> >>
> >>
>
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