Unable to block RFI

At FM frequencies, the h-field is blocked by aluminum just fine. Mu-metal is useful for audio-range stuff.

John

Are you using feed-thru capacitors? If not, the signals are being carried in and out on the power rail and others.

The aluminum box might shield electrical signals but what about magnetic? Mu-metal is used for some shielding.

Are you certain that it's pickup of emitted radiation, not conducted? If you share power supplies, the noise on the rails will kill your signal. Normal power filtering for radio circuits get quieter each stage as it goes from the noisy power supply through the intermediate signal stages to the extremely delicate first-stage input amplifier. Are you filtered?

What I've done in the past is to supply the radio stages with battery power (read this as "very quiet") and compare performance.

Good luck!

Hello Bob,

Aluminum may be kind of ok. Tin plated metal (thin) is better because you can solder it to the ground plane in several places. In a pinch you could sacrifice a cookie can, but not before discussing this with your wife.

Conducted emissions is what's going to get you as Charles wrote. You could filter the dickens out of that but this gets old. What I do in most of those situations is send the uC into low power mode with all clocks off. With many it can be done so that the pressing of a button or a twist of the encoder shaft wakes it up again. If you do that keep the routines as short as can be, else a frequency change is tedious because you always have to wait until the hash goes away. And yes, that can mean that is has to be done in ye olde assembler.

Regards, Joerg

Thanks for all the replies so far.

The problem is not conduction through the power rail. I know because a battery operated radio gets interference too, even when it is inside the aluminum case. The aluminum case is sealed pretty well, with the exception of a thin earphone cord coming out. (I needed the earphone to avoid the Shroedinger's Radio problem, where the radio is both clear and staticy at the same time, until I open the box and observe it). I tried wrapping aluminum foil around the seam where the wire was coming out, and that didn't help.

- Bob Alexander

What is your ground concept? Have you tried to use separate grounds, i.e. one gnd for the uC and one gnd for the radio?

Regards Dieter

does the micro always need to be active?

can you set up the freq of the FM part and put the micro to sleep?

JG

does the lid make contact all the way round?

Evidently, the RFI is on the band that you're trying to tune in. Quit trying to shield the radio, and make the uP quit radiating the noise.

Like, put _it_ in the box.

Good Luck! Rich

You need to probe the supplies with a scope. If it's the supplies try using inductive filters on the supply between the analogue and digital part.

Tam