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Unable to block RFI

R

realexander

Jan 1, 1970
0
Hi,

I've been working on a microcontroller-controlled FM radio, but the RFI
from the microcontroller is filling the radio with static. I've tried
sealing the microcontroller inside an aluminum box, but that didn't
work. Next, I tried taking the microcontroller out and putting the
radio circuit in. Still no joy. What am I missing?

Thanks,
Bob Alexander
 
J

John_H

Jan 1, 1970
0
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!
 
C

Charles Schuler

Jan 1, 1970
0
realexander said:
Hi,

I've been working on a microcontroller-controlled FM radio, but the RFI
from the microcontroller is filling the radio with static. I've tried
sealing the microcontroller inside an aluminum box, but that didn't
work. Next, I tried taking the microcontroller out and putting the
radio circuit in. Still no joy. What am I missing?

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

John Larkin

Jan 1, 1970
0
The aluminum box might shield electrical signals but what about magnetic?
Mu-metal is used for some shielding.

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

John
 
J

Joerg

Jan 1, 1970
0
Hello Bob,
I've been working on a microcontroller-controlled FM radio, but the RFI
from the microcontroller is filling the radio with static. I've tried
sealing the microcontroller inside an aluminum box, but that didn't
work. Next, I tried taking the microcontroller out and putting the
radio circuit in. Still no joy. What am I missing?

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
 
J

Joe G \(Home\)

Jan 1, 1970
0
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
 
D

Dieter Brozio

Jan 1, 1970
0
realexander said:
I've been working on a microcontroller-controlled FM radio, but the RFI
from the microcontroller is filling the radio with static. I've tried
sealing the microcontroller inside an aluminum box, but that didn't
work. Next, I tried taking the microcontroller out and putting the
radio circuit in. Still no joy. What am I missing?

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
 
Hi,

I've been working on a microcontroller-controlled FM radio, but the RFI
from the microcontroller is filling the radio with static. I've tried
sealing the microcontroller inside an aluminum box, but that didn't
work. Next, I tried taking the microcontroller out and putting the
radio circuit in. Still no joy. What am I missing?

Thanks,
Bob Alexander

does the lid make contact all the way round?
 
H

Heid The Baw

Jan 1, 1970
0
realexander said:
Hi,

I've been working on a microcontroller-controlled FM radio, but the RFI
from the microcontroller is filling the radio with static. I've tried
sealing the microcontroller inside an aluminum box, but that didn't
work. Next, I tried taking the microcontroller out and putting the
radio circuit in. Still no joy. What am I missing?

Thanks,
Bob Alexander
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
 
R

realexander

Jan 1, 1970
0
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
 
R

Rich Grise

Jan 1, 1970
0
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

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
 
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