EMC from AVR Mega32

I'm using an AVR Mega32 in the DIP40 package to interface with 35 MHz
remote control radio transmitters and have found that there is an
enormous radiated emission from the AVR at the second harmonic of the
16 MHz crystal. According to my spectrum analyzer the 32 MHz emission
is 40 dB at 25 cm, while the output of the transmiter at 1 m with the
antenna down is only 45 dB.
This noise is reducing the effective range of the remote control from
nearly 1 km to less than 100 m before the receiver can no longer hear
the transmitter. I've verified that the AVR alone is causing the
problem; on a breadboard with just the microcontroller, decoupling caps
and crystal the noise levels are identical. There is no radiated noise
at 72 MHz, so it works fine with the US transmitters.
I've tried a variety of replacement chips, chokes, caps and shields to
no avail. I even tried a Mega128 in TQFP packaging and found a
similar amount of noise. Does anyone have any further ideas?
-- Trammell
Reply to
Tramm Hudson
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Is it the micro or the crystal ?
Have you got enough decoupling capacitors and are they low esr caps? On Power supply and all io lines .
Shield the pcb and or use a metal case or complete mesh cage?
How are your ground planes on the pcb? And gnd plane under/around the xstal/osc ?
What about trying a different crystal ? or different capacitors connected to the crystal.
Do you need a 16MHz crystal ? Can you use a lower frequency like 4 or 8MHz instead of 16MHz and double/4x the frequency in the rf section ?
What about the ground / earth connection ? How are you powering the board and how are you grounding it ?
Have you had a read of the overview of EMC app note from Atmel ? AVR040: EMC Design Considerations
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Reply to
Alex Gibson
Different crystal frequency, although it may not help much, as your receiver is already desentisized by signal 3 MHz away.
If you are sure the the signal get in at the receiver antenna (and not via the powersupply or the ground) A standard 14,.. MHz crystal get the 2nd and 3rd harmonic 7 MHz away
32 MHz trap at the receiver input (serie resonance, parallel to the input or a parallel resonance in series with the input.
Additional selective filter at receiver input.
Reply to
Wim Ton
Try running with the on chip RC crystal, or try turning off the full swing option (Fuse bit P - osc. option).
Have you tried using a different crystal from a different manufacturer, our try grounding the case of the crystal.
Regards Ian Dobson
Home of the Atmel based UDP mobile web cam
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Reply to
ian dobson

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