EMC Emsissions - Are Pico Power devices better?

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Folks,

Just been testing some kit with a DSP banging away at 150MHz and an AVR at 8MHz and was surprised to find that the AVR (Mega64) is much noisier than the DSP. Although we've managed to stamp out most of the noise (by removing a cap from an input rather bizarrely), you can see sniffing about with the spectrum analyser, the Mega64 running 8MHz is producing a surprising amount of 8MHz noise and a rather odd 160MHz harmonic on a line connected te a switch on a rotary encoder.

Anyway, my question is really on whether other people see the Mega64 as a noisy device? (I have good ground planes, decoupling etc and as the 150MHz DSP is quiet as a mouse I don't believe it's a layout problem.) Also, would sticking a Mega325P - which looks like it's the same pinout - give me any improvment? I instinctively think that a lower power device might radiate less, but that could well be fallacious thinking!

Many thanks. :)

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Are there high frequency lines going to high-impedance components (like a cap without a resistive cathode path to ground)?

Annoying question, I know, but it's gotta be asked. :P

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tlucas wrote:
Are there high frequency lines going to high-impedance components (like a cap without a resistive cathode path to ground)?

Annoying question, I know, but it's gotta be asked. :P

Not annoying at all, it's entirely possible I've missed something obvious or am ignorant about something. :)

There aren't any high frequency lines at all coming out of the AVR. I have very short crystal tracks, but everything else is low frequency for driving an alpha numeric LCD, imnputs from a rotary encoder, UARTs running at 20k, various switches and fairly low speed SPI.

What's a resistive cathode path to ground?

The thing that really seemed to make a difference with the 160MHz spike was removing a cap to ground that was also connected to a pullup resistor that is connected to a switch. We noticed the spike diminishing when the button was pushed and the output of the switch/input to AVR was grounded.

Even with inductors on the 5V stubs to the AVR and good decoupling and good ground planes, we're seeing a lot of 8MHz noise around the AVR. (Having said that, it's now well below the limits, but we're dealing with high quality audio signals, so would like to lose as much as possible for the sake of audio quality as much as EMC.)

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This is in a breadboard, then, if you're able to remove a cap easily? Could be a piece of crud between the lines! The 160MHz spike is very interesting... I wonder if you've lucked out on building a good receiving antenna for a particular local VHF station. ("Maritime Mobile" and "Land Mobile" in the US - can't find the ITU chart.)

http://upload.wikimedia.org/wiki...

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No, it's a PCB; it's a piece of cake to remove an 0805 cap!

It was deinitely the AVR because stopping the clock removed the issue!