Are you aware that Wave Players (PCM) generate a lot of EMI ?

Marcello

All the Wave Players basen on PWM and Counters make a lot of RF noise in the 433 Mhz frequencies for instance.
It is very hard to get some Data from a 433 Mhz receiver while playing some wave file.

Has anyone noticed this problem before ?

I found that soldering a SMD restistor on the Wave output pin can help a lot, but the problem is only attenuated with this·kind of countermeasures.

Did somebody find a better solution to that ?

Ciao·

Kye

Mmm, well most wav players work by integrating a 40mhz signal over a low pass filter.

The duty cycle of the 40 mhz signal changes to the pcm sample to be outputted at that time then and is held.

... This shouldn't generate to much higher frequency noise. Interesting.

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Nyamekye,

Post Edited (Kye) : 11/20/2009 2:09:58 AM GMT

Peter Jakacki

If by PWM you mean the duty-cycle DAC then yes of course it will generate RF harmonics which is what you would expect from repetitive fast pulses. The DAC is supposed to be constructed with an RC filter on the output but if that resistor is placed at the end of a long track then all you have before it is an effective antenna. I say supposed to be because it sounded like you didn't have one as you mentioned soldering a resistor on the output to fix it. It is true that you can cheat and not use an RC filter and rely on the amplifier's self-filtering but the problem you mentioned is but one of the side effects.

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*Peter*

Marcello

I use a RC Filter and the PWM i meant is the 40 Mhz duty cycle.

If i·cut the Propeller pin and i solder the smd resistor directly on it i still
observe the 433 Mhz·receiver to receive and trigger less, but quite a lot !!

Some sound wave frequencies or the transitions of them cause more noise.
With the SMD·resistor I reduced the consequencies on my application,
but i·do not have a·quiet reception at all !!!!

I·did more: i used a pin near to the Propeller Package Center (DIL) to reduce the
antenna effect of long bonding and package wires.

With a small band receiver and going away from 433 Mhz i get much less noise.
Could be the 400Mhz·(40Mhz x 10) the critical frequency.

I would like very like much to see a spectrum of it !!!



Ciao

Marcello

Peter Jakacki

Well that probably explains it since you are using the DIP package. EMI is a problem with any micro, especially ones with external buses, so less so with the Prop. The Prop is probably one of the very few microcontrollers around though that can output such high frequencies on it's I/O pins. The duty-cycle mode will change frequency with a change in duty so the mid-range values have the highest frequencies. But normally on a SMD pcb with good grounds and layout this EMI is never really a problem. In the early days of computers I have heard some tell of radios being placed next to the computer and the sound pattern of the EMI would indicate the state of the software which was useful for debugging. I know that I have even done that before.

A square wave is spectrally made up of the sum of the "harmonics" of the fundamental (let's keep it simple) so if you combined all those sine waves you would end up with the square wave. Naturally this applies to all digital systems, even very low frequency ones, it just depends how "sharp" the edges of the square wave are. That's why may buses are slew-rate limited to minimize EMI and ringing.

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*Peter*

Marcello

I have been "hearing" with a radio a Sinclair Spectrum ...

Do you have a raccomandation to solve the problem beside not using DIL packages and PCBs with ground plane and "good ground" ?

mctrivia

i can't think of a solution. I know with a 4 layer pcb and qfn chip I can completely sheild the RF radiation. If you need in any large quantities it can be done cheap.

actually i can think of 1. use grounded tin around the chip and exposed traces. if you have good ground plane on bottom of board then EMI should be minimal.

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24 bit LCD Breakout Board coming soon. $21.99 has backlight driver and touch sensitive decoder.

Ariba

Marcello

You can use another PWM principle, with a fixed PWM frequency equal to the WAV sampling frequency.
With 80Mhz system clock and 44.1 kHz Wave files you get 10..11 Bit resolution of the PWM-DAC.

Can you post here the Assembly part of the Wave-player you use? Then I can show you the changes to make.

Andy