PWM and audio noise

[Charlie]

What are you using to connect to the jacks, how far apart are they, what does the board layout looks like, is there a ground plane, is there a decoupling cap on the PIC, how does the 9V ground relate to whatever you have connected, and probably a few more questions once you answer these. There are pretty sharp edges on the steps, so you may be generating RF that is rectified by whatever you are using to listen to audio.

[Dave]

Where is the .1Uf decoupling cap directly at the PIC power pins?

[keithv]

Where is the .1Uf decoupling cap directly at the PIC power pins?

I added at .1uF to the 5v and that did seem to reduce the noise a bit. Not to an acceptable level, but it did help.

[pedja089]

Add capacitors close to noise source(right across pin of PIC). 1,10 and 100nF in parallel.
Also Breadboard have large capacitance between 2 rows of pin. Try to put ground next to PWM pin.
Use short leads, as stray inductance plays large role. Also try to create star ground point.

[Dave]

I agree, Where did you place the .1Uf cap? At the supply? If it work's at all in reducing the noise, then you are on the right track.

[mpgmike]

On larger projects I may use over a dozen 0.1 uF (and one or two 0.01 uF) caps to filter board noise. I also add 10 uF and 100 uF caps near the power supply. I filter both the input to the LM7805 and the output. I use a 47 or 100 ohm resistor between my power source and voltage regulator to further filter the power supply. Invariably, whenever I'm getting unexpected behavior, it's often due to insufficient capacitive filtering. In other words, wherever you think, then add another one or two.

[mark_s]

Like someone suggested above. You should try to increase the frequency of your PWM. A 1khz square wave will have strong harmonics at 3khz, 5khz and 7khz. Thats right in the middle of the audio range. Then you start stepping thru the duty cycle, this will alternate between odd and even harmonics.

Easy fix unless I'm missing something

Change this

PWM 1, Duty, 1000

to this

PWM 1, Duty, 20000

[keithv]

What are you using to connect to the jacks, how far apart are they, what does the board layout looks like, is there a ground plane, is there a decoupling cap on the PIC, how does the 9V ground relate to whatever you have connected, and probably a few more questions once you answer these. There are pretty sharp edges on the steps, so you may be generating RF that is rectified by whatever you are using to listen to audio.

I'm using a small bread board. The PIC and power supply are on one side of the bread board. The jacks are on the other side. The jacks are your typical enclosed 1/4" stereo jack like a Switchcraft 112BX. I'm using some higher end shielded instrument cables to connect to a guitar and a little test amp on my bench.