PWM and audio noise

[keithv]

To make this as simple as possible, all this is, is a 5v power supply, a PIC, and two 1/4" jacks that share ground. The PIC is only connected to +5v and ground. The CCP1 pin isn't connected to anything. This is on a breadboard. I am getting a lot of noise that goes up and down in pitch as the duty cycle goes up and down.

Duty var byte
Main:

For Duty = 0 to 255 ' Gets brighter
pause 100
PWM 1, Duty, 1000
Next Duty

For Duty = 254 to 0 step -1 'Gets dimmer
pause 100
PWM 1, Duty, 1000
Next Duty
Goto Main

[mpgmike]

I would add a 0.1 uF cap to the output of your 78L05. I've had noise issues that affected the power supply, which in turn affected everything. A simple .1 ceramic cap fixed a multitude of gremlins.

[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.

[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.