Questions about PCB layout and Circuit Design for Analog Audio

[warpigs330]

I have been working on some designs for analog synth parts, and have a few PCBs ordered, and am trying to figure out exactly what is important and what isn't when designing a schematic and PCB for analog audio. I am trying to learn what causes interference, both external RF interference, and also potential power supply ripple induced in the circuit. I guess my question is what causes certain elements of an analog circuit to be more or less sensitive to interference? For instance I have a Voltage Controlled Filter, based on a diode ladder filter design, that has a resonance pot that will occasionally pick up some radio signals. At the moment, the signal flowing through the pot (set up as a simple voltage divider) is only roughly 60mv peak to peak, and the mounting points are floating. Is this susceptible because the peak to peak voltage is so low, and will grounding the mounting holes help? I know a bit about designing PCBs for signal integrity for fast digital systems, but I don't know as well the analog side of things. What are good rules to go by and things to avoid? Thanks for all the help.

[Terry Bites]

In essence, a radio receiver is a filter and a rectifier. Any trace or wire can act as an antenna. A good ground will help you recieve a stonger signal. So you have to track down where the RF gets in.
Long lines on the control pots?

You need a number of strategies to deal with unwanted signals. You need to understand the source of the problem and its remedy.

How does the interference get into your system. Electromagnetic, Electrostatic, Grounding errors, poor insulation, optical, vibration, proximity?

It’s hard to say if particular elements are more sensitive because it depends on the topology of the circuit. In general you need to keep the big signal stuff away from the small signal stuff. This is  requires a combination of physical and electrical isolation.

Avoid long loops for both signals and grounds. Use local decoupling. Don't connect opamp summing nodes to long lines. Current doesn't take the shortest path to ground, it takes all paths.
The lower the impedance the more preferential the path.


How to fix it. Shielding, Filtering, correct grounding, layout ,mounting.

[Grandchuck]

https://resources.altium.com/p/removing-noise-analog-signals-your-pcb