Ferrite beads

[banedon]

Hi guys

I've recently been having a look at the W65C816SXB datasheet and noticed that the circuit diagram shows ferrite beads being used inline between the +5V supply rail and the oscialltor cans (datasheet, page 3). I take it this is to protect the other devices connected to that rail from any noice generated by the oscillators?
What's odd is that I cannot remember seeing them being employed on most other 65C02/65C816 designs. Is this something worth bothering with? Maybe if you have some components especially sensitive to noise?

[Arlet]

Typically these aren't used to protect other devices, but to reduce electromagnetic interference from the board leaking into supply wires and causing it to fail CE/FCC tests.

[GaBuZoMeu]

Yes, they are used to prevent distribution of noise. They are commonly used in professional circuitry to achieve EMI levels (radiation and reception) that are required by some laws (CE/FCC).

In hobbiest circuits they are rare, because usually they would not help a lot and EMI isn't an issue - as long as you won't listen to radio while bredboarding

On the other hand, if you wish to have a high resolution (>= 12 bit) ADC working fine, then the self generated noise of a poor layouted µP system can cause significant noise which in turn prevents working with low signal levels. There you may have a "noise floor" which is nearly impossible to get around.

[banedon]

Many thanks for advising. I think I'll skip having ferrite beads in my projects then .

[BigDumbDinosaur]

I did some EMI tests with POC V1.1 prior to selling one to a hobbyist in Australia. I found I could get the computer very close to my plasma TV set and not pick up any signs of interference. I also tried that "test" with the radio in the kitchen. Same thing: no discernible interference.

My position on it is EMI is unlikely to be a problem unless the POC unit is right next to a radio or TV. Even so, I did take some precautions to reduce the possibility of some EMI getting into the power source, mainly by bypassing right at the point of power entry, using a low ESR electrolytic in parallel with a 0.1 µF capacitor of the same type used for bypassing chips (X7R MLCC). Scoping the power input showed no measurable RF energy present.

[ChuckT]

banedon wrote:

What's odd is that I cannot remember seeing them being employed on most other 65C02/65C816 designs. Is this something worth bothering with? Maybe if you have some components especially sensitive to noise?

It is kind of like this. The radio in my car is fine until I plug in my cell phone. If you add something to your SBC, you might not know the consequences or one day it might not operate as it should.

And XM Radio does have a wire with ferrite beads. I took it off because it interfered with the range even though they advise you to use it so the unit doesn't put out too much frequency.

[RichCini]

I know that ferrite beads were used on the Serial IEC circuit leading up to the DIN5 connector on the back panel of the VIC and C64. I haven't checked the others.