DC Line Filtering for Noise

[FlyingHacker]

I have a little voltage inverter I made for use in an Amatuer Radio application (<30MHz). The circuit uses a 555 to drive a charge pump. The 555 runs at ~3.2KHz

I would like to filter the INPUT 12V (to prevent this circuit from back-feeding interference) as the line is shared with a lot of other equipment. I want to keep the square wave from generating interference back to the shared power bus.

I am looking for some ideas, and some education regarding this topic (I am a hobbyist). Would an X and two Y capacitors be the best way to filter this? Which formulae would be used to calculate the values? I get output filters like basic low pass filters. How do those work in reverse to filter noise going back towards the supply?

Any info and suggestions/links appreciated. I would like to garner at least a basic understanding of the topic.

Thanks.

[Kleinstein]

The NE555 is known to produce quite some current spikes on the supply. So one might want to consider a less "noisy" solution to start with. Charge pumps also tend to draw current spikes - so they are not that attractive to a low noise circuit.

Filtering could be some kind of Pi type filter. So a capacitor (or 2 ) at the Chip, an inductor and than a capacitor at the supply side.
The filter would work in both directions. Other sensitive parts from the same supply might want a similar filter as well.

[FlyingHacker]

Thanks for the reply, Kleinstein.

The output will have almost no load on it (<10mA, likely less than 5mA).

OK. So a Pi filter will work in both directions. This is good to know. I know how to calculate that.

How much of an effect does bulk capacitance have on preventing these spikes? In other words if I have 1000-4700uF at the input is that going to reduce the spikes going back to the input line? Or once those capacitors are charged do those spikes just come off the supply rail?

I am also open to other ideas for producing this "negative" voltage (from a negative ground referenced supply to feed into a negative ground referenced radio). I am currently using batteries, which are clean and isolated, but a pain. This is also meant as a learning exercise, rather than simply a problem solving exercise, as the batteries are probably the simplest solution. I am trying to use something I build myself, rather than say a DC -DC converter IC (for isolation to let me reverse the polarity), or a pre-built inverter IC.

Thanks again.

[FlyingHacker]

Hmmmm.

If I want to calculate the L and Cs for a Pi filter I need the impedance of the line Zsub0. What would I use for that value if my "transmission line"  in this case is the input 12V power cable, like a 2-3ft long 22 or so gauge wire.

[sdouble]

pay attention to the resonance of the pi filters. If you use MLCCs, you may have to damp the resonance which typically match the switching frequency...

[FlyingHacker]

pay attention to the resonance of the pi filters. If you use MLCCs, you may have to damp the resonance which typically match the switching frequency...

Interesting. Do old school ceramic disc caps suffer from this too, or just the multi-layer ones?

[sdouble]

yep, same issue. linked to the very low ESR of ceramic caps.

[FlyingHacker]

Ok. So I want the resonance of the Pi network to NOT be near the 555 switching frequency, or a harmonic thereof, right?

When calclating a Pi filter for an application such as this do you use the switching frequency as fC? Or should I be using a lower frequency to make sure that fSwitch is more thoroughly attenuated? Is there a rule of thumb for this?

[sdouble]

I suggest to simulate your pi filter behaviour.
What I typically do to avoid any issue is the following :
1) use ferrite beads instead of inductor
2) instead of have,  say a 1 uF cap, I put 2 of those in parallele, one been connected to a 0.5 ohm resistor before been connected to ground.
That 0.5 ohm fully damp the resonance.
Another option is to use a tantalum cap with the proper ESR.

[cellularmitosis]

Here was my attempt to filter the noise from a switched capacitor negative supply: https://www.eevblog.com/forum/projects/an-evening-with-the-icl7660/

[FlyingHacker]

Here was my attempt to filter the noise from a switched capacitor negative supply: https://www.eevblog.com/forum/projects/an-evening-with-the-icl7660/

I had seen that before, but not since working on this project. Very useful. Thanks.

I was able to reduce my issues with a Pi filter on the input. The output goes into a simple Zener regulator. I think this is good enough for my application, but your tests show I can likely do better, even with a 555.

I may play with it in the future for learning purposes more than anything else. I want to add a couple of other simple features to my build (power LED and switch). So I need to go into the box anyway. I did do the construction ugly style with very short leads. So that is helping.