Sorry to get repetitive... I asked a question above, but I'm not sure I got the specificity of answer I was hoping for. Could I try again in more concrete terms please
I have a source of DC, 12 to 24V say. I have a switching PSU running off the DC. The sink takes current in bursts at 2us intervals. The 500KHz is approximate, but reasonably stable and accurate.
I'd like to be able to draw 15W in bursts, so assume average input current of 1 to 2A on full load.
That causes noise on the wire going back to the supply. That stops me getting CE approval for the box ;-(
I need to kill the noise.
All the PSU chips which run at fixed frequencies say "this makes the noise easier to filter" - Hmmm. how exactly?
I appreciate that there will be noise at 2us and harmonics at higher frequencies. We have already fitted a proprietary input filter, which sadly seems to provide little attenuation as far down as 500KHz.
It doesn't appear that [ after the filter get to them ] the harmonics are large enough to cause a problem.
Yes I know, shoot the designer who matched that filter with that switcher. The bullet is on it's way.
So I have just the one problem - noise at 500KHz.
I COULD treat it by just building/fitting a 'bigger/better' filter with a cut-off frequency well below 500KHz. But I have a PCB artwork already, and PCBS built, and that would require making new PCBs. I'd like a retrofit mod if I can work one out.
Now, back when I did exams, I seem to remember circuits which would exhibit a high impedance at a single frequency. usually consisting of a cap and inductor.
Could I "fix" the problem by fitting such a cct? Would the components required fit inside a small building if I did? Should I stop trying to be clever?
I can buy a 2A-rated inductor at about 33uH
to get the resonant frequency to be 500KHz I'd need a capacitor of
C= 1/( 4 * PI^2 * L * F^2 ) ???3nF???
Sounds a bit small... maybe I could use a smaller inductor and a larger cap?
Maybe I should just fit a 1R/inductor in series and change the input cap to 220uF low ESR instead of 22uF.
Is there a better way, maybe by building a suitable notch-filter circuit and fitting it into the DC supply lead outside the box for now?
All help appreciated.