# do passive rc filters have problems with 10GHz + frequencies?

#### ratstar

Aug 20, 2018
485
I was looking around on the net, and I stumbled apon a video which showed the rise time of a square wave oscillator causes ringing if its short enough, and is a problem in old logic boards/IC's.

Is this the same for capacitors? I mean is there some problem capacitors have with high frequency passive rc filters?

Here is the video I looked at->

It would also mean maybe, the faster you charge a capacitor, the more ringing it would cause? (cause it would be an increase in rise time.)

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#### ratstar

Aug 20, 2018
485
And another thing - do spark gap oscillators show ringing at gighert frequencies? If not, it means the ringing is a problem caused by the transistor in the oscillator.

#### davenn

Moderator
Sep 5, 2009
14,142
And another thing - do spark gap oscillators show ringing at gighert frequencies? If not, it means the ringing is a problem caused by the transistor in the oscillator.

Did you not understand what causes the ringing ? it seems not

#### ratstar

Aug 20, 2018
485
So, does it happen with spark gap oscillators too?

#### davenn

Moderator
Sep 5, 2009
14,142
So, does it happen with spark gap oscillators too?

#### ratstar

Aug 20, 2018
485
I actually dont understand, its got something to do with impedance, which is due to capacitance? But in the video the engineer showed he fixed it with what looks like a voltage divider... and I dont know how that would fix it, no.

#### davenn

Moderator
Sep 5, 2009
14,142
its got something to do with impedance,

Yes

which is due to capacitance?

No

But in the video the engineer showed he fixed it with what looks like a voltage divider... and I dont know how that would fix it, no.

He used the resistors to improve the impedance matching between the 2 circuits. This reduces the ringing and therefore the harmonics

#### ratstar

Aug 20, 2018
485
Thanks for the in-depth explanation, so do even spark gap oscillators suffer from this phenomena given the same circumstance?

What 2 circuits? dont you count it as 1, when the oscillator meets the logic/alu?

#### davenn

Moderator
Sep 5, 2009
14,142
Thanks for the in-depth explanation, so do even spark gap oscillators suffer from this phenomena given the same circumstance?

Honestly, I dont care about spark gap oscillators, they are illegal in any modern country and went out of fashion over 100 yrs ago

What 2 circuits?

between one stage and another

#### ratstar

Aug 20, 2018
485
So! That must mean it DOESNT HAPPEN!!! mhahahahahahahahaha.

#### ratstar

Aug 20, 2018
485
What causes the difficulty, too much resistance? and the capacitors are too small?

I only know how to make single pole filters, cause I'm only a beginner, and the maths is kinda complicated. (Ive copied alot of digital filter designers and got them to work but I didnt understand them.)

I know that a nice cut-off would be nice, but as a single pole filter - for 1 ohm resistance, you need a 1.5 nano cap, (I think, I could be wrong ) and they make those, bit hard to read the result tho as a charge passing through the metre.

but that only gives you -3db, but what would -3db do for your ringing? do you just filter it off? (but the oscillator has to be below the ring frequency for that to work.)

So is there any way to RAISE??? the ring frequency!?

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#### ratstar

Aug 20, 2018
485
So ok, I read a bit more, and ive got a solution already, but why is it called impedance, calling it parasitic inductance says it better.

#### Nanren888

Nov 8, 2015
623
Impedance is a general terms used somewhat differently in different areas, but generally it means Z = R + jX - so the general term including resistance and reactive contributions.

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