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Old 6th March 2012, 05:23 PM   #21131
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It shows otherwise. Even adding in 3nF for the leads, the impedance at 150MHz is a whopping 2.5 ohms.
And with a typical 70 nH? (My back of envelope says under 6 Mhz resonance.)

Last edited by simon7000; 6th March 2012 at 05:30 PM.
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Old 6th March 2012, 05:37 PM   #21132
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If you want a serious RF bypass solution, contact Scott Wurcer. He helped me 28 years ago with his suggestions, based on REAL measurements.
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Old 6th March 2012, 05:37 PM   #21133
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P5

There was an Elector article using BF*** parts, I guess this was best for 1979 and integrable but not hard to do with only 4 or so paralleled modern devices. The link got tiny, P5 above. Not a very special circuit, giant electrolytic at input yuk.
Thanks! Yes, via a search I also see that W & B are available but cost money, and I really don't need to know how they achieved the 1.5 ohms as I am not about to make transistors.

Yes, the Nordholt & van Vierzen schematic is not, errrrr, very appealing.

Brad
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Old 6th March 2012, 06:22 PM   #21134
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Originally Posted by simon7000 View Post
And with a typical 70 nH? (My back of envelope says under 6 Mhz resonance.)
Where does that number come from? Your reference from a few posts back is down the memory hole?

edit: I calculate maybe 20nH worst-case. That brings up the series impedance to a mind-boggling 12 ohms at 100MHz. Ed's erstwhile reference shows a smaller inductance. Who cares? 12 ohms is far and away sufficient.
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Old 6th March 2012, 06:52 PM   #21135
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Where does that number come from? Your reference from a few posts back is down the memory hole?

edit: I calculate maybe 20nH worst-case. That brings up the series impedance to a mind-boggling 12 ohms at 100MHz. Ed's erstwhile reference shows a smaller inductance. Who cares? 12 ohms is far and away sufficient.
The reference was for a capacitor in a typical circuit environment. I showed it to demonstrate that small leaded ceramic caps run out of bypass usefulness at fairly low frequencies. That showed a 1 nF cap the initial question was for a 10 nF. So we have more capacitance and much more lead length.

One end of the capacitor is connected to the case via some small (<5 nH) lead. The other end is connected to the shield. Pick your best guess for a 72" lead length (or 36" if you prefer.) That is where the resonant circuit would be.

If we were to treat the shield as just a freestanding length of wire .09" dia by 36" L the inductance would be 1 uH (CRC formula). Although the shield is hollow the inner conductor is not connected via a low impedance path to ground.

As to the loss for 12 ohms 20log(110/12) = 19.244 db. Which would help but really is not a guarantee of EMI proofing a circuit.

Small ceramic capacitors are effective for AM commercial radio filters. AM is the worst problem for audio gear. But you will not pass current EMI standards if that is the primary tool in your kit.
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Old 6th March 2012, 07:03 PM   #21136
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You're all over the map here. We're talking about a ceramic cap from input connector to chassis. 1/4" leads, max. If you put 72" leads on it, you're designing for Martians.
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Old 6th March 2012, 07:31 PM   #21137
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Sy,

We probably can agree if you have an RCA connector mounted in an insulated bushing with nothing connected to it, RFI will not be a problem even with a 10 nf ceramic capacitor with 1/4" leads.

Now what is the effective lead length when you plug something into the jack?
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Old 6th March 2012, 08:01 PM   #21138
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Still the same. That's the whole point of the cap to the chassis; that's why they call it a "bypass." I hope you're not suggesting to attach one lead to the sending end, the other to the receiver chassis.
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Old 6th March 2012, 08:03 PM   #21139
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Still the same. That's the whole point of the cap to the chassis; that's why they call it a "bypass." I hope you're not suggesting to attach one lead to the sending end, the other to the receiver chassis.
Then why bother worrying about the cap lead length to begin with?
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Old 6th March 2012, 08:25 PM   #21140
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That was your worry, not mine; I was just putting some numbers to it to try to figure out what you're on about. Bypassing the input plug to the chassis with a small ceramic cap with reasonably short leads is a standard method, and it's standard because it's effective. I think my Collins S-Line even used the same method. I put some numbers to your example, and that led to less physically reasonable examples.
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