Maybe you should have e-mailed Kurt.

Posted by jneutron (A) on October 25, 2004 at 11:13:27

In Reply to: PS posted by Jon Risch on October 21, 2004 at 21:29:09:

He would have answered your questions, and you would not have needed to harp on with "false representation", "not representative"...I already pointed out to Kurt that the article should have included that information as it is relevant, but he was scrambling to get ready for his brother's (I think) wedding.

At BJC, they terminated the opposite end with a source component that had been turned on, so a low impedance was presented to the far end of the cable..while a real amp input was used at the close end.

What struck me was the fact that they used a FLAT extension cord, not one that was twisted. That fact in itself means the stimulus was probably not consistent with typical applications, but was a worst case scenario.

As for rating based on hearing the hum??why not?..certainly not very accurate, but certainly meaningful with respect to relative magnitude.

As for the singly terminated shield, with cap at the other...I agree with you, that will help with hf while preventing loop currents at 60 hz. However, even if you connect the shield, it is not the shield currents that is coupling to the signal wires within the cable that is the issue, but rather, what the shield currents do in the equipment on either side of the cable. Because, a shield current does not produce magnetic field inside the shield, only outside..a consequence of one of those "laws" that was written by some dead guy..

As for amp current levels being lower by a lot??? Nope...the actual power factor of a cap input supply filter is very small, warranting lots of breaker overkill because of 3rd harmonic current draw heating....breakers are rated for the fundamental frequency draw, not the third harmonic. And the third and fifth harmonics are rather high. AND, don't forget, the coupling goes as the frequency, third harmonic couples 3 times more to shield, and 9 times more to input.

For coax, when it is far-field, it exhibits very good rejection of common mode coupling at any frequency....but, when a coax is very close to the dipole field of a line cord, the GRADIENT of the external dipole field will couple differentially between the shield and the center conductor..that is because the integrated field to the center is not the same as the average of the shield integrated field..if the field fell linearly, that would not be the case...This means, a voltage will be induced between the center and the shield..whereas, a twisted pair will average the effect out along the length.

You are correct in that shielding is useless at 60 hz...the mag field gets through anyway. Either common mode it away, or break it with the cap, as you stated.

The test they did is quite good when you have to drape the cable around real world components like amplifiers, televisions, and the such, where they broadcast mag fields.

Cheers, John

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Follow Ups Full Thread

Topic - Whether to terminate IC shield to ground with a Cap or not? - Tony Montana 15:30:13 10/20/04 (14)

Re: Whether to terminate IC shield to ground with a Cap or not? - Jon Risch 18:00:05 10/21/04 (9)

Where is Jneutron when we need him :) - Tony Montana 19:54:25 10/21/04 (8)

Re: Where is Jneutron when we need him :) - jneutron 11:20:54 10/25/04 (2)

Speaking of the devil :) - Tony Montana 21:56:00 10/25/04 (1)

Re: Speaking of the devil :) - jneutron 07:11:16 10/26/04 (0)

Re: Where is Jneutron when we need him :) - Jon Risch 21:14:08 10/21/04 (4)

PS - Jon Risch 21:29:09 10/21/04 (3)

Maybe you should have e-mailed Kurt. - jneutron 11:13:27 10/25/04 (2)

Re: Maybe you should have e-mailed Kurt. - Tony Montana 22:06:27 10/25/04 (1)

Re: Maybe you should have e-mailed Kurt. - jneutron 07:29:41 10/26/04 (0)

Re: Whether to terminate IC shield to ground with a Cap or not? - blackrock john 09:04:52 10/21/04 (0)

Re: Whether to terminate IC shield to ground with a Cap or not? - Top Jimmy 23:49:45 10/20/04 (2)

Re: Whether to terminate IC shield to ground with a Cap or not? - Tony Montana 20:03:10 10/21/04 (1)

Re: Whether to terminate IC shield to ground with a Cap or not? - Jon Risch 21:21:36 10/21/04 (0)

Follow Ups

Maybe you should have e-mailed Kurt. - jneutron 11:13:27 10/25/04 (2)

Re: Maybe you should have e-mailed Kurt. - Tony Montana 22:06:27 10/25/04 (1)

Re: Maybe you should have e-mailed Kurt. - jneutron 07:29:41 10/26/04 (0)

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