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In Reply to: Re: Hi John posted by Steve Eddy on June 22, 2004 at 10:34:09:
JC stated that he has put a poly across the output, supressing some of the harmonic distortion of the analyzer by pushing it to ground..If that causes ground bounce, well? Don't forget, we're talkin -120 dB or so down..
Cheers, John
Follow Ups:
JC stated that he has put a poly across the output, supressing some of the harmonic distortion of the analyzer by pushing it to ground..If that causes ground bounce, well? Don't forget, we're talkin -120 dB or so down..
Ok, but in that scenario, the distortion is inherent to begin with rather than being caused by the ground loop which your original post seemed to imply.
se
Steve: ""Ok, but in that scenario, the distortion is inherent to begin with rather than being caused by the ground loop which your original post seemed to imply.""I though I was being clear, obviously not..
Yes, it is the inherent distortion..and by pushing the distortion products into the ground with the poly cap, you are not getting rid of it, merely changing it's current path. If you are not careful, the current's mag field can couple to conductor loops that the field goes through. If the cable under test forms a loop that couples to that "shunted" distortion products, it will generate a voltage within.
That would make the distortion product at the analyzer susceptible to the shield guage, the connection resistances, etc..which was reported by JC...along with the TV scan signal susceptibility..which again confirms the possibility of a loop.
Cheers, John
'distortion' is essentially the same with or WITHOUT the cap to ground.
John...Thank you for trying it..Does the distortion change if you take the cable under test, and loop it tightly, to try to increase it's susceptibility to external fields? If the cable is actually intercepting fields, I would expect both the distortion and the tv scan frequencies to increase in magnitude.
Again, thanks..
Cheers, John
No, interference sensitivity changes, however.
Thanks..Since it's a coax, there should not have been an increase in interference from a far field source, the gradients across the coax diameter aren't large enough to support differential excitation..
The fact that interference increased means the equipment isn't fully rejecting the common mode signal increase between the shield and the hot..the two concentric loops formed by the coax are feeding different impedances, that result is logical.
If the distortion had increased along with the interference, I would have guessed that the ground current was returning along the panel between the input and output jacks..and the loop was sensitive enough to present them..
I don't think the inner conductor resistance is in play, I still remember you talking about the contacts affecting the reading, I just can't seem to get away from that.
Can you rerun a cable, but include some honkin big shorting conductor from output shield to input shield, in an attempt to change the ground loop resistance??
I'll try to get a good picture of the st1700, along with something in the way of internal layout and schematic. That way, I'll be able to provide some more intelligent possibilities to test..
Have you done any correlation between physical parameters of the cables and the results of your test? I was thinking about cable capacitance, resistance, inductance..and, although this might be stretching it a bit, the dielectric type?
With all the DA talk, is it possible your rig is more susceptible to that than the AP?
JC...Thank you for being patient and taking the time to try these things..
Cheers, John
"JC...Thank you for being patient and taking the time to try these things.."This is how we make progress i.e. by asking questions and sharing results. Two heads are better than one. That is, unless one of the heads is trying to bite the other one off : ) Sean
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