Electronics > Beginners
Common ground ringing
T3sl4co1l:
In fact, referring to page 16, I just recreated the diagram exactly as shown. Generator: function generator, 50 ohm source, 3m cable, signal pin to scope probe ground ring, probe set to 10x, scope set to 200mV/div.
Note, that's with the probe tip open circuit (this is justified below), clippy tip and ground clip removed. Just the coaxial pin and ring exposed.
I see about 20mV. The waveform shows it's approximately flat in bandwidth, probably either residual coupling from the open circuit tip, or actual leakage in the probe shield.
The signal amplitude by the way is 2.5V, so the shielding effectiveness is at least 40dB.
If I touch the probe tip pin, the signal becomes considerably stronger, and may become peaky as well. This is due to my body capacitance and resistance acting to ground the probe tip, at some frequencies or all, while the shield voltage moves independently of it. This proves that 1. the probe tip is moving with the shield when it's open-circuited, and 2. there is real, and substantial, voltage dropped across the probe shield, which does couple with the signal line within.
I sincerely hope that PowerPoint isn't based on a real book. I've heard loopy things about the Johnson book (but haven't reviewed it myself to see just how loopy it is), but this is just flat out, and very easily demonstrably, wrong, information.
Tim
schratterulrich:
I made the following measurement setup:
It shows the equivalent circuit diagram of the coaxial cable - the screen and the inner conductor are inductively coupled (L1, L2). however, it also shows an ohmic resistance (R1). And this is not coupled into the signal.
Simulation Results:
The difference signal of the coax shield and inner conductor is measured with channel 1 at the oscilloscope end.
I measured the signal across the shield with a 1:1 probe with channel 2.
The measurements show the same result as the simulation.
The signal of the coax inner conductor stays is at 1,5 mV indepentent of frequency. (yellow trace)
The signal across the shield is also 1,5 mV at low frequencies but increases at higher frequency (inductance) (blue trace)
And that is exactly what Johnson claims in the book.
T3sl4co1l:
Also a good introductory demonstration of creating an EMC simulation. :)
Tim
schratterulrich:
I searched the web for "transfer impedance" and found the following interesting link:
https://interferencetechnology.com/differential-transfer-impedance-of-shielded-twisted-pairs/
It shows how the signal of a coaxial cable can drive the shield into radiation!
I wanted to try this myself and connected the shield of a coaxial cable to port 1 of my VNA.
Then I connected the inner conductor to port 2.
Something like that
My guess was that if the cable length is in the quarter wavelength range, there will be a radiation resistance in addition to the ohmic resistance. This should lead to a larger coupling to the inner conductor...
The result shown as S21 parameter:
At 313 MHz the S21 (represents the shielding effectiveness in this case) peaks at -25 dB !!
My conclusion from this experiment is that the shielding effectiveness of a single braid cable can be very bad.
I haven't thought about this circumstance yet.
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