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Shielded/Unshielded Twisted Pair (STP...

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741
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Post Number: 798
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Posted on Friday, 15 July, 2016 - 08:30 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Evenin' all

I'd like to nail the different issues that (1) twisting & (2) shielding address, and the shared issues too.

So far, it seems to me that:

Firstly, twisting lowers loop inductance.

Also, if one 'wire A' is closer to interference than 'wire B' at some point, then the twisting means the converse is true on the 'next twist', thus keeping any external interference effectively common-mode.

Shielding (at one end?) can prevent EMI. If the outer cable is shielded the EMI excluded is external EMI, if pairs are shielded, the shielding also prevents crosstalk (though I can't really then see the point in also adding an over-all outer shield).
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twintub
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Post Number: 404
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Posted on Friday, 15 July, 2016 - 10:09 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

AFAIK....

Twisting reduces the crosstalk between the two wires of a pair. In the case of ethernet, the signals on the wires of each pair are in opposite phase (as line-driver/receivers are being used) therefore the reduced crosstalk results in less degradation of their signals. Any EMI will affect the two wires equally, so there is nil effect.

Any shielding should ideally be grounded at BOTH ends, but only if the two ends can be grounded to the same potential in order to prevent a groundloop. If that's not practically possible, then the cop-out is to ground one end only. If line-driver/receivers are being used, shielding is unnecessary unless the environment has very high EMI levels which can swamp the maximum voltage that the line-drivers/receivers operate at, thus causing an imbalance in their signal levels.

I could be completely wrong of course, and I'm sure there's gonna be several corrective responses!
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phonoplug
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Post Number: 430
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Posted on Friday, 15 July, 2016 - 11:39 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

In Ethernet cable each pair has a different number of twists per meter. This is to reduce crosstalk between pairs.

Twisting maintains impedance along the cable.

Sending a balanced/differential signal down a cable is intended to reduce the effect of interference as it affects both positive and negative parts of the balanced signal equally, so when you look at the difference it still the same.

Shielding helps in noisy environments, perhaps where absorbed interference could mean the signal received exceeds acceptable limits for the line receiver.

Hope thats useful info!
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james
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Posted on Saturday, 16 July, 2016 - 07:09 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hi

If pairs were not twisted then it is possible for two wires in adjacent pairs to be next to each other for the entire length of the cable. That could add cross talk to only one core of the pair. By twisting each pair any electromagnetic noise is added equally to each wire in each pair and this noise can therefore be removed by the line receiver.

A subtracter can be used as a line receiver to remove any common mode noise induced in each pair.

Subtracter

Vout = (R2/R1)*(V1-V2)

If all resistor values are equal then:
Vout = V1-V2

Consider a balanced signal coming in with no noise.
V1 = 5V
V2 = -5V

Vout = V1-V2 = 5-(-5) = 10V

The subtracter has had a doubling effect on the signal.

Now consider a noise voltage of +1V (for arguments sake) induced equally in each wire on top of the 5V signal.

V1 = 6V
V2 = -4V

Vout = V1-V2 = 6-(-4) = 10V
The same!

The common mode noise of 1V has been ignored.


Another way of looking at this is the output voltage will be at whatever reference voltage is connected to the bottom of the lower resistor, (in the above case 0V), when the two inputs are connected together. Independant of whatever voltage the inputs are commonly connected to.

Vout = V1-V2 and if V1=V2 then Vout = 0V.

In practical reality, with discrete resistors, it is difficult to get equal resistor values so the common mode noise cancellation effect of the subtractor is compromised somewhat.

Cheers

James
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rob_guyer
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Post Number: 204
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Posted on Saturday, 16 July, 2016 - 08:36 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

I found this application note from Analog Devices helpful. http://www.analog.com/media/en/technical-documentation/application-notes/41727248AN_347.pdf. To my mind, screening isolates the pair from an applied alternating magnetic field but not from an applied electric field. This because there are 2 series capacitances between an applied E field and the pair. The fix is t figure 23
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james
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Posted on Saturday, 16 July, 2016 - 06:32 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Using a more intuitive approach to determining the subtractor's output voltage rather than just using Vout = V1-V2.

Using the two basic DC opamp rules:
1) The output goes to a voltage which puts the two inputs at an equal voltage.
2) The inputs draw no current.



Subtractor

Assuming R1=R2=R3=R4.

R3 & R4 form an equal value resistor potential divider giving +2.5V at the +ve input.
R1 & R2 also form an equal value potential divider, (current in R1 must equal the current in R2), and -5V at the input to R1 forces the output to go to +10V in order to put the -ve input also at +2.5V.
+7.5V across R1 and +7.5V across R2.


Now using a similar analysis with +1V of noise added to each input.

Subtractor with noise

By the same potential divider action there is now +3V at the +ve input.
The output must still goto +10V in order to put the -ve input also at +3V.
Now +7V across R1 and +7V across R2.

Therefore the signal is amplified and the noise is cancelled out by the subtractor action.

This type of intuitive approach can be used with most opamp circuits for a range of input voltages provided the opamp's output is not saturated.

Cheers

James
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istedman
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Posted on Sunday, 17 July, 2016 - 10:11 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hi,

The other advantage to shielding, not covered is lightning protection. Take a look at the potential (pun intended) transient waveforms here:
http://www.microsemi.com/document-portal/doc_view/14681-lightning-protection-for-aircraft-per-rtca-do-160d-for-arinc-429-protocol

The old Arinc bus described in that appnote there has no shielding, hence the addition of transient voltage suppression devices. If a fully shielded cable is used, with high input impedance at the receiver and a series protection circuit at the transmitter, you can be confident of surviving the 1500V/60A transient without adding expensive TVS devices.

The other more benign advantage of shielded cable is it helps reduce the radiated emissions from the cable. I've used shielded, twisted pair on digital video cables. If there is a small break in the shielding, there will be a spike in the emissions, as the 40 MHz pixel clock caused an EMC failure.

Going back to Ethernet CAT5 cable, UTP cable has a differential impedance of 100 ohms, STP has a differential impedance of 150 ohms. Most modern ethernet devices can accommodate this.

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