Current transformer for coupling noise into a cable

[Paul Colby]

From a topology point of view one could wind a transformer using the transmission cable as a secondary and a single conductor as a primary. The noise would be injected in the primary. Your initial concept works from a topology point of view. The 100MHz bandwidth is centered on what frequency?

 

[Baluncore]

Wouldn't an ideal center tapped transformer generate no flux in the core if the master line is balanced.

I thought you were testing to see if the line was balanced and therefore immune to CM noise.

 

[Nikhil N]

I thought you were testing to see if the line was balanced and therefore immune to CM noise.

Actually my experiment is..The master and slaves are communicating using modbus protocol, and I need to analyze the effect of noise on data loss in this communication. The line length is about 1000ft and communication standard used is RS485.

 

[Paul Colby]

Okay, so maximum is 12Mbps so much less than 100MHz bandwidth centered on your baud rate should work, right?

 

[berkeman]

Actually my experiment is..The master and slaves are communicating using modbus protocol, and I need to analyze the effect of noise on data loss in this communication.

That still sounds like a low-frequency version of EN 61000-4-6. We do tests like that all the time here in our lab.

Driving the center tap does cause a common-mode signal to be propagated down the wire. The fluxes do cancel, but the center tap is galvanically connected to both wires. The common-mode signal propagates down the wires to the receivers, which tests their common-mode rejection at their inputs. Especially for a DC-connected network like this one appears to be, the CM rejection will be pretty low. Nothing like it is for a transformer-connected multi-drop network.

EDIT -- the Leakage Inductances (Lk) will limit the bandwidth of this drive technique, but for the frequencies you are asking about, and the modest common-mode capacitance, it shouldn't be much of a problem. It's worth calculating it, though.

 

[Paul Colby]

Driving the center tap does cause a common-mode signal to be propagated down the wire.

For sure. Just move the center tap to the device under test rather than what's shown.

 

[berkeman]

For sure. Just move the center tap to the device under test rather than what's shown.

But the network devices shown are DC-connected to the twisted pair network. The only transformer shown is the coupling transformer...

 

[Paul Colby]

View attachment 112743

Modified to inject a common mode voltage.

The thing with the blue circle is what I was referring to. Seems to me the center tap is shown on the wrong side but what ever. I'm going to bow out. Just don't have the expertise nor can I tell what the OP is really after. The charm of the inductive injection coil he originally posted would be not having to introduce new components into the system. If your going to modify the circuit I'm sure there 1000 ways to do it with a simple network.

 

[Baluncore]

If your going to modify the circuit I'm sure there 1000 ways to do it with a simple network.

I agree.
To select one of those 1000 circuits we need to know why the test is being done and what will constitute a pass mark.

 

[berkeman]

The charm of the inductive injection coil he originally posted would be not having to introduce new components into the system.

Exactly. That's why the EN 61000-4-6 industry-standard test uses a broadband CM noise injection coil.

 

[Nikhil N]

That still sounds like a low-frequency version of EN 61000-4-6. We do tests like that all the time here in our lab.

Driving the center tap does cause a common-mode signal to be propagated down the wire. The fluxes do cancel, but the center tap is galvanically connected to both wires. The common-mode signal propagates down the wires to the receivers, which tests their common-mode rejection at their inputs. Especially for a DC-connected network like this one appears to be, the CM rejection will be pretty low. Nothing like it is for a transformer-connected multi-drop network.

EDIT -- the Leakage Inductances (Lk) will limit the bandwidth of this drive technique, but for the frequencies you are asking about, and the modest common-mode capacitance, it shouldn't be much of a problem. It's worth calculating it, though.

Actually I don't have much knowledge about these, But I have one more doubt, the slave and master connected only using two wires and there is no ground wire for a reference. So in that case if I am using a center tapped transformer where will I connect the ground of my noise source ?

 

[Nikhil N]

From a topology point of view one could wind a transformer using the transmission cable as a secondary and a single conductor as a primary. The noise would be injected in the primary. Your initial concept works from a topology point of view. The 100MHz bandwidth is centered on what frequency?

I don't know at what frequency it is centered on now. I will check its spectrum and let you know that

 

[Paul Colby]

Unclear I can be of much further assistance. A measurement spec was given so I'd suggest following that if this is a work related question. One note, if the bore of the toroid is large enough the transmission line could be looped multiple times through upping the generated noise voltage and modifying the turns ratio.