Rogowski coil CWTUM03 (30 MHz) gives a 50 ohm resistance when I measured between its center pin and outer surface of BNC connector. When I measure any signal through a coax with 50 ohm selected on scope as input resistance, signal is attenuated to 50% which could only mean that there is a series resistance in rogowski probe as well. I could understand the parallel 50 ohm resistance at the output of rogowski before coax cable as a seperate terimation irrespective of what coax user connect, but why the series resistance??
\$\begingroup\$
\$\endgroup\$
Add a comment
|
1 Answer
\$\begingroup\$
\$\endgroup\$
6
The 50Ω resistance you measured IS the series resistance (source resistance) of the coil. Why would you think otherwise?
It is there to provide a good match to the cable, eliminating potential reflections that would confuse high-speed measurements.
You use the 50Ω termination on the scope for the same reason.
This combination of source impedance and load impedance creates a voltage divider that always results in a 50% reduction in voltage when compared to the open-circuit voltage. Sometimes test equipment takes this into account when displaying the data; sometimes it doesn't.
answered Dec 11, 2017 at 15:12
-
\$\begingroup\$ Thanks for replying Dave..first question, how could i have measured series resistance when I measured between inner pin and outer surface of BNC connector (at the output of rogowski probe without the cable), second question , would you say this ' If the line is fed with signals via a resistance equal to the characteristic impedance, it doesn’t matter if the far end is not terminated by the correct resistance; Any reflections from the far end will be absorbed by the source resistance.' is correct and the reason for series 50 ohm resistance? \$\endgroup\$– AutobotDec 11, 2017 at 15:34
-
\$\begingroup\$ and the parallel resistance of 50 ohm is to counter reflections of coil and internal circuit just before the cable \$\endgroup\$– AutobotDec 11, 2017 at 15:36
-
\$\begingroup\$ "How could i have measured series resistance when I measured between inner pin and outer surface of BNC connector?" I'm not sure how to address this. The coil itself has a low impedance, especially if it contains an integral integrator/buffer. Any additional resistance must be in series with it in order to bring it up to the cable impedance. \$\endgroup\$ Dec 11, 2017 at 15:43
-
\$\begingroup\$ "it doesn’t matter if the far end is not terminated by the correct resistance" Yes, that's mostly true. But you still need to account for the effects of the mismatch at the load end in terms of the voltage error. \$\endgroup\$ Dec 11, 2017 at 15:46
-
\$\begingroup\$ Ok, so that means there is just one resistance, that too in series with the coil and would be in series with cable when it is connected and it absorbs reflections from scope end. So when I look up datasheet of a current sensor and it says for example output resistance to be 50 ohms , that will always be a series one, right? \$\endgroup\$– AutobotDec 11, 2017 at 15:52