The RTA 4000 scope response has an positive gain. Am I doing something wrong?

Thread Starter

Kai03

Joined May 24, 2021
4
In my test setup, I have mounted 50 ohm BNC ports to the input and the output of an passive DM EMI filter. I have connected these ports to RTA 4000 with 50 ohms coax cable. I have set the function generator input impedance to 50 ohms.

With a Tee adaptor, the function generator splitted into two signals. One goes to the input of the filter and the other one goes to the channel 1. I have connected the output of the filter to channel two.

Unexpectedly, I am seeing positive gain. How can a passive filter show a positive gain?

The response:
1621845181465.png
 

Yaakov

Joined Jan 27, 2019
3,338
A bit hard to know what your setup actually is based on a narrative, but without the filter in place, that is, measuring only the coax, what do you see?
 

Thread Starter

Kai03

Joined May 24, 2021
4
A bit hard to know what your setup actually is based on a narrative, but without the filter in place, that is, measuring only the coax, what do you see?
I have forgot to mention that between the input of the filter and the output of the filter I have placed isolation transformers. These transformers are 1:1 transformers. All in all, the test setup is similar to
1621846282262.png
Fig. DM filter test setup in [1]

When I replace the DUT with a coax the response I get is

1621846168301.png

Thanks for your fast reply!

[1] I. F. Kovačević, T. Friedli, A. M. Muesing and J. W. Kolar, "3-D Electromagnetic Modeling of EMI Input Filters," in IEEE Transactions on Industrial Electronics, vol. 61, no. 1, pp. 231-242, Jan. 2014, doi: 10.1109/TIE.2013.2242422.
 

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Thread Starter

Kai03

Joined May 24, 2021
4
Just to be sure, you have the DUT rig calibrated, it shows no gain without the filter inserted?
I have not done any calibrations because in the user manual of rta4000 there wasn't a part where it mentioned calibration.

Yes It shows no gain without the filter.
 

Yaakov

Joined Jan 27, 2019
3,338
I will leave it to others to respond who might have an insight. I am thinking about it, but I don’t have any further input just now. I expect you will get some useful help in time.
 
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