Electronics > Projects, Designs, and Technical Stuff
How to use a Network Analyzer With Current Probes?
SniperFox:
Hi everyone,
I'm wondering how to set up current probes to use it with a network analyzer. I don't know how to calibrate them properly for short, open, and 50ohm terminations (oh and through, too. SOLT). It's straight forward with BNC or SMA connections but what about current probes?
What I tried:
Short -> I made a small wire loop.
Open -> left it open.
Load -> I made a small wire loop with a 50ohm resistor.
Through -> I Put a small wire loop between both probes.
This didn't seem to work, as the trace has lots of spikes and is bouncing up and down all over the place... so I'm seeing nothing but noise when I try to take measurements.
I'm trying to measure S11 and S21 btw.
What I'm trying to do is use a network analyzer with current probes to measure the impedance of a LISN and SMPS power supply. This is step towards systematically designing a conducted EMI filter.
I found this neat scholarly article explaining how to measure those impedances under operating conditions with clamp-on current probes. See link below.
https://www.researchgate.net/publication/260589392_EMI_filter_design_Part_II_Measurement_of_noise_source_impedances
If you scroll down a bit, you can view the article without downloading it.
They explain everything very well and derive the equations well too. The problem is that they don't explain how to calibrate and use the VNA with the current probes.
They also don't say what probes or VNA they used. Although I am 90% sure that they used CT1 or CT2 probes because a few of their references use those probes.
If anyone wants details on the instruments I'm using: I'm using E5061B VNA and two P6021 current probes.
(I tried searching the forums for this topic but could not find anything)
graybeard:
I made a video on how to do it with the bode plot function with a Siglent o-scope
https://youtu.be/T6mbTblw4QE
graybeard:
I read the paper. It was very interesting. However as they demonstrate with their measurements the frequency range is limited to about 2 decades with a high end in the 10s of MHz.
The calibration method is implicitly described in the paper.
You first need to establish your S parameter calibration planes at the current probes as shown in Figure 2. If you are using the typical 50 Ohm Tek CT1 or CT2 probes you will need to make a set of female SMB calibrations; short, open, load, and through to place the s parameter calibration planes as close as possible to the current probes.
You then need to make two calibration measurements with the current probes; one with a resistance standard whose value is near the impedance you expect to measure, and one with a short. You first get the ratio of the probe voltages at every frequency using equation 9 for both measurements; RSTD and short. Substitute that result into two equations 10 & 11 and solve for the complex quantities K and Zsetup at each frequency point.
Since you now have K and Zsetup you can measure your unknown impedance and using equation 8 calculate the unknown impedance at each frequency point.
This method will not work with the P6021 current probes you are using.
I hope that helps.
. . . Chris
SniperFox:
Thanks! I'll check out the video on Monday when I have time.
--- Quote from: graybeard on February 15, 2020, 12:59:44 pm ---I read the paper. It was very interesting. However as they demonstrate with their measurements the frequency range is limited to about 2 decades with a high end in the 10s of MHz.
--- End quote ---
This shouldn't be a problem as I only need to measure for the conducted EMI range of frequencies (150kHz-30MHz).
--- Quote ---The calibration method is implicitly described in the paper.
You first need to establish your S parameter calibration planes at the current probes as shown in Figure 2. If you are using the typical 50 Ohm Tek CT1 or CT2 probes you will need to make a set of female SMB calibrations; short, open, load, and through to place the s parameter calibration planes as close as possible to the current probes.
--- End quote ---
What is "SMB"? Is that the type of connection on the CT1 & CT2 probes?
Also, if I use a CT1 or CT2 probe: Where do I do the short, open, load, and through? At the green arrow or at the red arrow? (see the attached picture)
--- Quote ---You then need to make two calibration measurements with the current probes; one with a resistance standard whose value is near the impedance you expect to measure, and one with a short. You first get the ratio of the probe voltages at every frequency using equation 9 for both measurements; RSTD and short. Substitute that result into two equations 10 & 11 and solve for the complex quantities K and Zsetup at each frequency point.
Since you now have K and Zsetup you can measure your unknown impedance and using equation 8 calculate the unknown impedance at each frequency point.
--- End quote ---
Yeah, I understand this part and it's pretty cool how they came up with this.
I just don't know how to get the setup ready and prepared to do these Rstd and the short measurements.
--- Quote ---This method will not work with the P6021 current probes you are using.
--- End quote ---
How come it won't work with the P6021? This is what I suspect, but why? Is it because the P6021 requires a 1Mohm termination?
graybeard:
--- Quote from: SniperFox on February 15, 2020, 06:24:38 pm ---What is "SMB"? Is that the type of connection on the CT1 & CT2 probes?
--- End quote ---
The SMB is the connector on the current probes.
--- Quote from: SniperFox on February 15, 2020, 06:24:38 pm ---Also, if I use a CT1 or CT2 probe: Where do I do the short, open, load, and through? At the green arrow or at the red arrow? (see the attached picture)
--- End quote ---
The red arrow, the green area point to a mounting screw, not a connector.
--- Quote from: SniperFox on February 15, 2020, 06:24:38 pm ---How come it won't work with the P6021? This is what I suspect, but why? Is it because the P6021 requires a 1Mohm termination?
--- End quote ---
It is both high impedance (1MOhm) which is way to far away from 50Ohms and non-reciprocal (it has a built in amplifier) and thus S11 will be independent of the load and the current probe will not transmit any energy into the load for the S21 probe to detect.
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