Measuring impedance with lab scope and current probe - Page 1
Forum section: electronics / general

author: raff5184
author_rank: Regular Contributor
timestamp: February 21, 2021, 04:00:06 pm
content: Hi all,I have a Tektronix lab digital oscilloscope with most of the functionalities enabled, like math functions, FFT, signal gen, and many others, and a current probe (CT6https://www.tek.com/datasheet/current-probe/ct1-ct2-ct6).What would be the right way of measuring the impedance (both real and imaginary part) of a passive load at a specific frequency? Is it simply generating a 1V sinusoid with the signal gen, and measuring current into the load, voltage across it and phase shift between the two? And then using the standard formulas?How do I know if the load is capacitive or inductive? If I'm correct the scope can't distinguish between rhe delayed phase of a cap load and the early phase of an inductive one (I'm not sure about this though)Thank you

author: nnills
author_rank: Contributor
timestamp: February 21, 2021, 04:52:17 pm
content: Hi,The maximum phase shift of a inductor or cap between the voltage and the current is +-90deg. You should therefore be able to distinguish between a leading(cap) and a trailing(inductor) current.

author: bob91343
author_rank: Super Contributor
timestamp: February 21, 2021, 10:38:13 pm
content: There is the ambiguity of orientation of the current probe. Resolve that by testing a resistor first. When the two traces overlap, you have the setup correct.Then substitute the unknown impedance and see what you have.

author: octillion
author_rank: Contributor
timestamp: February 22, 2021, 03:52:44 am
content: Mag(Z) is given by the ratio of your measured voltage to current at the applied frequency, while Arg(Z) is given by the phase difference. In general, the frequency response and electrical delay of the voltage and current probes will differ, so depending on the desired accuracy you may want to calibrate against a reference standard (like a low inductance resistor of magnitude similar to the device under test). At every frequency of interest, use the standard to scale the V/I ratio and set the phase reference.This will be especially important if you are:1) measuring at relatively high frequencies where differences in probe electrical delay give a phase difference large enough to matter.2) sweeping around a region where one of your probes has a not-so-flat response. If the probe's amplitude response varies, so does the phase, and vice versa. Amplitude and phase are fundamentally linked through dispersion relations. This will certainly be true if you are measuring around the upper or lower frequency bounds of a probe.

author: Berni
author_rank: Super Contributor
timestamp: February 22, 2021, 09:03:46 am
content: Yep that would work.You can even simplify it a bit by using a series resistor on the output of the signal generator. The voltage drop on that resistor is the current so by using a normal voltage probe before the resistor and after the resistor can give you both the voltage and current. But even here one should be careful when taking it into higher frequencies, the probes themselves can start loading down the circuit and introducing significant measurement errors.But alternative ways of doing it is using a LCR meter or a network analyzer.

author: raff5184
author_rank: Regular Contributor
timestamp: February 22, 2021, 12:42:55 pm
content: unfortunately I don't have a network analyzer. My max frequency is 2MHz

author: David Hess
author_rank: Super Contributor
timestamp: February 22, 2021, 05:40:30 pm
content: You say that your DSO is a lab unit so it should support differentiation and FFT magnitude and phase. Use an input step, differentiate the measurement to produce the impulse response, and then calculate the FFT magnitude and phase. Tektronix and HP had application notes describing how to do this but I wish you good luck in finding them now.EDN sort of covered it here.In my opinion all modern DSOs should have the functions to support the above, but actually fewer DSOs support it now than in the past.For the swept measurement method, probing the voltage at two points across the source resistance can be used to return magnitude and phase, so no current transformer is required. This is effectively the bridge measurement method.

author: octillion
author_rank: Contributor
timestamp: February 22, 2021, 06:28:00 pm
content: I should have mentioned errors from parasitics in the instruments and fixturing (like probe loading). These can be calibrated out by external triggering the oscilloscope with the function generator, and adding short and open measurements.This is getting a bit academic, so depending on what you are really trying to do will dictate whether it is worth the effort to remove the errors or buy a better test setup. Understanding the errors will at least tell you when they are significant.The phase difference between an open and load (a good resistor) will tell you, for example, about parasitic capacitance across the test site. The phase between short and load tells you about series inductance. If these phases match within acceptable bounds, and the resistive load has magnitude comparable to the DUT, then these errors can be ignored. You will also have contact resistance (important for low Z measurements) and leakage (important for high Z measurements), which you gain insight from the magnitude of the voltage when shorted, and magnitude of current when open.Error terms from the calibration measurements are not completely orthogonal, and depend on the specific setup and error model. Good calibration relies on an accurate error model that accounts for the sources of error which are significant (this depends on the test setup and the properties of the device to be tested), and requires open/short/load references which are either good representations of ideal or are well characterized.

author: mawyatt
author_rank: Super Contributor
timestamp: February 22, 2021, 06:37:27 pm
content: If your intend is to measure an unknown impedance by any means (not just a DSO) and the impedance is moderate, consider the inexpensive but very capable nanoVNAs available in various flavors.Best,

author: raff5184
author_rank: Regular Contributor
timestamp: February 22, 2021, 08:08:36 pm
content: Quote from: mawyatt on February 22, 2021, 06:37:27 pmI didn't know about NanoVNA. Looks cool but I wonder where the catch is. In the resolution? Are they actually good? I saw they even sell them on Amazon, I'm a little suspicious. I would need something relatively in the low range of frequencies. Max I need to measure is at 10MHz

author: Andreas
author_rank: Super Contributor
timestamp: February 22, 2021, 09:02:00 pm
content: Quote from: raff5184 on February 22, 2021, 08:08:36 pmThe minimum frequency of my NanoVNA V2 is 100 kHz.The maximum is 3 GHz.For lower frequencies (0.1 Hz - 20 MHz) I use my PicoScope 5444A with the FRA tool.https://www.picotech.com/library/picoapp/frequency-response-analyzer-with-bode-plotsBut of course you can only measure the "through" parameters with a bode plot and not the "reflection" parameters.Many scopes with a integrated function generator nowadays have a bode plot integrated.with best regardsAndreas

author: Andreas
author_rank: Super Contributor
timestamp: February 22, 2021, 09:34:51 pm
content: Hello,here you find some NanoVNA measurements of chokes (up to 250 MHz or 3 GHz)https://www.eevblog.com/forum/metrology/emi-measurements-of-a-volt-nut/msg3347342/#msg3347342compared to a scope measurement (max 20 MHz)https://www.eevblog.com/forum/metrology/emi-measurements-of-a-volt-nut/msg3335704/#msg3335704with best regardsAndreas