Why use VCCS for impedance measurements vs just a voltage source

[middlehein]

I am working on trying to make some impedance measurements and have been reading through a bunch of research papers and everyone seems to be using a Voltage Controlled Current Source(VCCS) for their measurements. What I can figure out exactly is why they are doing that as compared to a Voltage Controlled Voltage Source(VCVS). To the best I can tell it may have to do with noise, but I have no clue, nor am I a electrical engineer by trade so I am learning a lot here and can find this answer.

 

[LvW]

Using a VCCS for impedance measurements means that you are going to measure the VOLTAGE across the device under test (DUT).
In case of a VCVS it would be necessary to measure the CURRENT through the DUT what is less convenient.

 

[crutschow]

Applying a voltage to an impedance and measuring the current gives a non-linear relation between impedance and current. This was used with the old analog multimeters which had a logarithmic scale for the resistance range.

Applying a current and measuring the voltage gives a linear relationship between impedance and voltage. This is the method used with most digital meters to give a linear output scale.

 

[D.A.(Tony)Stewart]

For reactive parts they use a current source with a signal like 1KHz 100KHz 1MHz to get a voltage proportional to current and thus impedance.

Other methods use bridges to match known values to unknown values and get equal results.

 

[Ratch]

middlehein,

What impedance, input or output?

Ratch

 

[WimRFP]

In addition to Ratch's comment, what frequency range?

Basically you can use whatever source, but this may complicate the math. I frequently use a source with known but not zero or non-infinite output impedance.

By measuring voltages and/or phase (oscilloscope), you can calculate the impedance with vector calculus. Depending on impedance and frequency, you may need to correct for the probe's input impedance.

At high frequency directional couplers are used to determine the complex reflection coefficient. With transmission line calculus the impedance can be calculated and corrections for transmission line length can be added. This is mostly done automatically in a (single port) vector network analyzer (VNA) via the calibration procedure with a short, open and load.

Before the advent of the VNA, HF impedance could be measured by determining the VSWR along a slotted transmission line (with diode detectors). From the VSWR itself and the shape of the voltage along the slotted line, the impedance can be calculated (or "calculated" graphically with a Smith Chart).

 

[FvM]

I completely agree with the comments about the "natural" way to perform impedance measurements, getting an output quantity that directly correspond to impedance.

Technical impedance measurement setups often take the opposite way. It's the case with most high performance RLC instruments. Also Analog Devices nice network analyzer chips are using voltage excitation and current measurement. https://www.analog.com/en/rfif-components/direct-digital-synthesis-dds/ad5933/products/product.html