I have a planar air core (no ferrite) transformer in a PCB. I need to characterize the transfer function of this transformer from DC to 500 MHz using a vector network analyser (VNA).

My first question is regarding measurement method selection: I’ve used a VNA before to characterize coils and capacitors so: first I would check the expected inductance or capacitance of this device under test (DUT) in a diagram like the one below, then I would use the measurement method with the best accuracy for the given frequency range.

enter image description here

Now my first question is, how to select the measurement method in the case of a transformer?

My second question is regarding the smallest signals that can possibly be measured with the VNA: I am afraid that if the voltage or current in the secondary are too small then the VNA won’t be able to measure them. My problem here is that no matter how I connect the transformer, one quantity (voltage or current) will be smaller than the primary, but as far as I know VNAs work with power (S parameters) measurements, so I don’t know how to decide (I’m dealing with a transformer with a very large ratio). How to determine what is the smallest signal the VNA will be able to measure?

Many thanks in advance!


Lets assume your noise floor is set by the termination resistors. These will n 50 ohms.

The noise density is 0.89 nanovolts/rootHertz; in 100Hertz bandwidth (a very slow frequency-sweep rate (so slow the VNA internal frequency synthesizer might set the noise floor), your nois floor will be sqrt(100) * 0.89nanovolt/rtHz

Or 8.9 microVolts rns

  • \$\begingroup\$ So any voltage above 8.9 microvolt should be measurable. Right? And how about the current? \$\endgroup\$
    – Ken Grimes
    Mar 5 '20 at 18:23

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