See how you can do fast TDR measurements using a network analyzer or VNA in this video. Whenever you are designing boards with specific characteristic impedances, you will want to be able to verify that on your fabricated boards. It’s recommended to put in test traces on each layer, but how do you measure this. Getting access to a good TDR may be difficult, as this is still a quite expensive piece of equipment.
How about using a low-end VNA instead? In this video, we set up the ZNL3 (up to 3GHz) VNA from Rhode & Schwarz to do this measurement to see how much time-domain resolution we get. Obviously the higher the frequency range, the more time/distance resolution you get. This is similar to how you get more resolution from a TDR with a shorter rise time.
Take a look and see the performance when using this ZNL3 network analyzer as a TDR on a simple trace impedance measurement.
(also available directly on YouTube)
The board used in the video is the test board we use for the signal integrity course at Danish Technical University (which btw just ranked 2nd in the world – not sure why, but I think it’s partly because of this course :).
As you can see, by using a test trace of only a bit more than 10 cm you can actually get a decent TDR measurement of the trace impedance with this low-end VNA.
Compare this to the measurements done with a “real” much more expensive TDR some time back.
DISCLAIMER: R&S provided the ZNL3 on loan for EE-Training.
M.Sc.EE, SI Consultant
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