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Frequency Response Analysis (FRA) First Point Glitch?

Question asked by glowitz on Jan 15, 2019

Is anyone on this forum using the FRA (frequency response analysis) tool on their X-series scope? I have a ticket pending with customer service, but in my case, the very first sample of the FRA sweep is often random. The image below shows a very simple case where the DUT is a simple BNC cable, being swept from 20 Hz to 10 MHz. As shown, the first sample in this case is showing 12.2 dB of gain and nearly 180 degrees of phase shift. Obviously this isn't the case. Increasing or decreasing the number of samples per decade still has the same symptom, but only for the very first sample.


I have been able to reproduce this issue on a wide range of DUTs (including just a straight BNC cable, amplifiers or simple attenuators), and it only affects the very first sample. It doesn't always do it -- but often will. My scope is an MSOX4034A and was recently recalibrated by Keysight, so I'm assuming (for now) that this is a firmware issue in the FRA application, and not related to my scope's performance. Sometimes I need run the plot several times in a row to get a "clean" one, which can be very time consuming when doing 100 samples per decade. It's as if the signal is still settling before taking a clean sample, but then after the first sample, it settles down. Either that, or something is stuck in a buffer from a previous sweep, or aborted sweep, and is getting re-insert on the first sample of the next plot.


I'm curious if anyone else can reproduce this symptom using just a simple BNC cable split off from the internal function generator to feed channel one and two of the scope. I've also tested with a range of voltages, but this doesn't seem to correlate either. For the purpose of this test, try voltages between 100 mV and 1 V, and 10 samples per decade (just for speed), but it can happen at other voltages as well. Then repeat a few times. Any insights appreciated!


FRA Sweep of BNC Cable Showing First Sample Glitch