Hi, I am new to signal processing. I am facing some difficulties in measuring the signal from an acoustic emission sensor for my project. I am using PXI-6115 with 1042Q and terminal block is TB2708. I used AI0 and AI1 for main signal and trigger respectively. I acquired signal from 6115 in SignalExpress (v3.0) and convert to linear spectrum (Hanning window and RMS conversion) with RMS averaging at 200. Same settings are used in oscilloscope (LeCroy LC564DL) too. I connected both signals to oscilloscope as well for comparison. I saved the spectrum data from SignalExpress and oscilloscope and plotted. You can see the graph in attachment. It is totally different. Why is that so? And one more thing is if I disconnect my connector to NI DAQ system, the spectrum in oscilloscope changed in magnitude at certain frequency and vice versa.Thank you in advance. Myo

My apologies for the late reply. I have been out sick for a couple of days. I generated a 1V amplitude (2V Pk-Pk), 100kHz, 10MS/s, 20ksample sine wave using Create Analog Signal. I processed this with Power Spectrum using 200 linear averages with RMS algorithm. The value at 100kHz was -3dB, as expected and as it should be. However, at one point in the process, SignalExpress coerced my frequency from 100kHz down to 50kHz (probably due to a mismatch in frequency and number of points). This would result in what you are seeing. Check your project to see if this happened to you. If it did, you would get -350dB at 100kHz (essentially the noise floor of a pure signal), and -3dB at 50kHz.

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Different spectrum from signalexpress with oscilloscope

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I got it. Thank you so much for your help. DFGray. 🙂

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Different spectrum from signalexpress with oscilloscope

Re: Different spectrum from signalexpress with oscilloscope