How to test circuit noise

[cainiao]

Hi, I'm building a low noise circuit. I collect the output with PCIe-6363 in Labview.(pic1) Usually people test noise with Signal Analyzer(pic 2) but I don't have one. What should I do?

[santo_13]

DSA (Digital Signal Analyser) is best suited for high dynamic range and retains frequency content very well. Depending on the resolution and accuracy of the measured noise, you may be able to use 6363 (general purpose analog input) to measure as well.

What are you testing? what do you intend to achieve?

[cainiao]

Hi, I got output noise spectral density of a circuit with simulation. The output noise spectral density divided by gain of the circuit is equivalent input noise, that's what I want. I search some information and it seems that I need to use FFT for my signal. 

[dsb@NI · Accepted solution]

Are you asking, "How do I connect signals to data acquisition hardware to measure my circuit response?" Or, "I have a time signal; how do I compute noise level?"

Depending on your questions:

Have you already found the data acquisition hardware to measure the response of your circuit? I also recommend NI DSA hardware. Then, use the (free) DSA Soft Front Panels to acquire data and measure the noise energy.

Do you want the noise as a function of frequency,  or are you looking for a single- number such as noise level? You can compute energy of signal in either the time domain or the frequency domain. The frequency domain makes it easier to differentiate between the tone energy and noise energy. LabVIEW Full includes many flavors FFT including power spectral density. You can integrate over the noise bandwidth to sum the energy - remember to account for the equivalent noise bandwidth of the window you use. If that seems daunting,  there are toolkits that perform spectrum measurements. Consider using the Sound and Vibration Toolkit, Spectral Measurements Toolkit,  or the Electrical Power Toolkit. 

[Bob_Schor]

Can you tell us a little more about your situation?  Here are some questions:

• What version of LabVIEW are you using?  Tell us something like "LabVIEW 2021 SP1 (32-bit), Full" (where "Full" can also be "Base", "Professional", or "Academic").
• What version of Windows?
• How much LabVIEW experience do you have?
• How much "signal theory" experience do you have?

Where do the two figures you show come from?  Pic1 seems to be a sinusoid at some unknown frequency, with some (small) amount of noise.  What's curious is the Time Base, where the time axis doesn't seem to change, so the frequency of the signal is unknown.  Pic2 looks like an FFT of a signal sampled at around 20 kHz, showing "noise peaks" at 60, 120, 240, and 480 Hz (probably 60-cycle pickup with some harmonics).  What are these really showing, and are they related to the signals you are trying to analyze?

Bob Schor