• 0
tdavismn

Measuring RMS noise with AD2 (scope/FFT) or SA

Question

Posted (edited)

renesas_an1560_making_noise_measurements_pgs_1-3.pdfI have a noise test setup that is measuring the en noise of jfets and is gaining it by 1000x to the AD2.  What tools exist in the AD2  - likely either scope with FFT or the SA where I can get reported the rms voltage value over frequency and the number of samples taken for the RMS.  I don't want pk-pk as it requires more work and samples to be meaningful to the analog circuit designer - me.

Please provide a best practice method as I am struggling at this time.

An alternate can be to move to another piece of test equipment if this isn't an intended feature of the AD2.

Thanks,

Tim

agilent_nz_5989-9087EN.pdf

Edited by tdavismn
Renesas app note. pgs 1-3 only, See fig 4.

Share this post


Link to post
Share on other sites

5 answers to this question

Recommended Posts

  • 0

Hi @tdavismn

I'm not sure how this measurement is performed, but with the WF SA you measure Vrms like this:
Under Magnitude gear drop-down you can select log vertical scale.

image.png.dd3cbd69474cca62d464055ea39bf312.png

Share this post


Link to post
Share on other sites
  • 0

Please see attached app notes.  Full from Agilent.  Partial from Renesas (based on attachment memory size).  This will explain the application and test setup better.  The AD2 is at the end of the signal chain.

Share this post


Link to post
Share on other sites
  • 0

This is the best reference I can find of how to make use of the rms value once it is measured.

This is for the relation of dBm/Hz in 50 Ohms and nV/Root Hz in 50 Ohms.

https://www.tek.com/support/faqs/how-can-i-measure-and-calculate-nv-root-hz-nanovolt-root-hertz-spectrum-analyzer

My suspicion is that this may not be easily supported by "waves" at this time.

If the feature were available, the AD2 with WAVE GUI/SW would probably find even more homes.

Share this post


Link to post
Share on other sites
  • 0
On 8/6/2019 at 2:53 PM, tdavismn said:

where I can get reported the rms voltage value over frequency

Hi,

a conventional spectrum analyzer shows average power observed within the resolution bandwidth (RBW) filter. I need a correction factor from the RBW filter to the equivalent noise bandwidth of the filter, either from the analyzer's manual or via the analyzer's firmware (e.g. 'noise density' marker or the like that performs the conversion automatically). I can ignore this factor but it won't be fully accurate then. The difference is between definitions e.g. -3 dB bandwidth vs. an equivalent brickwall filter with the same area under the curve.

For a "raw" FFT-based spectrum analyzer, the RBW is about the inverse of the capture time, e.g. 1 ms capture = 1 kHz RBW.

Knowing power in RBW, I scale down to power in 1 Hz. E.g. with a 300 kHz RBW (noise) bandwidth, divide the power (in units of Watts) by 300000.

Then convert to voltage over 50 ohms and there is the RMS value (since spectrum analyzer power readings mean "dissipated over the SA's 50 ohms input resistance).

This for the vendor-independent basics...

Share this post


Link to post
Share on other sites
  • 0

Hi @tdavismn

I will add such magnitude units to the application.
At the moment you can use the RMS/Peak (dB) units where you can specify a Reference value.

image.png.fe7a60642622900b85b6ddb8a02096b0.png

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now