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30 MHz AWG 33522A Sum Noise to Waveform

Question asked by schnuber on Sep 15, 2011
Latest reply on Oct 4, 2011 by lhornburg

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Hello
I have a sine with an amplitude of 1.4Vpp. Now I want to add 140 mVrms Noise to it. How do I make the adjustmentss? Since with the SUM function I can not say I want 140 mVrms noise, but can only define a percental fraction of the carrier. But the carrier is defined in Vpp whereas the noise is defined in mVrms. So how do I resolve the issue?
Thank you

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lhornburg Sep 16, 2011 1:31 PM
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Hello,

The problem is that the noise amplitude depends upon the setting of the carrier amplitude,
and is added as a %, and will vary as amplitude with the signal, and not remain fixed at xxx Vrms or yyy Vpp,
(as soon as the user knobs the main amplitude the “voltage” of the SUM signal changes in proportion).

Now to fix your problem. First set your amplitude to 1.4VPP, now press the units button on the front panel, at the bottom of the screen there is some soft keys, the third one from the left says "Ampl as". Press the blue softkey and change it from VPP to VRMS. The amplitude should now be converted to VRMS and should show 495 mVRMS.

You want to add 140mVRMS of noise: 140mVRMS/495mVRMS = 28.28%, so when you go into the modulate menu set the Sum Amplitude to 28.28%
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schnuber Sep 20, 2011 1:19 AM
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Thanks for replying
I am not quite sure if I understund you correctly, so let's do an experiment.

Assume I have my units set to Vrms. Now I set up a carrier sine with 140mVrms, 100kHz. Now with the SUM function, I modulate this carrier with Noise, 4MHz bandwidth and SUM Amplitude=100%.
I think I am supposed to get now a signal that is the sum of 140mVrms carrier sine and 140mVrms noise. But that is not what I see on the scope (MSO7054B) I see less noise than expected.
I even tried this out: I set the frequency of the carrier to 1uHz instead of 100kHz (this way I get virtually a DC signal) now with the StdDev function on the scope I can measure the amount of white noise which only figures to 40mVrms instead of 140mVrms. If in turn I turn off modulation and set up a single noise signal with 140mVrms then I actually see nearly 140 mVrms (StdDev) on the scope.
So either I still don't get it or there is some inconsistency out there.

Please support me
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lhornburg Oct 4, 2011 12:28 PM
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Hello,

I investigated the operation of SUM for both noise and Sine, and the results are the same, noise is handled no differently than SINE. Here is the summary of operation:

     1. The SUM signal is added numerically to the Carrier signal in the FPGA signal generators.
     2. The Output signal will increase in amplitude to handle the Carrier + Sum signal amplitudes. (carrier is not reduced to keep the same output signal amplitude)
     3. The adding is in linear units, with no conversion to Vrms, so using units of Vpp is the best way to quantify the different amplitudes of carrier and sum signal.

For example, if the carrier is 1.0 Vpp Sine wave, and you add Noise at Sum Amplitude of 100%, the resulting output waveform will have a maximum amplitude of 2.0 Vpp.
(Due to the nature of noise and the bandwidth limits and sampling of the oscilloscope, the Vpp displayed will vary and usually not appear to reach a peak of 2.0 Vpp, but this is more easily seen by adding two Sine waves of similar frequency, like 1000 Hz and 100 Hz).
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lhornburg Oct 4, 2011 12:40 PM
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With the way scopes sample they could miss the noise peaks, I would suggest turning on infinite persistance and let it run for a few hours. Then you should see the peaks. I know you were wanting to work in RMS but after speaking with the designers it would be wise to work in VPP instead.
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30 MHz AWG 33522A Sum Noise to Waveform

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