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MSO6104A data collected in single sweep

Question asked by NurS on May 29, 2008
I have been using Agilent MSO6104A 4 GHz Scope to capture and analyze signals. The scope is set to single trigger mode and collects the 8 megabyte data in raw format into a USB device. The waveform is adjusted to about 75% of the full scale screen display at the high signal level. I am applying the signal through a low pass filter at 200 kHz and set the sampling frequency to 800 kHz. While the signal is being captured I reduced the signal level by 30 dB. When I analyzed the data stored in the file, I found the signal and noise levels are reduced by about 30 dB after the change transient. The signal level change is normal but why there is a change of noise level.

I repeat the experiment with a Tektronix TDS3054 5 GHz sampling scope which has a memory of 10 kilobytes and analyze the data file. I found the noise level stays constant and signal amplitude is reduced by 30 dB.

I also displayed the signal with HP 4396B spectrum analyzer and did not find change in the noise level when the output of the signal generator is reduced by 30 dB.

I would like to find out what type of amplitude processing is being performed by the MSO6104 and is it possible to turn of any automatic operation? If I am measuring signal to noise ratio the information obtained does not represent the actual data.

Thanks for any help.  

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