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Confusing Amplitude Levels: Scope vs. Spectrum Mode

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Confusing Amplitude Levels: Scope vs. Spectrum Mode

Postby risto » Tue Jun 11, 2013 8:16 am

Hi there,

I'm using the PicoScope 4262.
When applying a 2 Volt DC Signal in scope mode I see a correct horizontal line at about 2 Volts.
But in Spectrum Mode (0 to 100Hz , linear,blackman, N=2048) at about zero Hz a peak with approx 4 Volts is measured.
Why is the Amplitude in spectrum mode two times higher than in scope mode?

Another case:
Applying a 1kHz sinewave with 1Volt amplitude from the picoscope AWG generator to Port A:
- In scope mode a Sinewave with 1 Volt Amplitude can be seen.
- In spectrum mode (0 to 2000Hz , linear,blackman) at about 1 kHz a peak with approx 0.7 Volts is measured.
Why is the Amplitude in spectrum mode lower than in scope mode?

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Re: Confusing Amplitude Levels: Scope vs. Spectrum Mode

Postby Hitesh » Mon Jun 24, 2013 9:19 am

Hi Risto,

The 4V is an anomaly as an FFT isn't meant to be applied on the zero frequency component.

The amplitude for the 1V sine wave is lower is because it is a Root Mean Square (RMS) calculation.

I hope this helps.
Hitesh

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