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-7 dBm innacuracy in FFT

I'm using a PCI-4474 DAQ card and Labview to create a spectrum analyzer.

 

I keep getting a -7 dBm result on my FFT graph.

 

For example if I input a sine wave from a signal generator at 0dBm into the DAQ the result will be -6.98 dBm after going through the FFT.

However if I look at the signal in the time domain in Volts the result is correct. 0dBm = 633mVp-p.

 

This also occurs if I use a simulated signal VI - I get the same discrepancy in the FFT 

 

Anyone else experiance this or know what I may be doing wrong?

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Hmmm

 

0.633Vp-p=  @0.208Vrms. Assuming 75Ohms  Power = V^2/R = @0.578mW or about -7dBm


"Should be" isn't "Is" -Jay
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