dBm to dBV - RMS ) dB mW = dB m = 20 Log...

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Electrical and Computer Engineering Department ECE 405L Electronic Communications Laboratory AG 54622A Digital Oscilloscope with FFT If a 1 V RMS sine waveform is measured in the time domain on the oscilloscope it will measure as 0 dBV in the frequency domain. This can also be measured as 1.414 V P or 2.828 V PP . Measuring in the frequency domain on the oscilloscope will perform like an AC voltmeter which reads in RMS. Looking at the definition of dB’s shows 1.0 V RMS (the measured value) dBV RMS = 20 Log 10 ------------- = 20 Log 10 (1) = 20 (0)= 0 1.0 V RMS If the voltage measured in through a 50 Ω terminator then you can convert to dB M , which is considered the industry standard unit for frequency spectrums, and is usually defined for a 50 Ω system. So, V (measured in V
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Unformatted text preview: RMS ) dB mW = dB m = 20 Log 10------------------------------- PR V 2 Where P = ---- or V = PR R Vmeasured Vmeasured dB m = 20 Log 10----------------------- = 20 Log 10 ------------------------- (50 ) ( 1 mW) (.05) Vmeasured dB m = 20 Log 10 ------------------ = 20 Log 10 ( 4.472) V M 0.223606 Then, if V M = 1 V RMS dB m = 20 Log 10 (4.472) (1) dB m = 20 (0.65) = 13.01004 Therefore, if you measure the dBVs from the oscilloscope/FFT you add 13.01 to that value to get the equivalent in dB m , or simply: dB m = dBV + 13.01004 from oscilloscope to spectrum analyzer, dBV = dB m-13.01004 from spectrum analyzer to oscilloscope, dB mV = dB m + 46.98996 from milliwatts to millvolts. R. Cockrum 06/24/08...
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This note was uploaded on 11/28/2010 for the course ECE 405L taught by Professor Cockrum during the Fall '10 term at Cal Poly Pomona.

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