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Bandwidth definition

Question asked by LeonardHayden on Dec 13, 2011
Latest reply on Jun 3, 2013 by mkelly
An Agilent oscilloscope accessory called an Optical Reference Receiver manual describes the use of the LCA to measure the 3 dB bandwidth. What is unclear to me is if this 3 dB bandwidth is an optical bandwidth or an electrical bandwidth.

For an optical bandwidth a 3 dB roll-off measurement would correspond to an O-E receiver output voltage that is one-half of the low frequency value (since the output voltage is a measure of the input optical power and calculating 3 dB from the indirect power ratio).

For an electrical bandwidth the 3 dB measurement would correspond to an O-E receiver output voltage that is 0.707 of the low frequency value (calculating 3 dB from a voltage ratio).

When the LCA shows an S21 of -3dB which case is implied?

-Leonard

PS - This app note (http://literature.agilent.com/litweb/pd ... 7808EN.pdf) almost tackles the topic, but fails to define things well enough to be definitive.  

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