We have the FG's low Z output amp (assumed to be of unchanging amplitude and phase) driving a 50R non-inductive resistor connected to an unknown load. We are measuring across that 50R resistor.
For the sake of analysis, we can assume that the output impedance of the FG's DAC+amp is 0Ω. Now let's consider a situation in which the DUT has stored some energy in the past and now at this particular instance in time is presenting 10V to the FG. Which way will the current flow through that internal 50Ω resistor ? To know this, you'll need to know what voltage the FG's DAC+amp is outputting at that moment: 1) If the FG's DAC+amp is outputting >10V then the current will flow from the FG to the DUT. 2) If the FG's DAC+amp is outputting 10V then no current will flow between the FG and the DUT. 3) If the FG's DAC+amp is outputting <10V volts then the current will flow from the DUT to the FG. ...but, but, you don't know what the FG's DAC+amp is outputting at this moment so you can't tell which way the current is flowing nor the sign of the energy flow (power). For your power measuring method to work, you could hack the FG and get at the voltage signal before the internal 50Ω resistor (which is not a bad idea) or with a FG like Itsu's you could mirror the Ch1 signal on Ch2 and get at the internal driving voltage signal that way, without opening the FG.
« Last Edit: 2017-05-09, 06:43:45 by verpies »
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