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E4443A Measuring DC voltages, need Help

Question asked by locktofer on Sep 22, 2009
Latest reply on May 28, 2010 by locktofer

I am using an E4443A Spectrum analyzer, and I have a detector connected directly to the 50 ohm input. The detector is an Agilent 83440C lightwave detector. The problem is this. I would like to measure the DC voltage across the terminals of this detector with the spectrum analyzer. It woudl be the equivalent measurement of putting a resistor of exactly 50 ohms across the detector terminals and measuring the voltage across it. In order to do this, I switch the analyzer input coupling to DC, and then I look at the signal at 0Hz (DC). It reads a value in mV when the measurements units are in Volts. Fruther, if I use the 0 span function at 0Hz, I get the same reading, which is good I suppose. However, if I disconnect the detector and put a resistor of 50 ohms across it, I get a different result completely, different by abou 2mV or so. So, I am not even sure if this is acceptable.

Note that I am aware we don't want to put more than +/- 0.2 Volts maximum into the SA input port, in our case this is not happening as the voltage is 10s or millivolts at most, so we are in no danger of damaging the analyzer.

I woudl like to know why this measurment has this discrepancy and if perhaps I am doing something wrong or there is another way to do it? It wold be best if the analyzer did the whole thing as I can then automate it all without disconnecting the detector.

Thanks and regards,