I am using an electronic load to draw current into my DUT, but when I draw current the voltage drops. Why is this happening?
The output impedance of the DC bias source is 50 ohm (nominal).
A voltage drop will appear across this impedance as current flows through it in accordance with Ohm’s Law (Voltage = Current * Resistance).
For example, a 0.5 V drop would appear if the load were to draw 10 mA of current through the output.