I am doing phased array measurements by using voltage controlled amplifier, attenuators and phase shifters, those are used to provide the required input signals (in the form of amplitude and phase) to the array elements for beam forming. I need to know how can i find the voltage at the port of the VNA because, this will be the reference voltage that i ll be using in the whole scenario. Gratitudes in advance.
Step 1: Specify the output power of the VNA.
Step 2: Measure the reflection loss and the input impedance (i.e. Rin+jXin) of the DUT
Step 3: Derive the actual power fed into the DUT based on the reflection loss measured and the VNA output power specified: Pinc=Pout*(1-|RL|^2)
Step 4: Calculate the voltage at the VNA port with the formula: Pinc=0.5*V^2/Rin
There is a bit of a semantics issue here depending on the intended application of the DUT. The procedure above is essentially determining the power transferred to the DUT and then calculating the corresponding real component of the voltage based on the real component of the impedance of the DUT. This accounts for the impact of mismatch and is an important number if for example you're concerned about the actual voltage that a component will see at its input (although I'd think you'd want to use the magnitude of the complex impedance to get the worst case voltage, not just the real component). However, in most RF circuit analysis, the goal is to evaluate performance at the intended characteristic impedance of the system (i.e. 50 Ohms) and not the actual input impedance of the circuit that might have a mismatch. Thus, the VNA measures Vin/Vout at 50 Ohms to determine S21. You'll get a different answer if you account for the mismatch at Port 1 to determine Vnet = Vout*sqrt(1-S11^2) and calculate Vnet/Vout. That's not S21 by definition. Note too that there's likely to be a similar mismatch issue with Vout for whatever the characteristic impedance is of the circuit relative to the 50 Ohms of the VNA.
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