Many RF ICs use differential inputs and outputs. This article describes how a two-port network analyzer is used to measure differential impedance. Gamma is calculated from the measured S-parameters. The transformation between series and parallel models is given.
The following measurement method will prevent the use of baluns and transformers, which introduce errors and will also provide a more accurate and repeatable measurement method of the differential impedance.
This example will use a mixer, which uses a single-ended input and a differential or balance output.
- The first step is to do a full two-port calibration on the network
- Activate the mixer to its normal operating conditions
- Measure all parameters S11, S12, S21, and S22
- Finally, use the equation below to convert the differential measurement to a single-ended result
From this equation, calculate the real and imaginary components to arrive at a parallel equivalent circuit:
Figure 2. Parallel equivalent circuit of the series circuit.
The fastest way to prove that this method actually works is by performing a simulation of a one-port differential circuit with two measurement ports single-ended and one measurement port differential. Calculate Γ d for circuit #1 and compare to Γ of circuit #2. The result should be the same.
The derivation of Γ d can be found in the January 1999 edition of RF Design Journal.