Many power supplies use optocouplers in the feedback loop. The stability and overall performance of the power supply are often dependent on the CTR and the location of the poles (and zeros) of the optocoupler. It is also common for optocouplers to have a minimum specified CTR, but not a maximum. CTR curves vs. LED current are also not always provided. Therefore, measurements are required to determine the expected performance range. These measurements also support the creation of a SPICE model. The OMICRON Lab Bode 100 Vector Network Analyzer can be used in conjunction with Picotest injectors to measure the CTR and frequency response of optocouplers. The DC Bias injector can be used over a reasonable frequency range of 10Hz -40MHz, and limits the IF current up to approximately 4mA. The objective is to provide a DC+AC bias to the optocoupler while the Bode 100 controls the AC bias, as well as measures the CTR over frequency.
In this measurement the J2130A provides a DC bias to the Optocoupler LED, which returns to ground through a termination resistor. The optocoupler transistor collector is powered from a separate power supply and the emitter is terminated through a termination resistor to ground, we use equal valued termination resistors in order to normalize the CTR to 100%. The resistor value is not as critical as it is for the resistors to be the same value and type.