J2121A

While the injection transformer used for Bode plots is a very wideband transformer, it is not useful for measuring ripple rejection (PSRR) of a power supply or even an opamp. Doing so could permanently damage the transformer. This is because the attributes that make the injection transformer perform so well also result in a transformer that is intolerant of DC current. Even very small DC currents (5mA or less) can greatly reduce the signal capacity or even totally saturate the transformer. For this reason, the Picotest line injector (J2121A) is another essential test adapter for power supply testing.

The J2121A is especially suited to high power applications such as those associated with military and satellite busses. It can be used with input bus voltages up to 400V and supply up to 20A at 1MHz. The line injector allows the input DC supply voltage to be modulated by the network analyzer source signal (oscillator). The J2121A line injector output is DC regulated and provides a fixed voltage drop of 750mV from the input bus voltage fed into the injector in addition to the AC modulation signal.

The J2121A can be connected to the power supply input under test and, depending on the test probe connection points, can be used to measure either PSRR or the power supply’s input impedance The line injector can also be used to measure the impedance/inductance of an inductor under bias.

While a current probe can be used as part of a test setup, the J2121A includes a current sense monitor output that can be connected to the VNA.