Picotest J2120A linear injector 10Hz-10MHz, 5A

Product overview

Although the injection transformer is a very broadband adapter, it is not useful for measuring ripple suppression (PSRR) in a power supply or even in an op amp (operational amplifier). This is because the features that make the injection transformer work so well also result in the transformer not tolerating DC. Even very small DC currents (5 mA or less) can significantly reduce signal capacitance or even saturate the transformer completely. For this reason, the Picotest linear injector (J2120A) is another essential test adapter.

The linear injector allows the input dc supply voltage to be modulated by the source signal of the network analyzer, as in the case of PSRR measurement. The linear injector must be able to operate in a frequency range well below the AC line frequency and at least above the control loop bandwidth of the circuit under test.

The linear injector can only be powered by current, so the output amplitude can be significantly affected by the operating current and total load storage capacity. The network analyzer has very high selectivity, so distortion at the output of the linear injector generally does not affect the measurement. Again, this is a small-signal injector, so oscillator signals should be kept as low as possible above the noise floor. As a guideline, the amplitude of the input signal should be below 50mVpp (-20dBm). In some cases, we want to further attenuate the source signal, so we have included attenuators with the injector kits. Some analyzers, such as the Omicron-Lab-Lab Bode-100, allow the amplitude of the injected signal to be shaped as a function of frequency, which helps optimize the signal level.

Key features

• Useful bandwidth of 10Hz-10MHz
• Low-loss design
• Maximum current of 5A
• Max input voltage of 50 VDC
• Easy measurement of input filters and PSRR

Input impedance measurement

The linear injector can also be used in conjunction with a current probe to measure the input impedance of a power supply. The input impedance of a switching power supply or regulator is negative, which in combination with an EMI filter poses a stability problem, so, measurement is a very important part of the design, analysis and verification process. The current probe must be set to 1A/V or the results must be scaled accordingly for different settings.

Selected parameters