In the vast majority of cases, the line injector is almost certainly working correctly. It is often the case that the test setup impacts the results and that a large voltage drop at its output is seen. We do get many questions about the J2120A and its operation. After reading the following if you are still having issues please let us know (instructions below).
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Measuring Power Supply Rejection Ratio (PSSR) - on a Tektronix Scope
To get the right results you need to understand what you are asking the J2120A to supply, in terms of current, when it is connected to a load. If you are loading the J2120A too heavily, its voltage drop can be significant.
The amount of load current (out of the J2120A) is FREQUENCY DEPENDENT because the input impedance of the load (power supply input) varies with frequency and can fall if there is an input capacitor on the input of the power supply circuit. This is usually the case in normal designs.
Why is this the case? Because the impedance of the input capacitor is too low in general and especially at its resonance point and this causes the circuit (load on the J2120A) to pull too much AC current through the J2120A at some frequency (not DC).
As shown on the https://www.picotest.com/products_J2120A.html page in the Detailed Specifications Tab, you will see the reason for the issue. The more current, the more drop from the input of the J2120A to the output. What you have with the J2120A and an input capacitor is a very effective low pass filter. The cable impedance of any interconnects also affects the drop. The solution is to, if possible, remove the input capacitor, or replace it with a smaller cap, so that the impedance is not too low at higher frequencies.
For further support…
If you are still having issues we need to understand your setup. If you could send the following to firstname.lastname@example.org it would help us to help you debug what is happening and what you are seeing.
A picture of the test setup
A description of the circuit loading the injectors you are trying to test PSRR including the input capacitance and/or input impedance of the loading power supply
What is the operating current of the load?
Can you make an impedance measurement at the input to the power supply loading the J2120A?