One of the difficulties of measuring active devices like LNA and IF amplifiers with the Nano VNA is the care needed to avoid over-driving the amplifier input and, more importantly perhaps, to avoid blowing up the CH1 port during S21 (gain) measurements.
A common approach to avoid these problems is to place attenuation on both the input (CH0) and amplifier output into CH1. This seems reasonable and I'm sure it'll work just fine, although I haven't tried it myself (yet).
However, I recently purchased a low-cost Return Loss Bridge from a vendor in the Ukraine (see image below) and I was wondering if I could press it into action to help measure amplifiers.
While browsing for information about this subject, I came across an interesting thread (Nano VNA Forum Thread). About halfway down this thread a user makes the following comment:
This is a tricky case as the power out can overload the device.
The solution I use is attenuate the output, run S21 and use an external return loss bridge with an amplifier on the return port to the channel 1 input. With known gains and attenuation its easy to get return loss, input impedance, and then input SWR
Same methods used for receiver inputs, with care to insure the input is below overload.
It's an old thread, so I can't ask the OP for clarification but I'm hoping someone more knowledgeable than I am can parse this comment and help me understand how the RLB is helping in this situation.
Some of the questions I would have are:
What does the layout look like? What are the connections? When he says: "attenuate the output" does he mean the output of the VNA (i.e., CH0) or the amp? What exactly is the "return port" of the RLB?
What exactly can you measure with this technique? Can you measure gain, for example?
Is this external RLB method superior to using the attenuation method? Why so?