I would like to measure the efficiency of an HF (3-30 MHz) class C power amplifier. In order to do this, I have to accurately measure AC+DC RMS current in the order of 1 A or so. Most instruments are not designed to measure AC current above ~100 kHz. One option I'm aware of is to connect the power supply through a 1 Ohm resistor and measure Vrms on this resistor with an oscilloscope and a differential probe. The probe is needed, because otherwise the PSU will be shorted to ground through the oscilloscope. Unfortunately, the differential probe I have is a high voltage and noisy one. It shows ~200 mVrms even if there is no current.
How would you solve a task like this? Should I look for a low voltage differential probe, a special current probe, or some other tool? Is it difficult to home-build a suitable probe? Or maybe there is a completely different way to solve the task?