Suggestions about RF current measurement

[Debeli]

I would like to measure delivered power in a system with impedances that are signicificantly different from 50Ω (dominantly capacitive impedance with small real part) and with time variable load impedance. I think that the most accurate way is to measure Voltage and Current wave effextive values and phase difference between them and than calculate the power. I hope that I have a solution how to build the voltage probe, but I am not sure in which direction to go to pick up a current, since the current ranges from 0.1-200 A and working frequency is in range 1-20 MHz.
Is there anybody who has tried to use Rogowski coils in such applications?
Or someone has another idea how to inductively couple probe with the RF signal keeping the dynamic and phase response under control?
Any experience using bidirectional Wattmeter in such applications?

I appreciate any response.

Thanks,
D.

 

[flatulent]

You can use a current transformer with a resistive load to get a voltage proportional to current. Then use a resistive divider to get the voltage. Run both of these to a MPYxxx four quadrant multiplier IC. (xxx is the model number, I cannot off hand remember the one that operates at 20 MHz.) This multiply will take the phase angle into account and you will end up with a DC (low pass filtered) level proportional to real power.

 

[Debeli]

Thank you very much on your response, however, I have following concerns regarding RF current transformers:
1. Size of the core and linearity response
2. Dynamic range, since primary current is 0.1-100 A.
3. Secondary winding must have large number of turns and than parasitic effects will be dominant at 20 MHz.

Do you have any manufacturer specialized in this application area?

 

[IanP]

You can try to construct a simple peak detectors based on fast diode, fast comparator and a capacitor and measure a voltage drop across a power resistor ..
Just different idea ..