27 March 2014

Understanding the RF Current sensor

I thought it might be instructive to set up a bench experiment to try to understand the performance of the RF Current sensor. To keep it simple I measured the sampled RF current at just 1 frequency 3579KHz.

In order to simulate an end-fed halfwave input impedance, I terminated my Z Match in a 3000 Ohm resistor. I then linked the termination through the RF current sensor. After measuring the K2 output at 1.2 Watts I then connected the K2 to the Z match and tuned the Z match for a 1:1 SWR. Best I could get was in fact a 1.1:1 SWR. I measured the RF current sensor output at the RF point across the 270 Ohm resistor using my scope with a 10X probe. I measured the DC output using the Keithley DC DVM.

I then compared the results with some theoretical calcs which assumed that the system was 100% efficient. ie all power from the K2 is transferred to the 3K load AND all power from the current sensor is transferred to the DVM and Scope readings. Here is a summary of the results obtained.

DC voltage output (calculated) from the sensor = 463mV. Measured DVM DC voltage = 255mV
Voltage output (calculated) across 270 Ohm resistor = 763mV peak. Vpeak measured = 490mV.

In the end the only observation would be that the measurements were 'in the ballpark' of the expected results.

There could be many reasons why the discrepency. Listed as follows.

1) K2 Power output measurement error on the Power Meter
2) 1.1:1 SWR is not a perfect match.
3) Losses in the Z Match.
4) Load not exactly 3000 Ohms at 3579 KHz (did not measure the actual resistor values)
5) Core losses in the current transformer
6) Incorrect theoretical calculations
7) DVM and Scope measurement errors
8) Loading effect on the circuit from the DVM/Scope
9) Assumed forward voltage drop of 300mV across diode is incorrect

Looking at item 6 it is not clear how accurate the assumption is on current transformer core action. In practice this could be widely different to the theoretical 10:1 turns ratio. I assumed that all available current would be extracted across the resistor? This is probably not correct. I don't know how to calculate this value. More education required on transformers. Without knowing what the actual efficiency is of the Z Match it is not possible to narrow down the discrepancy.

The most practical next step would be to perform the test with another 2 transmatches. This way it would be possible to get some idea of the difference in efficiency of the transmatch section of the test.

Test Setup showing the Z Match terminated in 3000 Ohms and with the RF Current sensor inline. 

DVM = 248mV and Scope showing peak to peak RF across the 270 Ohm resistor of 980mV

RF Current sensor with scope probe in place

No comments:

Post a Comment