RFPM2 – current probe

This article describes a current probe for use with a power meter calibrated in dBm (eg RFPM1 and RFPM2).

For use with RFPM1 and RFPM2, both of which read to 16dBm max, it is convenient that the scaling factor for the probe is 0dBA/dBm, ie that those meters read dBA directly, implying a current range of -75-16dBA or 0.186mA-6.3A. In fact for low measurement noise, the effective range would be more like -65-16dBA or 0.56mA-6.3A.

The probe comprises a ferrite cored transformer that is clamped or placed over the conductor(s) of interest, and uses a 10t secondary which has a low value resistive load, across which the power meter connects.

Above is a screen shot of a spreadsheet calculation of relevant design values.

So, for 0dBm on a 50Ω power meter, the secondary circuit voltage is 0.223V, and current into the 50Ω power meter is 0.004472A. The shunt resistor current is the residual of the 0.1A secondary current at 0dBA, 0.095528A, and calculated shunt resistance is 2.34Ω, which conveniently is very close to half of a preferred E24 value of 4.7Ω. In fact the error is calculated at 0.4%. The power dissipated in the shunt at +16dBA is calculated as 0.85W, so a 2W resistor will be used.

The shunt resistor will comprise two 1020 SM 1W 4.7Ω resistors.