Why is RF current measurement better then simple voltage measurement?

Why is RF current measurement (as in our ESU-2050 and ESU-2300) better then simple voltage measurement?

The answer to this question very quickly dives into electrical theory and a lot of heavy-handed calculations.  Before all of that, consider the fact that the electrosurgery manufacturers have used RF current measurement as the industry standard for over twenty years now.  When you visit a manufacturer such as Covidien/Valleylab, Conmed, Erbe, Bovie, etc., you will see instrumentation (typically until now a Fluke Model 8920A Wide Band Digital Multimeter) in place that directly measures RF current flow through an external load resistor via a wide band current transformer.  The reason?  It is the most direct and consequently the most accurate measurement available.

To dive into some of the specifics, all ESU analyzers take measurements with some type of test load attached to the generator.  This test load is rated at a certain resistance at DC (0 Hertz).  Let’s say it is a 50-ohm load connected to the bipolar outputs of a Conmed Excalibur generator for this example.  The test load should be a “non-inductive” resistor, but in the real world, there are no “ideal” non-inductive resistors.  Even non-inductive load resistors have a certain inductance to them due to the windings that are an internal part of the resistance at DC (remember, a transformer is simply a set of windings around an iron core and a transformer is an almost purely inductive device).  This inductance results in an electrical characteristic called inductive reactance, which changes with frequency.  So let’s take a look at the actual characteristics of our 50-ohm non-inductive load resistor with this in mind.

• At DC (0 Hertz): Load resistance is purely 50 ohms
• With our Conmed Excalibur generator in bipolar mode:  Load Resistor “Impedance” is R + jwl (where w=2 pi f and L = inductance in Henries)
• Conmed Bipolar output frequency is approx 1 MHz (f)
• Let’s use a low level of inductance at 10 micro-Henries for our “non-inductive” load resistor
• The calculated inductive reactance of the load resistor is 62.83 ohms
• Combining the 50 ohms of pre DC resistance with the 62.83 ohms of inductive reactance, we get a load impedance of 80.3 ohms!

Now let’s look at the two methods of measurement and see how they differ.

Current Measurement Device (ESU-2050 & ESU-2300):

The current measurement device actually measures the current flow through the load at the actual frequency of the RF energy being applied to (through) the test load.  Therefore, this actual impedance of the test load is taken into consideration as part of the measurement, and it is a measurement that is correctly using the 80.3 ohms at the 1 MHz frequency of the applied signal.

Voltage Measurement Device (RF303 & QA-ES):

The voltage measurement device measures the voltage across the test load or across a voltage divider circuit that samples the voltage across the load in a reduced form. This is in itself a source of measurement error. It then applies Ohm’s Law to calculate the RF current:

I = V/R (where I = RF current, V = measured voltage and R = resistance of the test load)

There are multiple sources of error in this approach, the very first being the actual value of the DC resistance.  A 50-ohm resistive load with a 5% tolerance can be 2.5 ohms off as compared to the 1% precision tolerance resistors used in the current measurement method above.  Next, we have the error being introduced by the voltage divider circuitry in the analyzer that is used to step down the measured voltage to a more manageable level for the measurement circuitry used in the analyzer.  Finally, the largest degree of error involved the AC impedance of the test load.  Our 62.83-ohm impedance (at 1 MHz) resistor from the current measurement method above is not even taken into consideration with the voltage measurement device.  The Ohm’s Law calculation uses only the 50-ohm rated DC value for the calculation of current in the voltage measurement device.   It totally ignores the inductive reactance component of the load resistor.  This results in a very significant error, which gets larger and larger as the frequency of the applied RF waveform climbs.

The trend in the electrosurgery market is moving towards higher and higher waveform frequencies of output.  The Conmed Excalibur produces a bipolar output that is slightly above 1 MHz.  The Ellman International Surgitron is a 4 MHz device.  Using our 50-ohm test load example from above, the inductive reactance of our 50-ohm (DC) test load resistor would be 251.33 ohms!  The total impedance of our test load would be 256.26 ohms at 4 MHz!  Try making this Ohms Law calculation and see just how far of the voltage measurement device would be when using the DC value of 50 ohms as the “R” value when calculating the current!

When it comes to electrosurgery generator testing, current measurement is inherently more accurate than voltage measurement, especially with higher frequency generator outputs!  Just do the math!