Power Measurement at RF and Microwave Frequencies


  This application note reviews the fundamentals of power measurements from DC to MicroWave.  Also covered is how test equipment, circuit, and coaxial cable interact and influence the accuracy of power measurement.


  In DC circuits, power measurement is relatively easy.  For example, if the voltage V across and current I through a resistance R is measured, the power is easily calculated by using the following.



  In an AC circuit containing inductance or capacitance, power measure becomes complex because the voltage and current are out of phase.  Power is calculated by using the following.


P = e i= e i


Where  is the phase angle difference between e and i, and the magnitudes of e and i are the RMS values.


  At RF/MW frequencies power measurement presents more of a challenge.  Instruments used to measure voltage or current cause de-tuning of the RF/MW circuit or coaxial transmission line.  A RF voltmeter would add pico-farads of capacitance and a current measurement would add nano-henerys of inductance to the circuit or transmission line thereby making measurements inaccurate.  However, power causes the same amount of resistive component heating regardless of source frequency (DC, 60 Hz AC, or RF/MW).  Accurate power measurement of RF/MW circuits (amplifiers, oscillators, filters, etc) is possible because of the output-matching network used to couple the circuit to a load or coaxial cable (usually 50 or 75 ohms).  Voltage and current vary with the length of a lossless coaxial cable, but power is not a function of length of a lossless coaxial cable.


Thermistor Mount

  Power measurement at RF/MW frequencies is accomplished by connecting a load capable of responding to the applied RF/MW power.  One such RF/MW load is a small bead thermistor that responds with a change in resistance when RF/MW power is applied.  A diagram of a RF/MW thermistor mount is shown below.

Figure 1, RF/MW Thermistor Mount


  The thermistors in the RF Bridge are biased so that their individual resistance is 100 ohms thereby presenting a 50-ohm load to the RF/MW power because of the surface mount capacitor connected between the DC bias line and ground.  The surface mount capacitor is an open circuit to DC, but at RF/MW frequencies the value of the surface mount capacitor is chosen so that a low impedance (short circuit) is presented to the RF/MW signal.


  Refer to Figure 2 for an explanation of the DC Bridge operation.


Figure 2, Power Meter Block Diagram


  When RF/MW power is applied to the thermistor mount through the coaxial connector, additional thermistor heating is created and the resistance of both thermistors is lowered.  To bring the bridge back into balance, an equal amount of DC power is automatically subtracted by the bridge and displayed on the power meter indicator as a power measurement.

  Since ambient temperature changes will cause the thermistors to change resistance and create zero drift, temperature compensation is required.  A separate bridge and thermistors are incorporated into the measurement system to respond to temperature only.  With no RF/MW power applied to the thermistor mount, the term (Vc-Vrf) = 0 over ambient temperature variations and the meter remains zeroed.


Schottky Barrier Diode Detector

  Schottky barrier diodes are also used to measure RF/MW power, but do not use the heating effect of the applied power as an indicator.  RF/MW diode detectors depend on the non-linear diode junction to generate a DC voltage when a RF/MW signal is applied.  A diagram of a circuit used to measure RF/MW power is shown in Figure 3.  The detector diode is biased at approximately 27 uA to lower video resistance to a point where it can drive a DC load.


Figure 3, Power Measurement using a Schottky Barrier Diode


  The graph of Figure 4 shows the square law and linear region of a typical Schottky Barrier Diode.  In the square law region Vo = KPin with input RF/MW power levels between -60 and -20 dBm.  Above about -20 dBm input power level, the diode operates as a rectifier and is in the linear region.


Figure 4, Schottky Barrier Diode square law and linear region


  To remove the DC component from the measurement and provide temperature compensation, an additional diode of the same type as the detector is needed.  With the same bias and physical proximity, the DC level on each diode should be approximately equal.  A matched diode pair is preferred.  The differential configuration shown in Figure 5 can be used to measure RF/MW power in the -60 to -20 dbm range providing the DVM is capable of displaying a fraction of a mille-volt.

Figure 5, Differential RF/MW Power Measurement


  A coaxial attenuator can be used to extend the upper power limit on any of the power sensors described in this application note.