Power Measurement Using Non-Thermal Devices

There are three primary methods for measuring RF power. They are diode voltage detectors similar
to the Bird 43 series of wattmeters or RF volt meters similar to the Booton 9200B. Both of these 
devices read RF voltage and their meter scale is calibrated to read in dBm or Watts. Thermocouple
and thermistor devices similar to the HP/Agilent 438/432 series respond to heating power and read
actual average power less mount conversion losses.

Theory of Diode/Voltage Detector Operation

Let's examine the diode and voltage detectors. Since diode and voltage detectors are voltage 
sensitive devices they are calibrated by using the known relationship between RMS and peak 
voltage. This relationship is only true for distortion free sine waves or amplifiers and or 
transmitters with harmonic filters installed. Their usefulness is limited to measuring CW 
signals. Both conventional AM and SSB (two tone) signals in addition to harmonic distortion will 
cause unpredictable results. The measured voltage is a function of the addition and subtraction 
of the magnitude and phases of the carrier sidebands.

Diode/Voltage Detector Limitations

This is a fundamental limitation of using diode devices to measure RF power. Both thermocouples 
and thermistor devices use heating as a method of measuring power. This is true RMS* power and 
although the implementation of the devices is slightly different, both are unaffected by 
harmonics and the number of signals or sidebands present in the RF signal.

Explanation

The reason for the measurement error when using diode type detectors is that the complex voltages
generated by vector addition of all the signals present distorts the signal from a pure sine 
wave and the relationship between average power and peak voltage is no longer valid. To 
illustrate this point if one is trying to measure the power level of a signal with two tones and 
one tone is 20 dB less that the first tone, the thermal type wattmeters will read 1% higher than 
with one tone. This is because the power in the second tone is really one-hundredth that of the 
primary tone. A diode type power meter would read the voltage. Now a tone 20 dB down from the 
reference tone will have an amplitude of 10% of the reference tone so depending on the relative 
phase of the two tones the resultant peak to peak voltage could result in and an error of +/- 10% 
in voltage or +/- 20% in power. This is a possible error of almost +/- 1 dB and compared to an 
error of just .043 dB error for the thermal measurement method.

Conclusion

In general, the diode type of power meter is accurate and useable for any amplifier testing 
that is CW with the harmonic level below -50 dBc. If this is not the case, then thermal type 
meters should be used to avoid power measurement errors. Diode or voltage detection type power 
measurement should not be used where harmonics and multi-tone (asymmetrical) signals similar but 
not limited to SSB are present.

* average power