Seizing power accurately: a single-parameter modulated RF power measurement is just so 20th century.(RF POWER METERS)(Cover story)

The first step in making accurate RF power measurements is choosing the sensor most appropriate for the signal. Obviously, you wouldn't use a sensor limited to 1 mW (0 dBm) for a 1-W (30-dBm) signal. Nor can you use a thermal sensor to measure the peak power of a fast pulse. And, a simple diode sensor calibrated to read the correct power level of CW signals can't be used for digitally modulated signals above about -20 dBm.

In addition, RF power measurement is subject to several sources of uncertainty. These include instrumentation accuracy, noise, zero set and drift, reference power uncertainty, reference-to-sensor mismatch, sensor linearity, sensor calibration factor uncertainty, sensor-to-signal mismatch uncertainty, and temperature effects. Of these, the two mismatch components often dominate.

If the signal to be measured is relatively large, you can minimize the mismatch uncertainty by placing a precision attenuator in the measurement path. Power reflected because of the impedance mismatch will be reduced by the attenuator on its way back to the signal source. If the attenuator characteristics are very stable and well known, inserting the attenuator should reduce the measurement uncertainty even though another element has been introduced into the circuit.

Many of the other measurement uncertainties relate to the power meter with which the sensor is used. Zero set and drift have been improved by design so that both effects are small in new meters. And, at very low levels, many power meters reduce their bandwidth and use averaging to enhance the signal-to-noise ratio. At higher signal levels, noise generally is not a problem. Sensor linearity remains as the next largest error source.

Sensors

Thermistors, diodes, and thermopiles are the more popular types of terminating RF sensors. Thermistors and thermopiles, as their names suggest, measure the amount of heat created by the RF signal. As such, these devices are not limited by video bandwidth or type of modulation. They accurately measure average RF power. Unfortunately, thermistors have only a limited dynamic range. New, integrated thermopiles are very linear but too slow in responding to a signal change to measure peak power, for example.

Diode-based sensors now have video bandwidths in excess of 35 MHz and, separately, a 90-dB dynamic range. However, there are many types of diode-based sensors, and they are not all created equal.

Basic Diode Sensor

A Schottky metal-semiconductor diode naturally exhibits a square-law characteristic for low signal levels up to about -20 dBm. This means that the current through the diode is directly proportional to the square of the voltage across it. Sensors using two diodes in a full-wave rectifying arrangement, as shown in Figure 1, develop twice the output signal of a single diode circuit.

For low-level signals, this sensor circuit works well, generally requiring averaging and a chopper-stabilized amplifier to detect the very small signals corresponding to -70-dBm power levels. Nevertheless, the output is linear with power so accurate measurements result for any type of modulation if the peak power level remains below -20 dBm.

[FIGURE 1 OMITTED]

Bird Electronic, a producer of in-line-type RF wattmeters used in the broadcast industry, has developed the diode-based BPM Series Broadcast Power Monitors. …

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