12 Jul 2010

Our editor, Ian Poole interviews Thomas Reichel, Head of Power Meter Development at Rohde and Schwarz about RF power sensor technology and looks at key elements in their design.

The diode based types need very careful design, but they offer many advantages in terms of functionality, speed, and dynamic range - they can actually measure down to levels of around -60 or -70 dBm whereas the thermal power sensors typically have a lower range limit of between -30 to -35 dBm. Reading the peak power or the rise time of a pulse and making statistical evaluations of the envelope within some milliseconds are typical tasks for the new designs. Reichel added: "For about eight years, so-called USB power sensors have been available. These are in fact, miniaturized power meters that are operated directly from PC's or other measuring instruments. If properly designed, these sensors outperform their traditional counterparts in terms of accuracy, speed and versatility. Rohde & Schwarz was first in this field and feels dedicated to go this way further."

RF power sensor design challenges

I asked Reichel about the key challenges in the design of an RF power sensor? He replied by saying: "There are many challenges in power sensor design, but probably the main ones in the RF area revolve around the impedance matching required to provide the very high accuracy levels that need to be achieved. If there is a poor match, then there will be multiple reflections of considerable amplitude between the power sensor and the DUT, resulting in the DUT emitting more or less power than in the ideal case of a perfectly matched load. Therefore the measurement result of the sensor will be different from the expected one, even if the sensor itself performs very well. The power loss at the sensor input due to sensor mismatch doesn't cause mismatch errors at all, since this loss is included in the calibration factor of the sensor. In contrast to the effect of multiple reflections, reflection loss is independent from the DUT and can therefore be determined very well"

R&S Power Sensors

"There are other challenges as well. For thermal sensors, enabling the sensor to remain accurate when the environment is changing presents real problems. As the sensor basically operates by detecting the temperature change resulting from power being dissipated in a load, external temperature changes need to be removed from the equation." "Another challenge is in achieving the dynamic range needed. It took a lot of development work to be able to achieve a dynamic range of 55dB with a thermal power sensor. In previous times, linearity was a problem at the top end, which has been solved meanwhile by accurate numeric compensation. The low power end of the scale is still a challenge due to the influence of noise, zero offset and zero drift."

Accuracy

I asked Reichel about power meter measurement accuracy, and wondered what some of the major pitfalls were when making measurements.

He answered by saying that: "One of the major problems we find is that people use the wrong type of sensor. Besides a selection by frequency and power, it is necessary to look at the required measurement and its uncertainty as well as at the DUT and the waveform being measured to select the correct sensor - thermal, multiple-path, wideband. CW power sensors shouldn't be used any more, since they don't like any modulation of the envelope or even spurious, delivering unpredictable results in these cases."

"Another major problem we find is that the RF connector on the power sensor becomes damaged with use. As the RF connection directly to the sensor is very important any damage to the connector can result in a poor connection being made and this will have a significant effect on the measurement."

"There are other problems that can occur - the frequency input may not match the signal frequency. Power sensors make a lot of frequency-dependent numeric compensations. Therefore they need a numeric frequency input - that is until they can make their own frequency measurements."

"There may also be a large zero offset if the zeroing has been forgotten - it is surprising how often this happens. The zero offset is an additive power contribution independent from the power level, leading to large relative (dB) errors at low levels. Zeroing reduces this effect by considering the outcome of a test measurement without signal, executed by the user. Zero offsets mainly stem from un-symmetries in the signal path between the RF transducer and the first amplifier stage, generated by the sensor and the base unit as well in traditional designs. Since integrated power meters like all the sensors of the Rohde & Schwarz NRP family contain all the analogue circuitry, these offsets can be measured in the factory and stored in the data memory of the sensor. They are automatically considered from the first measurement, so that an additional zeroing by the user can be omitted in many cases."

RF Power Sensor

"Another important consideration is the mismatch uncertainty and whether this has been considered. At least 'guessing' the amount of mismatch uncertainty should be a must for any power measurement, since this contribution to total uncertainty often exceeds the uncertainty specified for the sensor alone. This is especially true, when frequency rises and the matching of the DUTs and the power sensors as well get worse. However, if the impedance mismatch of the DUT and the power sensor are known in a complex notation, a proper numerical compensation is possible. The R&S NRP power sensors support this by a measurement function called 'gamma correction', which only needs the complex reflection coefficient of the DUT to be input to the sensor."

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