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Multipath Diode Power Sensors vs Thermal Power Sensors for Modulated Signals

I am often asked about measurement of modulated signals using diode sensors / meters. There is a view that Thermal Power Sensors / Meters are more accurate than Diode Power Sensors / Meters when it comes to measuring modulated signals. There is a trust in the industry that drives this – of course, heat dissipated in a resistor is an absolute measurement of power. From an R&S perspective our thermal sensors (NRP-Z5x series) are able to get the match so good that our linearity is proven to the be just about the best there is with a thermal power sensor in the market today.

Within the square law region the detector provides a linear relationship between incident power and output signal. The detector works as an RMS (voltage) rectifier and delivers a signal that is proportional to the average RF power. Outside the square law region the relationship is non-linear and this causes the problem if you cannot compensate these non-linearities in real-time anymore.

Even if the sensor is a multipath device with switches – during the switch points, this can cause distortion and large in-accuracies in the switch points. A diode sensor should give a good average result, but like the Thermal sensor it will not track the envelope (rise / fall time etc). This is a topic for a different post! R&S multipath diode sensors have a patented design that does not include switches for each power range. All ranges are measured simultaneously and a soft weighting is applied to each path. Effectively the switch linearity can be corrected so the whole sensor is operating in a linear fashion.

Below are some measurements on various modulated signals with an R&S diode sensor vs the results from a thermal sensor. Firstly, it can be seen that all results of the diode sensor are within 0.02dB of the thermal sensor (which is within the uncertainty due to mismatch, and in these examples I did not use gamma correction – s-parameter correction which we support) but when measuring linearity the NRP Diode Sensor (NRP-Z11) is within 0.1dB of the thermal sensor (NRP-Z51) over a 30dB dynamic range from +10dBm to -20dBm.

The images below show the difference in power measured for the same signal through a calibrated resistive splitter for various test signals. The power at the top is of a diode based multipath power sensor NRP-Z11, the power at the bottom is from a thermal NRP-Z51.

Linearity of NRP-Z11 vs NRP-Z51 for 802.11ac, 80MHz BW, 13.2dB crest factor – fixed averaging of 256

Its also worthwhile checking out our Power Meter Tutorial