RF and Microwave Power Meter / Sensor Tutorial
January 20th, 2011. Posted in Test and Measurement, TutorialsRF Power Sensors are the maintstay of RF and Microwave test. There are few points of test more fundamental than for an engineer to need to measure RF power accurately. From the output power of a transmitter, to reflected power say on a reciever. RF power sensors are needed in all micrwave labs and come in a number of flavours. Here we give an overview of the types of RF power sensor and their applications, as well as some of the points to keep in mind when choosing your sensor.
RF Power Meter or RF Power Sensor?
In former times the items power sensor and power meter where two seperate devices. The meter took the readings and the sensor performed the task of sensing the power and transferring that nformation back to the meter for storage.
More recently, these two instruments / units have been combined into a single unit, now just the Power Sensor. Modern power sensors are able to use USB technology to connect with a PC, or instruments or a base unit if the user wishes (but this is not necessary) to capture all data and perform the processing needed before producing a result.
A good example of this technology in use if the first sensors of this type, and currently the only sensors on the market from DC to 67GHz as a USB type power sensor from the Rohde and Schwarz NRP series.
But there are many ways to measure RF power, right? What makes RF Power Sensors so special?
Before choosing an RF power sensor the application that the sensor is to be used for must be carefully considered. It is true that other methods of measuring RF power are avialable, such as the use of a spectrum analyzer. Looking at the typical measurement accuracy specifications shows the main difference here and ultimately whether an RF power sensor is required, will depend upon what level of power measurement accuracy is needed.
| Accuracy | |
| Spectrum Analyzer | ~0.2 dB to 3GHz, >3GHz 1.5dB |
| Thermal Power Sensor | ~0.06 dB DC to 18GHz |
It is clear to see from the above typical specifications / accuracy that the RF power sensor is significantly more accurate at measuring power and at higher frequencies. The main reason for this is that with specturm analyzers it is very difficult to get a very good power match so that most of the power is absorbed into the detector, rather than being reflected. This results in a degredation in level measurement accuracy especially at microwave frequencies (>3GHz) where normally a different path is used in the spectrum analyser to convert the signal to something more usable.
By contrast, the RF power sensor is designed specifically to provide a very good power match to the signal source. This attention to detail for power matching is largely responsible for the performance of an RF power sensor in measuring RF power. There are other additions to be considered, but this is the main point.
The fact that sensors are typically quite small has other advantages such as allowing them to be used in probing stations to connect the end of the sensor directly to the DUT ensuring that power is transferred efficiently. This would be very difficult, if not impossible with a Spectrum Analyzer.
Power Sensors need to be able to provide results within a very tight tolerance for measurement uncertainty. For that reason alone a vendor for sensors should be chosen that is able to provide a fully traceable history of measurement uncertainty for calibration.
What types of RF power sensor are there?
Thermal RF Power Sensors
Although thermal sensors are slow over a relatively large portion of their dynamic range and they only can measure the average power of a signal, they still deliver the most accurate measurement results. One reason is the effect of harmonics and non-harmonics can be predicted very well
Diode RF Power Sensors
Diode power sensors are the most common type of sensors. In general, much their design is much more critical when it comes to measurement of modulated signals as the signal being measured is fed through a diode detector to produce a video volage and then sampled. Below is a diagram of the makup of a diode sensor which incorporates a chopper to invert the polarity of the signal prior to being fed into the op-amp. This design reduces pink noise and also removes the need for zeroing a power sensor as the ADC offset voltage can be determined as part of the chopping process. This makes measurements much faster to perform, without needing to remove any power from the input of the sensor to perform its zeroing function. This unique design is specific to Rohde & Schwarz NRP universal power sensors.
Peak / Pulse RF Power Sensors
Peak or Pulse RF power sensors are designed to be able to measure the pulse characteristics of signals, such as peak power, rise and fall time etc. Universal sensors such as the diode sensor and also thermal rf power sensors are not able to measure pulse characterstics. Either due to the sample rate of the sensor and rise time of the diode detector or the requirement to heat an element in the thermal sensor.
Peak or Pulse RF power sensors must be able to sample the video voltage at the output of the detector at a much wider bandwidth than the standard diode sensor such that the pulse characteristics can be obtained. Other rmeasurements may also be obtained including:
- Rise Time
- Overshoot on rising edge
- Fall Time
- Overshoot on falling edge
- Pulse Duration
- On Power, Off Power
- Pulse Repetition Frequency
