Product How To: Use multipath diode sensors to make precision power measurements

Thomas Röder, Rohde & Schwarz -September 05, 2012

For a long time, power meters were built using a combination of a base unit and an external power sensor connected via a cable. The RF signal was converted to a voltage signal in the power sensor, amplified and then digitized and displayed in the base unit. Purely analog signal transmission was used between the power sensor and the base unit in this type of power meter. This approach has the benefit that a power sensor appropriate for the task at hand can be selected without requiring a new base unit. The obvious disadvantage is that a power sensor can never be used without a base unit.

Today, the situation has changed due to the advancing miniaturization of components as well as the availability of small, high-performance, energy-efficient processors. A power meter can now be manufactured as a small, integrated unit and connected directly to a PC or base unit via a standard USB interface. In this case, the base unit does not perform any analog signal processing. Instead, it is used primarily for control and measured value display.

This solution has a clear advantage: The integrated power meter does not consist of multiple components and can be characterized in full during the manufacturing process. This eliminates the need to calibrate the combination of the sensor and base unit with a reference signal prior to usage. Moreover, signal processing is less vulnerable to unwanted influences since it is performed inside an integrated device, and zeroing is required only for very small signal amplitudes.

Sensor technologies
Power meters are available based on different technologies that cover a frequency range extending beyond 100 GHz and a power range from 100 pW to several tens of watts.
Nowadays, mainly the following technologies are used in power meters:
•    Thermo-electric detectors
•    Multipath diode detectors
•    Wideband or peak sensors with a diode detector
•    CW sensors with a diode detector and integrated logarithmic detectors

Thermal sensors convert the applied RF power into heat using a resistor. The RF power can then be computed based on the temperature difference between this resistor and its immediate surroundings. The major disadvantage of thermal sensors is their low measurement speed and thus the inability to display the power envelope. Owing to the way thermal sensors work, they can only be used to measure power starting at approximately 300 nW. Their dynamic range is therefore limited.

Diode-based power sensors extend the power measurement range far below 300 nW. They offer a dynamic range of up to 90 dB. Depending on their implementation, these sensors can also measure the power envelope up to a bandwidth of several tens of MHz.

A diode-based power sensor converts the RF signal to a DC voltage signal using an RMS detector. At power levels below –20 dBm, the detector exhibits a linear relationship between the RF signal and the output voltage. This is known as the square-law region. Here, the diode detector behaves more or less like a thermal detector and is largely immune to harmonics and amplitude modulation. Above this signal level, the linear relationship between the RF signal and the detector output voltage no longer exists. Precise power measurements are possible in this region only if the signal's bandwidth is smaller than the detector's bandwidth. In addition, each measured value must be linearized numerically before it can be used for further computations.

The solution: multipath diode power sensors
Multiple techniques are required in order to exploit the positive characteristics of the diode detector in the construction of a universal power meter with a wide dynamic range.

First, several diodes are connected in series to form what is known as a stack. This results in a dynamic range improvement of 10 dB·log(N), where N is the number of diodes. Moreover, two or three independent measurement paths with different attenuation levels are integrated into the detector.

The path with the best performance for the applied RF level is selected. Hard switchover between paths is one possibility, but it is associated with hysteresis. Smooth transitions can also be attained, as is the case with the Rohde & Schwarz NRP-Z sensors. The latter approach has many advantages, including the avoidance of signal steps, better reproducibility due to elimination of hysteresis, and the ability to measure the power envelope without interruptions. Moreover, there is an improvement in the S/N ratio of up to 6 dB in the transition region.

Figure 1: Improvement in accuracy in transition region due to path weighting.

Figure 1 shows the measurement uncertainty in the transition region of two paths for hard switchover and smooth transition. The blue curve characterizes the sensitive measurement path, which is operated at the switchover point at its upper measurement limit. At this operating point, measurement uncertainty increases rapidly due to harmonics or modulation. The red curve characterizes the less sensitive path. This path is operated at the switchover point at its lower measurement limit. Its measurement uncertainty increases due to noise and zero offset as the level decreases. Smooth transition results in better performance along with faster measurement speed in the transition region.

The extensive effort that has gone into the development of today's multipath diode power sensors has brought about a certain degree of success. These sensors now nearly match the accuracy of thermal sensors while providing better dynamic range and higher measurement speed. Integrating multiple measurement paths that operate simultaneously has become possible with the manufacture of integrated power meters.
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