Figure 1 Custom thermocouple RF power sensor.
Figure 2 Integrated thermocouple diagram.
RF power sensors are essential for precisely measuring RF signal power. Their applications span various industries, including avionics test systems, semiconductor testing, telecommunications infrastructure, medical systems, satellite communications, radar and 5G technology. Systems typically measure power using various complex waveform adaptive modulation techniques, such as OFDM, CDMA and QAM. Often, systems incorporate an internal reference sensor to establish an absolute power level for verifying proper operation. An example of a LadyBug Technologies thermocouple RF power sensor is shown in Figure 1.
Understanding RF power levels at various points within a system ensures the safe and proper functionality of test systems. In addition to stringent performance requirements, size and weight constraints are essential in systems. LadyBug Technologies manufactures custom thermocouple-based RF sensors with better than 1 percent linearity and less than 0.2 percent readout movement per degree Celsius to enable high performance, lightweight system requirements. These sensors maintain precision performance over a wide -40°C to +80°C operating range.
TECHNOLOGY OVERVIEW
Modern RF sensor transducers are typically either diode-based or thermally-based. Diode-based sensors rectify an RF signal to produce a DC voltage, while thermally-based sensors convert RF energy to thermal energy for measurement by a secondary transducer. Thermal sensors are desirable due to their true RMS nature, allowing for high-accuracy power measurements independent of waveform characteristics. LadyBug has developed a process for producing custom thermal power sensors based on thermocouples. With this process, LadyBug can create custom products to meet industry needs at relatively low volumes.
Thermocouple-based sensors leverage the Seebeck effect, a well-characterized phenomenon where a voltage is generated at the junction of two different metals when there is a temperature difference between the junction and the ends of the metals. This effect occurs because the electrons in the metals diffuse from the hotter to the colder metal, creating a potential difference. In LadyBug sensors, the RF load has an integrated thermocouple that generates a voltage proportional to the RF power input. A diagram of the construction of this integrated thermocouple is shown in Figure 2.
Due to the inherently small signals being measured, the thermal mass that is being heated must be minimal. As shown in Figure 2, a 3 micron silicon membrane device serves as the transducer substrate to minimize the thermal mass the RF energy must heat. A 50 Ω termination resistor is built on top of this substrate and serves two purposes. It is an RF termination with the matching characteristics necessary to meet the intended system requirements. It is also a thermocouple built from metals selected to meet the system output voltage requirements. The thermocouple device is manufactured using standard MEMS processes and then flip-chip bonded to a low loss substrate.
Figure 3 Linearity error versus power level at different frequencies.
Figure 4 Output voltage versus frequency.
DESIGN CONSIDERATIONS
Key requirements for custom thermocouple-based RF power sensors include:
Sensitivity: The sensor’s typical output of 0.25 mV/mW is well-suited for high-resolution measurement of any RF signal within its range.
Linearity: The sensor output should be proportional to the power input over a wide range. LadyBug sensors achieve exceptional 1 percent linearity due to our innovative thermocouple-based technology. An example of linearity error at three different power levels over a broad frequency range for a thermocouple-based RF power sensor is shown in Figure 3.
Temperature Stability: The sensor maintains accuracy over a wide temperature range. Thermocouple sensors are inherently immune to ambient temperature changes as thermal gradients within the sensor remain intact, resulting in exceptional temperature stability. The output voltage variation versus frequency for a typical thermocouple-based RF power sensor is shown in Figure 4.
Maximum Power Rating: Thermocouple-based power sensors generate voltage proportional to RF power through thermal transduction. For every 3 dB power increase, the thermal gradient across the detector doubles in temperature. Consequently, the thermal gradient within the sensor can exceed hundreds of degrees Celsius. Careful balancing of dynamic range and sensitivity is crucial.
LadyBug thermocouple power sensors meet these requirements and provide optimal performance when integrated into embedded systems.
IMPLEMENTATION
The custom thermocouple-based RF power sensor uses a thermocouple to detect temperature changes, which are then converted into voltage changes. To prevent device temperatures from exceeding safe levels, which will occur with RF input levels of approximately 20 dBm, a protection diode is added to mitigate risk and enable the full dynamic range. This protection diode shunts high-power RF signals to ground to maintain device temperature and protect the silicon and metal films.
When bonding the device to the carrier PCB, a highly thermally conductive silver solder compound is used. Thermal energy dissipated within the thermocouple device is efficiently transferred. After mounting to the RF circuit, devices are encased in a waterproof package, maximizing durability. Following final assembly, each unit undergoes testing to ensure proper functionality and is accompanied by a full traceability report.
LadyBug Technologies uses a unique development and assembly process to design and implement custom thermocouple-based RF power sensors to meet customer specifications. By utilizing cutting-edge industry standards for calibration, design and implementation, LadyBug provides customized solutions to address unique customer requirements in RF power sensing.
LadyBug Technologies
Boise, Idaho
www.ladybug-tech.com