By their nature, instruments such as power meters and power sensors are used in applications where they are exposed to high RF power measurements. If handled correctly these instruments are very reliable and rugged. However, manufacturers receive a significant number of damaged power sensors every year. Based on an assessment of those repairs, this article highlights practices that will protect and prolong the life span of power meters and power sensors.
Practice 1: Avoid Overpowering
Figure 1 shows the basic method of measuring high-frequency power using a power meter and power sensor. The power sensor converts high-frequency power to a DC or low-frequency signal that the power meter can measure and relate to an RF power level. The meter displays the detected signal as a power value in dBm or watts.
The maximum measurable power for a power sensor can vary from low power, –60 dBm, to high power, +40 dBm. To provide a safety margin, the maximum power rating is slightly higher. Even with the safety margin allowance, over half of the failed sensors received at the Keysight Technologies, Inc. Service Center are caused by overpowering. Inspections typically reveal a destroyed component in the bulkhead thin-film circuit.
The bulkhead is the metal part of the power sensor and is the most expensive module in a power sensor, costing around 80% of the price of a brand-new unit of the same model (Figure 2). Inside the bulkhead, there is a plastic bead supporting a fragile, high precision RF input connector center pin, a cartridge with a thin film circuit populated by termination and attenuator resistors, and a sensing element. The function of the bulkhead is to convert the RF input to a low DC voltage which varies in proportion to the input power.
To prevent overpowering the bulkhead:
- Know the approximate signal level you are measuring
- Make sure the measured power is well within the dynamic range of the power sensor
- Use an RF limiter to attenuate power that exceeds the power sensor’s limits