The only significant aging mechanism found so far is thermal aging of the tantalum nitride. A group of devices were stress tested, producing the results of Figure 3-7. These curves predict that, if a device is left at 300 mW for one year, the resistance should increase by about 3.5 percent. Nine days at a half watt would cause an increase in resistance of 2 percent. The aging accumulates. 
With such a design it is relatively easy to adapt the basic design to other requirements. It is easy, for example' to change each tantalum-nitride resistor to 150 ohms so that the thermocouple can work in 75 ohm systems. It is also easy to enhance low frequency performance by using larger blocking and bypass capacitors. This usually compromises high frequency performance due to increased loss and parasitic reactance of the capacitors. The low frequency HP 8482A Power Sensor is designed for 100 kHz to 4.2 GHz operation in contrast to the 8481A which operates from 10 MHz to 18 GHz. 

Power Meters for Modern Sensors 
Design contributions do not stop at the sensing element itself or with the rf structure. The HP 435A Power Meter will be used to describe the basic instrumentation (Figure 3-8). This meter has full scale ranges from -25 dBm to +20 dBm for use with the 8481A. With other sensors the same dynamic range is translated up or down. With the semiconductor diode 8484A Power Sensor, to be described in the next chapter, the full scale ranges go from -65 dBm to -20 dBm. 
The thermocouple dc output is very low-level (approximately 160 nV for 1 microwatt applied power), so it is difficult to transmit on an ordinary cable. Small, undesired thermocouple effects creep into the system -- especially at connections and points of wear and corrosion. This problem is multiplied if the user wants a long cable between his sensor and power meter. For this reason it was decided to include the low-level dc circuitry in the power sensor, so only relatively high-level signals appear on the cable. 
The only practical way to handle such low dc voltages is to "chop" them to form a square wave, amplify this with an ac-coupled system, then synchronously detect the high-level ac. For reasons mentioned above. it was decided to include the chopper and part of the first ac amplifier in the power sensor. The chopper itself