heat that does enter the diode area is directed uniformly to both ends of the diode package. With both ends changing temperature uniformly, the temperature gradient across the diode is almost nil, thus minimizing drift. Any thermal gradient is also reduced by the heat sinking action of the outer housing (see Figure 4-5). 
Like other Hewlett-Packard power sensors, the 8484A is supplied with calibration factor vs. frequency data. Diode detectors sometimes have calibration factors at microwave frequencies that are larger than those at 50 MHz. This means that the calibration factor dial on the power meter should be adjusted to the proper 50 MHz value before going through the calibration adjustment procedure for open-loop power measurements. The reference calibration factor is clearly marked on each 8484A Power Sensor. 
The power emerging from the 50 MHz reference output on the power meters is l mW (0 dBm), which is near the center of the dynamic range of most power sensors, but outside the range of the sensitive 8484A. Since the range of any power sensor may be translated upward by a suitable attenuator, a special attenuator of 30 dB at 50 MHz is supplied with each 8484A. When that attenuator is attached to the power reference output terminal on the power meter, the emerging power is 1 microwatt (-30 dBm). Attenuator design is such that a maximum error of 1 percent is added to the calibration step.
 
The sensitivity of the diode detector in the 8484A is somewhat influenced by ambient temperature. Compensation circuitry is included in the sensor, but, at extreme ambient temperatures, the instrument should be recalibrated using the reference oscillator and the 30 dB attenuator to obtain the most accurate results. Typical sensitivity variations with temperature for uncompensated and compensated detectors are shown in Figure 4-6.