The 8484A power sensor can accept average and peak overloads of 200 mW ( +23 dBm) without permanent damage. The maximum level for accurate measurement, limited by the diode square-Iaw region, is 0.01 mW (-20 dBm). This results in a safety range of 20,000:1 (43 dB). 
Amplitude Modulation 
There is a somewhat popular, but erroneous opinion that diode detectors cannot measure the average power of an amplitude-modulated wave. This results from the concept of a diode detector as a peak detector. Proponents of this belief say that only sensors that convert energy to another form, such as a thermocouple or thermistor that measures heat, actually measure the true average power. In actuality, diode detectors do properly measure average power so long as operation is confined to their square-law region. This can be verified experimentally by measuring amplitude modulated waves with thermocouples and then with diode detectors. The validity can also be demonstrated by a mathematical analysis of equation (4-1) in the circuit of Figure 4-1. Such an analysis is beyond the scope of this application note. 
The erroneous belief about the diode detector arises from mistakenly applying the principle of peak detectors. The principle of the peak detector is that voltage stored on bypass capacitor Cb of Figure 4-1 during the peaks of the modulation cycle is quite large. It is so large that, in the valleys of the modulation cycle, the rf peaks are not large enough to switch the diode into conduction. There is no measurement of power level in the valley because the rf source is effectively disconnected from the voltmeter by the back-biased diode. 

Figure' 4-7 shows the square-law range of the i-v characteristic for a typical low-barrier Schottky diode. If there is -20 dBm of GW power applied to the circuit of Figure 4-1 that i-v characteristic is traversed every rf cycle. The voltage across the diode swings from about -35 mV to +25 mV. The capacitor Cb is charged to about 5 mV and supplies 1 uA of dc current to the voltmeter amplifier. The average source impedance that charges Cb is about 2.5 K ohms as found from the slope of the i-v characteristic at v = -5 mV. 
Now if the rf amplitude is reduced by 20 dB to -40 dBm, the voltage across the diode swings only ±3 mV, at first about the -5 mV position but eventually about the 0.05 mV position. Note, however, that the diode impedance is still about 2.5 K ohms during every portion of the rf cycle. The diode is never "off"; it never disconnects Cb. from the rf source. In summary, the diode is just as efficient in discharging Cb during valleys of the amplitude modulated signal as it is in charging Cb during crests of the amplitude modulated signal. Only when rf voltage levels are so large that the average diode resistance during crests is significantly different than during valleys of the modulating signal is the peak detector "on-off concept" valid. 
Diodes can, in principle, be used for absolute power measurements outside the square-law region by using appropriate shaping or non-linear amplification. Such shaping, however, is only valid for GW and constant amplitude waveforms. For general amplitude modulalion, shaping is invalid. Even for GW waveforms, the shaping required for accurate measurement is likely to be a strong function of temperature. 
The principal advantage of the low-barrier Schottky diode is the ability to measure power levels down to 100 picowatts (-70 dBm) from 10 MHz to 18 GHz. This level is about 30 dB (1000:1) lower than the power levels needed by the crystal radios from the early days of electronics. The sensitivity at low levels is achieved while maintaining a low-reflection coefficient and therefore also maintaining accurate, convenient power measurements.