as far as the dc voltmeter is concerned. For rf frequencies, the two thermocouples are in parallel, being driven through coupling capacitor Cc. Half the rf current flows through each thermocouple. Each thin-film resistor and the silicon in series with it has a total resistance of 100 ohms. The two thermocouples in parallel form a 50 ohm termination for the rf transmission line. The lower node of the left thermocouple is directly connected to ground and the lower node of the right thermocouple is at rf ground through bypass capacitor Cb. The dc voltages generated by the separate thermocouples add in series to form a higher dc output voltage. The principal advantage. however. of the two thermocouple scheme is that both leads to the voltmeter are at rf ground; there is no need for an rf choke in the upper lead. If a choke were needed it would limit the frequency range of the sensor . 
The thermocouple chip is attached to a transmission line deposited on a sapphire substrate as shown in Figure 3-5. Coplanar transmission line is used to allow the line dimensions to taper down to the chip size, while still maintaining the same characteristic impedance in every cross-sectional plane. This structure contributes to the very low reflection coefficient of the 8481A sensor, especially at microwave frequencies. 
The principal characteristic of a thermocouple sensor for high frequency power measurement is the sensitivity in microvolts output per milliwatt of rf power input. The sensitivity is equal to the product of two other parameters of the thermocouple, the thermoelectric power and the thermal resistance. 
The thermoelectric power (not really a power but physics texts use that term) is the thermocouple output in microvolts per degree centigrade of temperature difference between the hot and cold junction. In the HP 8481A thermocouple mount, the thermoelectric power is designed to be 250 uV/°C. This is controlled by controlling the density of n-type impurities in the silicon chip. 
The thickness of the HP 8481A silicon chip was selected so the thermocouple has a thermal resistance 0.4°C/mW. Thus the overall sensitivity of each thermocouple is 100 uV/mW.Two thermocouples in series, however, yield a sensitivity of only 160 uV/mW because of coupling between the thermocouples; the cold junction of one thermocouple is heated by the resistor Qf the other thermocouple giving a somewhat smaller temperature gradient. 
The thermoelectric voltage is almost constant with temperature. It depends mainly on the temperature gradients and only slightly on the ambient temperature. Still, ambient temperature variations must be prevented from establishing gradients. The chip itself is only 0.8 mm long and is thermally short-circuitecl by the relatively massive sapphire substrate. The entire assembly is enclosed in a copper housing. Figure 3-6 depicts the better thermal behavior of a thermocouple compared to a thermistor power sensor . 
The thermoelectric' power is not exactly constant, but does vary somewhat with temperature. At large powers, where the average thermocouple temperature is raised, the output voltage is larger than predicted by extrapolating data from low power levels. At a power level of 30 mW the output is 3 percent high, at 100 mW it is about 10 percent high. The circuitry in the HP power meters used with thermocouples compensates for this effect. Circuitry in the sensor itself compensates for changes in ambient temperature. 
The thermal resistance limits the maximum power that can be dissipated. If the hot junction rises to 500°C, differential thermal expansion causes the chip to fracture. The HP 8481A is limited to 300 mW maximum average power. 
The thermal resistance combines with the thermal capacity to form the thermal time constant of 120 microseconds. This means that the thermocouple voltage falls to within 37 percent of its final value 120 uS after the rf power is removed. The response time for measurements, however, is usually much longer because it is limited by noise considerations in the voltmeter circuitry.