A PTC thermistor and a wire-wound low-temperature coefficient heater are bonded to the outside of the 30° conical receiver cavity. The PTC thermistors are polycrystalline barium strontium titanate with dopants added to make them semiconductors. A sharp rise in the resistance R with increasing temperature T occurs above the Curie temperature associated with a ferroelectric phase transition. This results in dR/dT values of up to 1000 Ohms/K with an operating resistance of only 10 kOhms.
By careful tradeoff between several critical parameters, the NISTAR ACRs were designed for optimum power measurements in the tens of microwatt range. The optical signal incident on the receiver is only 1 microWatts cm-2 , however the emission from the receiver cavity to space is estimated to be 30 microWatts cm-2 when the shutter is open.
There are four digital control loops, three receiver cavity control loops and one for the heat sink. The PTC temperature sensor resistance measurements are performed with AC-Bridge circuits operating between 35 and 155 Hz. The AC-Bridge circuit enables the measurement to take place in a very narrow bandwidth about the excitation current frequency, effectively rejecting noise at all other frequencies. This low-noise arrangement is able to measure resistors such as the PTC thermistor at noise levels comparable to the intrinsic thermistor Johnson noise. The NISTAR AC-bridge circuit has been designed to have a full scale of 20kOhms with a resolution of 0.010 Ohms or 0.5 ppm.
The PID digital control loops are realized in software where the error signal from the AC-bridge is used to control a DAC driving the heater circuit for each device. An A/D converter measures the voltage across the receiver heater. The measured electrical power is derived from the square of this voltage divided by the known resistance of the receiver heater. The NISTAR electronics have both a circuit to apply power and an independent circuit to measure the voltage drop across the heater, thereby improving the reliability of the measurement. The time series of this electrical power during shutter cycles is used to deduce the optical power. The measured irradiance is then determined by dividing the measured optical power by the measured area of the precision aperture mounted in front of the receiver cavity.