Automation of Direct Comparison Technique and Study of long term Performance on the Calibration factor of the Thermistor Mounts in N- type Coaxial line upto 18 GHz

National physical laboratory (NPL) of India is the premier research & development center and the National Metrology Institute, which provides traceability in measurements by calibration throughout the country. Radio Frequency (RF) power is one of the most important quantities in RF metrology. The measurement of calibration factor for thermistor mounts by direct comparison technique in the frequency range of 50 MHz to 18 GHz and its automation is presented in this paper. The automation of this technique makes calibration faster and more accurate through GPIB cables using IEEE-488 interface card. The software for this technique has been developed using the VEE Pro application, which is a graphical programming environment optimized for use with the electronic instruments. It also minimizes the human involvement and therefore reduces the chances of errors. The stability performance study carried out on three different thermistor mounts by the direct comparison technique is reviewed in this paper. The results show that the maximum deviation in the calibration factor of the thermistor mounts from the reference value over the years is within plusmn1.5% from 50 MHz to 18 GHz.