Discovery and Resolution for Measurement Anomalies in Type-N Coaxial Mount at 10 MHz

This article analyzes the comparison data of the Type-N coaxial 8478B thermistor mount from the report CCEM.RF-K8.CL. An abnormally low calibration factor at 10 MHz is identified, which is more than 3% lower than that at 50 MHz. Even if combined with reflection coefficients for analysis, the calculated effective efficiency at 10 MHz is still more than 1% lower than that at 50 MHz. However, from the perspectives of loss and substitution errors, the value at 10 MHz should be much closer to that of 50 MHz. After careful analysis, we found that this anomaly is attributed to the radio frequency (RF) signal leaking from the mount to the power meter during measurements at 10 MHz. Then, the direct current (dc) bias provided by the power meter to the mount became abnormal, and the mount’s operating resistance dropped below <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$200~\Omega $ </tex-math></inline-formula>, which should have remained consistent. Therefore, the power meter supplied higher dc power, causing a greater temperature rise in the thermopile, which resulted in a lower measured effective efficiency. According to the above analysis, we designed a plug-in isolator with an inductor-capacitor (LC) circuit to isolate the leakage signals. The anomaly elimination by the proposed isolator for the Type-N coaxial mount at 10 MHz dramatically enhances the accuracy of the calorimetric primary standard in the full frequency range.