For many years, MEDTEQ work in the HF area has been backed up by thermal measurements - using the temperature rise of a resistor to infer the rms voltage, which in theory is independent of frequency. This method is not new, and thermal based methods are a key foundation for accuracy in high frequency measurements.

The MEDTEQ systems have been excellent for detecting relative change, but until now have been sensitive to changes in ambient temperature, requiring frequent re-calibration against a reference meter at low frequency.

Attempts to create a meter with absolute accuracy have fallen for the assumption that bigger is better - larger and larger heatsinks to eliminate the effects of ambient change, more and more thermal insulation. However, it was finally realized that larger heatsinks only exacerbate the problem by creating longer time constants, and most importantly, different time constants between the two sensors used to detect a temperature rise.

Ultimately it was realized that smaller is better - and the final solution even includes fan forced cooling. Using a fan reduces the time constants of the system below the potential changes in ambient temperature. The result is a system that responds accurately to temperature rise of the resistor, from which Vrms can be determined.

Early attempts have found that this new fan forced sensor is accurate to within 0.2% for absolute measurements, and still able to show short term variations of 0.1%.

The smaller size of the sensor makes this new measurement system easy to incorporate into the HFTEQ range meters. Being a thermal-meter (with relatively slow response), and displaying only to Vrms, the meter is not intended for day to day measurements. Rather it is intended a background tool for reference. The area of high frequency is such that having two independent methods is important to provide overall confidence in the measured result.