Calibration of Thermal Converters:
To calibrate thermal voltage converters, the unit under test (UUT) and the reference TVC are connected in parallel using a Tee structure, and, for measurements at frequencies below 100 MHz, the plane of reference for the measurement is the center of the Tee. TVCs have historically been constructed with General Radio Type 874 connectors for the voltage input terminals, and with two pin connectors for the output emf; however other types of connectors, particularly Type N and BNC are often used for the input voltage. In most cases, the Tee is matched to the input connector of the UUT, so that a Type 874 Tee will be used with Type 874 input connectors; however, occasionally a different Tee will be used and an adapter required to connect the odd converter to the Tee. At frequencies below about 100 kHz, errors due to the additional transmission line length to the plane of reference are negligible, but at higher frequencies, the additional transmission line length, and skin effect in the adapter may create significant errors in the measured ac-dc difference. In these cases, corrections must be made to the measured ac-dc difference in order to reference the UUT to the plane of reference.
Thermal current converters are connected in series so that both converters see the same applied current. Connectors for TCCs rated at 5 A or less are usually either UHF type or binding posts for spade lugs. 10 A and 20 A shunts are usually supplied with UHF connectors, and shunts for currents up to 100 A typically have either large pins or Type LC connectors. Adapters are rarely used with current shunts, and have little effect on the ac-dc difference.
For both types of converters the input signals (the unknown ac and both polarities of dc) are applied in a timed sequence to the input of the converters, and the output emf measured at the output terminals. Using a timed sequence is important because equally-space measurements cancel out linear drift in the converters. The ac-dc difference is defined as the difference in the difference in the response of the thermal converters between the application of the unknown ac signal and the average of both polarities of the dc reference. The measured ac-dc difference must then be corrected for the ac-dc difference of the standard.
To cancel any sequence effects in the measurements, an even number of data points should be acquired.