Holt TVC Model 12

Accurate calibration of low AC voltages presents several challenges not encountered at voltages above 1 volt. The introduction of new high resolution high acccuracy AC digital voltmeters with band widths up to a megahertz requires improved calibration standards.

The Holt LOW VOLTAGE THERMAL VOLTAGE CONVERTER, Model 12, is designed to meet these challenges while adhering to Holt's principle of extending the known state of the art of conservative metrology. This device is a passive TVC based on the principle of the Micro Potentiometer which was developed by Myron C. Selby at the National Bureau of Standards in 1951. The original micropots were developed for the generation of low voltages at frequencies to 1000 MHz. Holt has adapted this technique to a multi range TVC which provides continuous coverage of voltages from 1 millivolt to 200 millivolts at frequencies from 6 Hz to 1 MHz.

The AC/DC differences are very small and stable and the uncertainties are as low as NIST traceability will allow. A passive instrument with no amplifiers, the long term stability of the Model 12 is a function of it's vacuum thermoelement and thick film resistors. The AC/DC difference is not a function of applied voltage level so that ranges can be inter-compared for long term stability eliminating the need for frequent recalibration. When used with the Holt Model 68 Thermal Transfer Voltmeter as the readout device a constant resolution of better than 5 ppm is provided. The LVTVC provides stable output voltages at the frequency desired with low output impedances. The loading factor is less than 0.1% at 1 MHz when calibrating an instrument like the Hp 3458A at 100 millivolts. This loading factor is easily measured and a correction factor may be used to compensate if even higher accuracy is desired.

OUTPUT VOLTAGES

The Model 12 provides seven output voltage ranges with the following full scale voltages:
200 mV., 100 mV., 50 mV., 20 mV., 10 mV.. 5 mV., and 2 mV. Each range is usable from 120% to 40% of full scale.

TYPICAL AC/DC DIFFERENCES

The table below shows typical AC/DC differences of the Model 12. The frequency response curves of each range are the normal smooth responses of a resistive element with low inductance and a small capacitive load. There are no second order resonances or discontinuities