Electron transit time and other effects in a valve voltmeter operating at extremely low current

The theory of the effect of electron transit time on the comparison of v.h.f. voltage with a direct voltage, using a thermionic diode operating in the exponential region of its characteristic, is presented. The resulting transit time correction factor is shown to be dependent on the value of the peak voltage measured as well as on frequency, except for the lowest quarter of the usable frequency range where the voltage level has little effect on the correction factor. Agreement between theory and experiment is sufficiently close to enable the particular diode used (EA 52) to measure peak values of sinusoidal voltages between 0.1 and 2 V to about 3% at frequencies up to 1200 Mc/s, under laboratory conditions using a new method of comparing the r.f. voltage with a direct voltage that is independent of such variable things as total emission and contact potential. At frequencies up to 300 or 400 Mc/s, an accuracy in voltage measurement as good as ± 0.3 % can be obtained using the diode. It is concluded, however, that apart from frequencies up to about 400 Mc/s where the frequency correction is small, it is simpler to determine the frequency correction experimentally if suitable apparatus is available. This is because the accurate calculation of transit time effect which is necessary for the larger correction factors at frequencies around 1000 Mc/s requires difficult subsidiary measurements to determine all the constants required for the particular diode used.