The Measurement of Fluctuation Noise by Diode and Anode-Bend Voltmeters

The response of a diode voltmeter to a C.W. sine-wave signal and to noise is analysed for three typical diode characteristics: discontinuous linear, discontinuous parabolic and exponential In the first type the indication is proportional to the r.m.s. value of the input voltage but the constant of proportionality is different for noise and for C.W. and depends upon the ratio of the load resistance to diode resistance For small input signals the curvature of the diode characteristic is important and the voltmeter tends to a square-law behaviour with the result that C W, noise or a mixture of C.W. and noise can all be measured in terms of a single calibration. The input conductance of a diode voltmeter is in general greater for noise than for C W It can be very much greater in practical conditions and this represents a possible source of error in the measurement of noise The response of the anode-bend voltmeter to C.W. and to noise is considered for a discontinuous parabolic and for an exponential characteristic The departure of the C.W. calibration from a square law and the difference between C W. and noise calibrations are evaluated in terms of the characteristics.