326 ABSTRACTS OF PAPERS

THE EFFECTS OF AN UNWANTED SIGNAL MIXED WITH THE CARRIER SUPPLYOF RING AND COWAN MODULATORS

By D. G. TUCKER, Ph.D., Associate Member.

(ABSTRACT of a Radio Section paper which was published in May, 1948, in Part III of the Journal.)

It is shown how the performance of a ring or Cowan modulator,when the carrier supply has an unwanted signal mixed with it, canbe investigated by a frequency-analysis of its modulating function.Results obtained for particular cases lead to certain general con-clusions, among which are the following:—

(a) The primary modulating effect of an unwanted signal mixedwith the carrier is largely independent of the resistance of thecircuit supplying the carrier, even though this affects the shapeof the modulating function very considerably. The amplitude ofthe unwanted component in the modulation is, relative to thewanted component, only about one-half of that in the carrier input.

Dr. Tucker is at the Post Office Research Station.

(b) Ring and Cowan modulators give a similar performance inthis respect.

(c) When the frequency of the input signal is equal to thecarrier frequency, the output of the difference frequency betweenthe carrier and unwanted signal is zero for an ideal modulator,and very small for one with a carrier supply of high resistance;as the resistance is reduced, the output of difference frequencyincreases, until with zero resistance it has a ratio relative to amain sideband of about one-quarter of the amplitude ratio ofthe unwanted signal relative to the carrier in the carrier-supplycircuit.

A VOLTMETER FOR THE MEASUREMENT AND COMPARISON OF CREST AND R.M.S. VALUES OFRECURRENT VOLTAGE WAVES

By D. E. M. GARFITT, M.Sc.Tech., Associate Member.(ABSTRACT of a Measurements Section paper which will be published in 1948, in Part II of the Journal.)

The voltmeter consists essentially of an electrostatic instru-ment in combination with a diode rectifier and a reservoir con-denser. With the condenser in parallel with the instrument andthe rectifier in series, the instrument will indicate peak voltages,of polarity depending upon the sense of connection of therectifier. With the rectifier short-circuited and the condenserdisconnected, the instrument will respond to the r.m.s. value ofthe applied voltage. The measuring circuit is shown in Fig. 1;

Fig. 1 .—Circuit for measurement of peak and r.m.s. voltages.For peak-voltage measurement, Si is in position 1, S2 in position 2 (negative)

or 3 (positive).For r.m.s. voltage measurement, Sj is in position 2, S2 in position 1.

the quantity measured is determined by the setting of the switchesS2 and S2, which may be operated whilst the voltage to be

Miss Garfitt is with the Metropolitan-Vickers Electrical Co., Ltd.

measured is applied to the input terminals of the circuit. Thuscomparisons may be made rapidly, permitting the accuratededuction of peak factors and the detection of harmonics—par-ticularly even ones, which result in unequal values of the twopeaks.

The circuit is accurate to within ± 2% of full-scale deflectionup to a voltage of 1 kV, and to frequencies of a few kilocyclesper second. It is applicable also to the measurement of thepercentage ripple on d.c. supplies, which can be deduced froma comparison of readings obtained in the "peak" and "r.m.s.'1

settings of the circuits.The range may be extended up to 100 kV or more by the use

of suitable potential dividers. Oil-immersed resistance dividersare described, with which an' accuracy of ± 3 % is attainable inthe measurement of direct or 50-c/s voltages. Standard typesof voltmeters in general use have accuracies of a higher orderthan this, but may be insensitive to impurities of waveform—e.g. circuits which employ moving-coil instruments for themeasurement of direct voltages will take no account of super-imposed ripple, and peak voltmeters of the rectified-condenser-charging-current type cannot normally be used with a waveformhaving multiple peaks or even harmonics.

The most useful application of the present circuit is thereforenot as a substitute for voltmeters of standard types. Its chiefvalue lies in its ability to detect small distortions of waveformand to enable the deduction of correction factors which may thenbe applied to readings obtained with these standards.