short communication time jitter phase controlled thyristor...

Electrocomponent Science and Technology1975, Vol. 2, pp. 213-214

(C) Gordon and Breach Science Publishers Ltd.Printed in Great Britain

SHORT COMMUNICATIONTime Jitter in Phase Controlled Thyristor Circuits

J. K. HALL and D. S. PALMER

Dept. ofElectronic and Electrical Engineering, University of Technology, Loughborough, Leicestershire

(Received July 7, 19 75)

INTRODUCTION

It is frequently assumed that radio interferencegenerated by thyristor circuits is entirely due to theharmonics of the output waveform. However a closerexamination of the higher frequency spectrum oftypical phase controlled circuits reveals that thespaces between the harmonic lines are infilled withwhat appears to be noise, This communicationpresents some of the early findings of an investigationinto this phenomenon.

POSSIBLE SOURCES OF NOISE

Three sources of noise may be postulated at theoutset, the thyristor, the firing system and the mainssupply. Noise may also be produced by interactionbetween these sources in some cases. Of particularimportance in this respect is the time jitter in theperiod of conduction of the thyristor, and betweenthe conducting periods. This jitter is caused byrandom variations in the time the thyristor com-mences and ceases conduction.

The possible types of noise produced in semicon-ductor devices are well documented.2 In so far astime jitter due to the thyristor turn-on process isconcerned, only random effects in the avalanche cancause variation in turn-on time from cycle to cycle ifall the external parameters of the thyristor such asgate voltage and current, and its rate of rise, anodevoltage and junction temperature are held constant.Any variations in timing of the gate signal providedby the firing circuit will produce jitter in the outputwaveform.

Two variable properties of the mains supply mayinfluence timing jitter. Changes in load flow in thesupply system impedance cause fluctuations both inthe timing of the zero crossings about a mean

position, and in the amplitude of the voltage. If thezero crossings of the supply voltage are not fixed atprecisely regular intervals in time, neither can be theturn-on nor the turn-off of the thyristor. In mostsimple firing circuits the time delay is produced bycharging a capacitor from the mains through aresistor. Fluctuation of the mains amplitude willtherefore cause further turn-on timing errors.

EFFECTS OF TIME JITTER

The precise effects of time jitter upon the output of aphase controlled thyristor circuit and upon thesupply current drawn by that circuit will dependupon its source. If all the jitter present is introducedby the firing circuit, then only the steep front edge ofthe waveform will move in time, the rest of thewaveform being stationary. On the other hand if allthe jitter is produced by mains irregularities thenboth leading and trailing edges of the waveform willmove randomly in time.

In the latter case the resultant waveform may beexpressed mathematically as:

32rn

F(t-Dq(T)) , An cos---f- (t-Oq(t)) (1)n=O

where F(t) is the unjittered time waveformD is a constant with the dimensions of timeq(t) is a dimensionless function of time

so that F(t Dq(t)) is the time jittered waveformAn is the peak amplitude of the nth harmonicT is the fundamental period of the waveformt is time.

The nth harmonic may be written as

2?rnFn(t’Dq(t)) An cos (t-Dq(t))

213

214 SHORT COMMUNICATION

A phase modulated carrier wave may be written as

)y(t) C cos \-c + mpq(t) (3)

Where C is the peak carrier amplitudeTc is the period of the carriermp is the phase modulation index.

Comparing Eq’s (2) and (3) it can be seen that, ifthe period of the nth harmonic Tin is substituted forthe carrier period Tc and if 2rnD[T is substituted forrnt,, the two equations are identical in form. Thisdemonstrates that each harmonic of the jitteredwaveform is phase modulated. An important conclu-sion which may be drawn from this is that the phasemodulation index mt 2zrnD/T is proportional to theharmonic number. Thus more pronounced phasemodulation effects may be expected in the highfrequency spectrum.

Consider the case where the fundamental wave-form is jittered sinusoidally, the jittering frequencybeing appreciably lower than the fundamental fre-quency. The phase modulation produces a series oflines spaced by the jittering frequency on either sideof the main spectral lines. The higher the mainwaveform harmonic, the more sideband lines willappear. If the jitter is random, the sideband lines arereplaced by noise.

Jitter does not increase the total power containedin the spectrum; it merely redistributes it so that athigher frequencies the inter-line spaces become filledin with noise while the main lines fall in amplitude.

If the mains supply is fluctuating in amplitude, i.e.amplitude modulated, the effect on the spectrum willbe that all the harmonics will be amplitude modu-lated with the same modulation index.

a) thyristor avalanche variation has little effect.b) the firing circuit introduces only a small

amount of noise when driven from a stable source;the "cheaper" circuits produce a higher noise level.

c) by far the worst source of jitter noise is themains supply.

Confirmatory tests for (c) were made by severelydipping the mains and inverter voltages to producevirtually identical rectangular waveforms. No thy-ristor control was used. The interharmonic spaces inthe clipped mains waveform spectrum showed highnoise level, particularly at the higher frequencies,while the dipped inverter waveform did not.4

CONCLUSIONS

Most of the noise found between the higher harmoniclines is due to mains imperfections. Since there islittle that the circuit designer can do about this, it isperhaps fortunate that when the usual steps are takento remove the harmonic from the waveform theinterharmonic noise will also be removed. The re-moval of the harmonics may be compared with theswitching off of the carrier of a radio transmitter. Ifthere is no carrier there can be no sidebands. If thereare no harmonics there can be no noise due to jitter.

ACKNOWLEDGEMENTS

Acknowledgement is made of the experimental facilitiesprovided by the Department of Electronic and ElectricalEngineering, Loughborough University of Technology and tothe Science Research Council for financial support. Theadvice and encouragement of Professor D. S. Campbell isgreatly appreciated.

REFERENCES

RELATIVE IMPORTANCE OF THE JITTER NOISESOURCES

Experimental results obtained by comparing thespectra of thyristor output voltage waveforms whenmains driven and when driven by a very stableinverter have demonstrated that:

1. J. K. Hall and N. C. Jones, Electrocom. Sei. and Tech., 1,1, 43-49, (1974).

2. A. Van Der Ziel, Noise: Sources, Charaeterisation, Meas-urement, Prentice-Hall, (1970).

3. D. S. Palmer, R.F. Components Generated by ThyristorGate Pulse Jitter, M.Se. Project Report, LoughboroughUniversity of Technology, 1973.

4. D. S. Palmer and J. K. Hall, LE.E. Conf. Publ. No. 123,38-4 3, (December 1974).

International Journal of

AerospaceEngineeringHindawi Publishing Corporationhttp://www.hindawi.com Volume 2010

RoboticsJournal of

Hindawi Publishing Corporationhttp://www.hindawi.com Volume 2014


Active and Passive Electronic Components

Control Scienceand Engineering

Journal of



RotatingMachinery


Hindawi Publishing Corporation http://www.hindawi.com

Journal ofEngineeringVolume 2014

Submit your manuscripts athttp://www.hindawi.com

VLSI Design



Shock and Vibration


Civil EngineeringAdvances in

Acoustics and VibrationAdvances in



Electrical and Computer Engineering

Journal of

Advances inOptoElectronics

Hindawi Publishing Corporation http://www.hindawi.com

Volume 2014

The Scientific World JournalHindawi Publishing Corporation http://www.hindawi.com Volume 2014

SensorsJournal of


Modelling & Simulation in EngineeringHindawi Publishing Corporation http://www.hindawi.com Volume 2014


Chemical EngineeringInternational Journal of Antennas and

Propagation




Navigation and Observation



DistributedSensor Networks


short communication time jitter phase controlled thyristor...

Documents