IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. IM-19, NO. 1, FEBRUARY 1970

maximum of u(t) [see Fig. 3(b)] is shifted only slightlyfrom the true value. When the noise is strong, the ob-served maximum may be shifted so far away from thetrue value that an accurate measurement is not available.This phenomenon is called anomaly [2], which willappear if the mean-square error is less than -15 dB. Itis also noted that the maximum-likelihood estimate of mhas low variance (mean-square error) though not neces-sarily the minimum. iMinimum conditional variance isobtained by Bayes' estimate. In this paper, the noise isassumed to be Gaussian. If the random variable m is also

normally distributed, the maximum-likelihood estimate mis also a minimum conditional variance estimate.

REFERENCES[1] K. S. Lion, Instrumentation in Scientific Research-Electrical

Input Transducers. New York: McGraw-Hill, 1959.[2] J. M. Wozencraft and I. M. Jacobs, Principles of Communi-

cation Engineering. New York: Wiley, 1965, ch. 8.[3] M. Schwartz, W. R. Bennett, and S. Stein, Communication

Systems and Techniques. New York: McGraw-Hill, 1966.ch. 6.

[4] C. H. Chen, "Digital accelerometer performance analysisusing waveform communication concept," AVCO-MSD, In-ternal Memo., November 8. 1967.

Programmable Current Regulator for

High-Voltage Operation

P. J. KINDLMANN, MEMBER, IEEE

Abstract-A current regulator for the operation of gas dischargesis described, capable of delivering peak currents up to 1 ampere andaverage currents of up to 0.5 ampere with voltage excursions to9 kV. Completely floating two-terminal operation accomodatesdifferent load configurations. By means of externally suppliedtiming pulses the output current can be programmed to generaterectangular or trapezoidal current waveforms. For rectangular out-put pulses, switching speeds of 5 ,us into low-impedance loads andslewing rates of 500 V/,s for high-impedance loads can be obtained.The output resistance is in excess of 107 ohms, shunted by 50-250pF, depending on regulator configuration.

I. INTRODUCTIONE XPERIMENTS in plasma and laser physics in-

volving the stable operation of low-pressure gasdischarges frequently require stable current sources

with large output voltage range. It is often importantto maintain the high output impedance of the source evenat frequencies of 100 kHz to reduce the discharge currentnoise.' Conventional high-voltage supplies operating inthe constant-current mode usually employ a filter capaci-tor across the output. Series resistors or inductors arethen needed to raise the output impedance at high fre-quencies.A current source offering good frequency response by

Manuscript received September 18, 1969. This work was sup-ported in part by the National Science Foundation.The author is with the Department of Engineering and Applied

Science, Yale University, New Haven, Conn.I L. J. Prescott and A. van der Ziel, "Gas discharge modulation

noise in He-Ne lasers," IEEE J. Quantum Electronics, vol. QE-2,pp. 173-177. July 1966.

virtue of a fast feedback loop offers the additional possi-bility of high-speed current programming. Such program-ming may take the form of modulating the discharge cur-rent for experiments involving phase-sensitive detection,or fast linear sweeps of the current to study dischargeparameters on a time scale short compared to thermaleffects in the discharge.The apparatus described here offers these programming

modes at currents of up to 1 ampere peak and at voltagesup to approximately 9 kV. It represents a floating two-terminal device to be used with a conventional 10-kVpower supply.

II. GENERAL DESCRIPTION

The basic current regulator is shown in the blockdiagram of Fig. 1. An operational amplifier senses thedifference between the negative voltage from the current-sensing resistor and two positive referenice voltages, onefrom a reference supply determining the minimum outputcurrent and another from an externally triggered wave-form generator determining the modulation waveform andamplitude. This arrangement allows a low sustainingcurrent to be passed through the discharge, avoiding thediscontinuous behavior that would result from having tofire the discharge at the start of each modulation cycle.The plasma physics experiments for which the regulator

was originally designed required the "floating" two-termi-nal operation as shown in Fig. 1. This means that theexternal programming input must either be transformercoupled or optically coupled to the regulator. In thepresent instance the required waveforms were simple

68

69KINDLMANN: PROGRAMMABLE CURRENT REGULATOR FOR HV OPERATION

+

JNEG.10REG.kV A MINIMUM OPERATIDIINPUT ICURRENT AMPLIFIE0OkV MAX.I +4 ADJUST

_a

Fig. 1. Block diagram of the current regulator.

(triangular and square wave) and readily generated withinthe regulator in response to a transformer coupled ex-

ternal trigger signal. More complex programming wave-

forms will either require voltage-to-frequency frequency-to-voltage conversion with transformer coupling, or

optical couplers of adequate stability and linearity.The error signal from the operational amplifier is further

amplified by driver stages that produce a 600-volt excur-

sion for driving the grid of the triode regulator tube. Al-though the plate characteristics of a tetrode are more

desirable for constant-current operation, it does requirean additional floating supply for the screen grid. Thetriode clhosen in the present instance (Amperex 8269)has the advantage of low-cost high-power dissipation,and simple grid drive requirements. With forced-air cool-ing, it can dissipate the full 5 kW implies by a short-circuit load current of 0.5 ampere and a 10-kV unregulatedsupply voltage, allowving safe operation for all possibleloads.With the exception of the triode-regulating element, all

circuitry was solid-state. This reduced the power require-ments of the regulator electronics to a level where it waspossible to generate conveniently all required supplyvoltages from the 12-V filament winding for the triode.This then is the only power transformer required to havehigh-voltage insulation and low capacitance betweenprimary and secondary windings. The total capacitancebetween the regulator "common" and the grounded en-

closure housing the regulator is 250 pF, making possiblefast slewing rates in the floating mode of operation, even

at low load currents.

III. CIRCUIT DESCRIPTION

A. The Regulating LoopThe regulator feedback loop is shown in Fig. 2. The

summing junction of a high-speed operational amplifier(Nexus FLS-6) compares the voltage across a low-in-ductance 20-ohm current-sensing resistor against reference

voltages from the "minimum current set" potentiometer(adjusted to give a range of 0 to 0.5 ampere) and thewaveform generator. A common emitter stage (2N3497)translates the error signal via three Zener diodes (lN3049B)to a pair of common-base transistors (TRS35OMP). Dualemitter-followers, each consisting of a TRS350MP and a

2N3585, form the actual output stage supplying the re-

quired grid drive. The collector voltage ratings of readilyavailable transistors is generally less than 600 volts,necessitating the "stacked" configuration shown, wherethe collector supply voltage is always equally dividedamong the series transistors. Negative feedback around thedriver stage (330-kQ resistor) limits the gain.The emitter-follower feeds the grid over forward-

biased diodes with high-inverse-voltage rating. Thesewere included to prevent damage to the driver transistorsin case of arcing between plate and grid. This mighthappen if excessive voltage were to be applied from theunregulated power supply, but so far this has never

happened. A simpler approach might be to place a 10-kVspark gap across the tube. A plate choke suppresses

parasitic oscillations that are readily possible since the8269 tube is useful at frequencies in excess of 50 MiHz.The driver circuit as described supplies an approxi-

mate range of grid voltage between -550 volts (completecutoff for 10-kV plate voltage) and +75 volts (approxi-mately 1-ampere plate current at plate voltage of 400volts). The grid current at +75 volts is approximatelv0.3 ampere.

The loop frequency response is dominated by thecapacitive feedback around the operational amplifier.Due to the finite plate resistance of the tube, the loopgain will depend somewhat on the impedance of the load,the gain being highest for a short-circuit load. If thefeedback trimmer capacitor is adjusted with square-wave

programming for minimum overshoot under short-circuitconditions, the loop response will generally be dampedfor actual loads. Optimum risetime for a particular load,however, can be obtained by readjustment of the feedback

+

CURRENTOUTPUT

O.5A RMS MAX.

0IEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, FEBRUARY 1970

+80

IOOK

MIN.CU RRENTSET

COMMON

2N 3497

NOTES: I)ALL RESISTORS I/2W,5% 3-lN3049BUNLESS OTHERWISE INDICATED. 47KMF MEANS METAL FILM. ,_

2) ALL VOLTAGES SHOWN WITH IN 914RESPECT TO COMMON

3) ANY LOW POWER, FAST RECOVERY .005

DIODE (S) GIVING 12 kV TOTAL 480V 3.3KPEAK INVERSE VOLTAGE RATING |

4) * INDICATE DEVICES REQUIRING -562 -55SMALL HEAT SINKS.

Fig. 2. Schematic of the regulator feedback loop.

capacitor. The use of a tetrode as regulating elemenit wouldresult in less dependence of loop gain on load conditions,due to its higher plate resistance.

B. The Waveform Generator

The original design of the apparatus included only asquare-wave generator. Recently, a ramp generator wasincorporated to produce triangular or trapezoidal currentprogramming, but retaining the square-wave modulationfeature. The waveform generator is shown in Fig. 3.A square-wave current supplied to the "trigger input"

is coupled via a well-insulated (10-kV minimum) pulsetransformer chosen to differentiate the input. These pulsestrigger an ordinary flip-flop in such a way as to define thestate of the flip-flop in terms of the trigger input slope.The amplified flip-flop output is clipped to a symmetrical±6.2-volt square wave by a Zener reference diode (GEF16H1).The function of the /1A741 operational amplifier de-

pends on the position of the "slope" switch. The closedposition results in an integrator configuratioin. The rateof change of the integrator output (pin 10) depends onthe setting of the slope control. Two differential ampli-fiers that supply "hard" negative feedback limit theamplitude of the integrator output to values betweenzero and a negative voltage set by the "modulationamplitude" control. This results in a negative-going sym-metrical trapezoid waveform with independently ad-justable slope and height. The trigger-input waveformcontrols the repetition rate and the duration.The slope control is the ordinary volume-control type

with coupled ON-OFF switch. In the open position of the

-2N 3585

AMPEREX'SEE 8269

12 V

10- 3.9K2 W

50

switch the operational amplifier generates a precisionclipped unipolar square wave.The second ,uA741 operational amplifier serves as buffer

amplifier. The range of the modulation amplitude controlis set to correspond to 1 ampere full scale.

C. Overload IndicatorsTwo overload indicators have been found extremely,

useful in operating the regulator, one indicating excessivecurrent, the other a saturated condition of the seriesregulator tube. Since in each instance only a simple over-voltage indicator is required, the circuits need not hegiven here.The maximum safe current through the regulator could

be determined by a wattmeter circuit monitoring theregulator tube dissipation. At the time this apparatus watsdesigned (early 1966) such an approach was not sufficientlveconomical and a simple average-current indicator waisused. Referring to Fig. 2, the voltage across the currentsensing resistor (point C) is averaged by an RC filter.When the filter output exceeds -10 volts, correspondingto 0.5-ampere average regulator current, an indicatorlight is activated.The present low cost of multiplier circuits makes possi-

ble the economical computation of regulator tube dissi-pation. A high-impedance voltage divider between plateand cathode would not seriously compromise the regul.a-tor output impedance. An overload indication based onaverage power dissipation would give the greatest flexi-bility of current programming waveforms.Depending on the characteristics of the load, the regu-

lator tube can be easily driven into saturation during

70

71KINDEMANN: PROGRAMMABLE CURRENT REGULATOR FOR HV OPERATION

+15

oP=F71(J~I¶5IHH N9I1I 41KPHASE< 5 t' IN1 01K 1lx I 2.2K

-15 -15 -15 -15

Fig. 3. Schematic of waveform gei

the current peaks of the rectangular or trapezoidal sweepwaveforms if the load impedance is too high. This has theeffect of breaking the regulator loop, causing the outputof the operational amplifier to jump to a negative satura-tion value. Under closed-loop conditions the operationalamplifier output (point B, Fig. 2) is never more negativethan approximately -1.5 volts. An open-loop conditionwill result in a jump to an output in the range of -5 to-10 volts. It must be kept in mind that the duty cyclefor this occurrence may be quite low (as in low-repetitionrate high-current pulsing). A satisfactory indicator forregulator saturation involves a simple diode-capacitorpeak detector monitoring the voltage at point B, followedby an overvoltage indicator set at, say, -2 volts.

D. Power SuppliesAs mentioned before, all supply voltages for the floating

regulator circuitry are generated from the 12-volt secon-dary winding for the regulator tube filament. Thus, onlyone transformer with low capacitance and high-voltageinsulation between primary and secondary windings isrequired.Two high-current 6.3-volt windings in series are used

as the primary windings of a standard power transformer(Stancor P-8170). The 117-volt winding supplies a com-mercial modular dual 15-volt supply, as well as the recti-fier, filter, and Zener diode combination constituting the+80-volt supply.The -550-volt supply voltage is derived from the

760-volt winding, the -562 volts from the series connec-tion of a 12-volt supply operating from a third 6.3-voltwinding in series with a 5-volt winding. Both of thesesupplies are also Zener diode regulated.The input power to the regulator power supply is ap-

proximately 100 watts (12 volts at 8 amperes).

IV. CONSTRUCTION

The regulator is contained in a standard cabinet rack107 cm high. A rear view is seen in Fig. 4.A Rotron model CXH33 blower is mounted in the

bottom compartment into which air is taken through afilter at the front of the cabinet. The regulator tube on itsceramic insulator pedestal is seen in the middle left com-

aerator for current programming.

Fig. 4. Rear view of regulator cabinet showing compartmentedconstruction. The operating controls of the regulator circuitryin the top compartment are brought to the front panel viainsulated shafts.

partment, which also contains the plate choke. Air ex-hausts through a grille in the side of the cabinet. Sincethe temperature of the exhaust air can reach 80°C at fullregulator tube dissipation (5 kW), the exhaust ductingshould be leakproof so as to prevent excessive heating ofthe regulator electronics.The low-capacitance filament transformer is mounted

in the middle-right compartment, while the top compart-ment contains the chassis with the solid-state circuitry,supported by standoff insulators. Convection coolingthrough louvers in the side of the cabinet is adequate forthe filament transformer and the chassis.

Wiring between compartments is routed so as to mini-mize capacitance to the grounded cabinet. All construc-tional details follow standard practices for high-voltagecircuitry (safety interlocks, etc.).

V. PERFORMANCE

The dc stability of the regulator is determined essen-tially by the stability of the 20-ohm current-sensing re-sistor (see Fig. 2). Self-heating effects must be minimized,

IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, FEBRUARY 1970

Fig. 5. Regulator configuration used for measurement of responsespeed.

10 20 30 40 50t (, s)

20 40 Et (/ s)

a 0.6

- 04

0

0.2

0 20 40 60 80 100t (,Ls)

(a) (b) (c)

Fig. 6. Regulator response for resistive loads. (a) Short-circuit load: rise and fall times < 4 ,us. (b) Effect of 4.1-kQ load:risetime ; 7 ,us, fall time 10 ,us. (c) Effect of 12.1-kQ load: risetime Z- 12 ,.ss, fall time - 13 As. The figuires for (b)and (c) include the inductive effects of the wire-wound load resistors.

5 0 5

,,1 .0 KV -|C3! 0KV-

05

> 0 I0

40 80120

160 200

(a)

a) L

vr- a)Qo cn 0

0 10 20 30 40 50t (,us)

(b)

0.75A /

0.5,, 0.25

oC 0Vl.OKV05

0 0.4 0.8 1.2 16 2.0t (ms)

(e)

Fig. 7. Regulator response with gas discharge as load. This particular discharge tube contains hydrogein at a pressureof about 1 torr, has 4 cm in diameter and 25 cm in length, with heated cathode and annular anode. (a) Dischargecurrent and voltage with a rectangular programming waveform (repetition rate 1 kHz), with the current starting fromzero. (b) Expansion of leading edge of waveforms in (a). The transient in the current waveform results from the dis-charging of the regulator's distributed capacitance to ground ( 250 pF), which appears across the load. (c) Dischargecurrent and voltage for a triangular programming waveform (repetition rate 200 Hz). A current of 10 mA is main-tained between pulses.

yet the resistance value cannot be made arbitrarily lowsince this will allow the input offset voltage instability ofthe operational amplifier to dominate. While low in-ductance wire-wound resistors are the most suitable, carbonresistors can be used if high-dc stability is not required.In the present case 19 resistors of 390-ohm 2-watt ratingin parallel and heatsunk to the regulator chassis givelong-term stability on the order of 1 percent. Improve-ment by a factor of 10 or more is readily possible withbetter resistors. The regulator's loop gain is sufficientlyhigh so that the measured dc output resistance of the regu-

lator is in excess of 107 ohms.The speed of response of the regulator in the externally

programmed mode of operation is easy to specify for re-

sistive loads, but much more difficult for the highly non-

linear loads represented by gas discharges. However, inview of the diverse gas-discharge types encountered inplasma and laser research, regulator behavior for linearloads is a useful basic specification.

To maximize the effects of regulator capacitance toground (250 pF between regulator "common" and thegrounded enclosure plus about 50 pF total plate capaci-tance) in the tests to follow, the configuration shown inFig. 5 was chosen. A commercial 10-kV supply (unregu-lated, dropping to 8 kV at 0.5-ampere steady-state loadcurrent) was the power source. Any pulse generator withconveniently variable duty cycle and repetition rate can

be used to program the waveform generator described inSection III-B. The ac component of the load current was

monitored with a current probe. Fig. 6 shows the resultsunder various load conditions. As expected, the fastestresponse is obtained for a short-circuit load [Fig. 6(a)],resulting in a rise and fall time of about 4 its. The slewingrate of the driver stage generating the required 600-voltgrid-voltage excursion is the limiting factor under theseconditions.The response with load resistances of 4.1 and 12.5 kQ

[Fig. 6(b), (c)] includes the effect of inductance of the

0.75

0.50

o 0.25a0

0L

72

a}

.I_

IEEE TRNSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. IM-19, NO. 1. FEBRUARY 1970

standard wire-wound units used since low-inductancetypes could not, unfortunately, be located for this test.Somewhat faster switching speeds could be obtained bychanging the regulator configuration so as to feed the loadfrom the anode, since this greatly reduces the outputshunt capacitance.

Finally, Fig. 7 shows the waveforms obtained with agas discharge for both the rectangular pulse and tri-angular sweep waveforms available from the waveformgenerator. In the case of the discharge, the current- andtime-dependent load impedance leads to less ideal wave-forms than for resistive loads. Fig. 7 can only serve as anexample since other types of discharges will give some-what different waveforms, If high-speed operation is ofparticular importance, the capacitance to ground of the

interconnections between regulator and load must be keptto a minimum.The regulator has been in operation for over two years

in the Plasma Physics Laboratory, Department ofEngineering and Applied Science, Yale University, andhas not required servicing during this time.

ACKNOWLEDGMENTThe author wishes to express his thanks to Prof. J. L.

Hirshfield for providing the primary stimulus and supportof this work, and to the members of the Department ofEngineering and Applied Science Electronics Laboratory,Yale University, where the apparatus was constructed,particularly to H. G. France who was responsible for allmechanical design and wiring.

A New Method of Measuring Dielectric Property ofVery-High-Loss Materials at High Frequencies

BUNJIRO ICHIJO AND TOMOKAZU ARAI

Abstract-This paper presents a circuit system functioning as acapacitance meter suitable for very-high-loss material with a widerange of application in various fields of scientific research and in-dustrial operation.The minimum equivalent parallel resistance of the specinen to

be measured reaches as low as 50 ohms and the measuring range ofcapacitance is from about 0.1 - 1000 pF at 2 MHz. Some experi-mental data are given for the appreciation of its characteristics.

I. INTRODUCTIONR ECENTLY reactance-measuring equipments have

come into general use as error detectors in auto-matic control systems of industrial operation.

For dielectric analyzers used in chemical works, it isrequired that the capacitance readings not be affected bythe fluctuation of resistance components of specimenscaused by the various kinds of impurities.

In the following, the principle of a resistance-insensitivecapacitance meter together with some of the experimentaldata is described.

II. PRINCIPLE OF THE CIRCUIT SYSTEM ANDPROCEDURES OF MEASUREMENT

Fig. 1 illustrates the circuit of the new capacitance-measuring system, where L, L2, C, C0, and C, constitutea series resonant circuit. C2. and C' are variable standard

Manuscript received August 7, 1969.The authors are with Shizuoka University, Hamamatsu, Japan.

C>,,

Dtco 'C2 21 Nospcimien

Fig. ~~~1. Mesuin tir2Tt't

Mrcpctsthcat Ce is connr. C,te C, pra

L a

Detector ctrcuit Di

epVI

Dtetector circuit D nFig. 1. Measuring circuiit.

air capacitors; the capacitor C' is connected in parallelwith L,. They are mechanically joined together and soconstructed that the changes of0dc and C' per dial divi-sion are exactly equal.When the circuit is at resonance, the phase angle be-

tween the voltagese= and e,, is 900 and the output meter ofthe phase-sensitive detector indicates null point.

Specimens to be measured are connected in parallelwith 02 and C' as indicated by C,, in Fig. 1. To make ameasurement, the meter indication with the specimens incircuit is again adjusted to zero by decreasing 02, and C'2i.e., AC,,,= C., and ACI -C, Then the unknowncapacitance C. can be read on the dial that is calibratedbeforehand.