The Electrical and Electronic Tools of the Analytical Chemist

REPORT FOR ANALYTICAL

CHEMISTS

The Electrical and Electronic Tools of the Analytical Chemist: OSCILLATORS by S. Z. Lewin, Department of Chemistry, New York University, Washington Square, New York 3, Ν. Υ.

4 Ν OSCILLATOR is any device that -t\- hiis an output which varies in a periodic manner as a function of time. In the case of oscillators em ployed in chemical instrumenta tion, this periodic variation may have a variety of shapes, from sim ple sinusoidal waves to abrupt step functions, and may range in fre quency from a fraction of a cycle per second to many billions of cycles per second. The methods of generating oscillations, as well as their properties and applications. depend upon wave shapes and fre quencies, and these parameters commonly serve as the basis for the classification of oscillator types. Types of Oscillators

Low frequency oscillations, par ticularly in the subaudio range of frequencies below 20 cycles per sec ond, are best generated by mechan ical or electromechanical means. Examples are: a clockwork mech anism driving a commutator shaft on which are mounted contacts which make and break a circuit; a synchronous motor rotating a per manent magnet in the vicinity of a coil and thereby inducing a periodi cally varying voltage in the coil; a vibrating reed, the end of which bears a contactor that makes and breaks contact with a stationary member. Oscillations in the audio to radio frequency ranges 120 cycles per second to hundreds of megacycles

This is the second article by Professor Lewin on the subject THE ELECTRICAL AND ELECTRONIC TOOLS OF THE ANALYTICAL CHEMIST. The first, on AMPLIFIERS, was published in the February issue, page 25A. The same author wrote PROPER UTILIZATION of ANALYTICAL INSTRUMENTATION, page 23A, No. 3, 1961. He is well known for his teaching and writing in the general field of electronics and instru mentation. Professor Lewin's aim is to acquaint the analytical chemist with these tools for application in his everyday work, and to give him a better understanding of modern analytical equipment.

per second) are very conveniently produced by the application of posi tive feedback to the operation of a vacuum tube circuit. By suitable design of the circuit (as will be shown in detail below), the tube current can be caused automatically to increase to a maximum value, then decay to a minimum, with this cycle repeating itself continuously at a rate determined by the magni tudes of the various capacitances, inductances, and resistances present in the circuit. At audio frequen cies, the current and voltage oscilla tions are readily confined to the physically obvious components of the circuit. However, at the higher frequencies corresponding to the radio range, the oscillating signal tends to leak away through distrib uted —i.e., intangible, or not pur posely added—capacitance and in ductance, and the manipulation and measurement of such signals pose special problems. The distributed capacitance and inductance associated with mount ing wires, connectors, etc.. impose

an upper limit on the frequency that can be attained with ordinary radio tubes. For the generation of oscillations in the ultrahigh fre quency and microwave ranges, spe cial tubes, such as the klystron and magnetron, are commonly em ployed. In these devices, the oscil lation is produced by controlling— i.e., modulating—the paths and velocities of electrons within the tube, rather than through feedback of a portion of the tube output to the control grid, as in the audio and radio ranges. Special circuit arrangements make possible the production of oscillations which are nonsinusoidal in wave form. For example, by the use of a gas-filled tube which does not conduct appreciably until a minimum firing potential is applied across it, an oscillator can be de signed which yields a sawtooth wave. This is an example of a re laxation oscillator. Two vacuum tubes suitably interconnected are involved in another form of relaxa tion oscillator known as a multiviVOL. 34, NO. 8, JULY 1962

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27 A

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Table I.

Types of Oscillators

Classification Principle Subclasses of Oscillators Subaudio: less than 20 cycles/second Frequency Audio: 20-20,000 cycles/second Intermediate (superaudio, ultrasonic) : 20-500 kilocycles/ second Broadcast band : 500-1500 kilocycles/second Short wave band: 2-25 megacycles/second Ultrahigh : 30-3000 megacycles/second Microwave : 3000 megacj'cles/second and greater Alethod of generation

Triode oscillator: Armstrong, Hartley, Colpitts, Clapp, Meissner, TPTG, electron-coupled, Wien bridge, phase-shift Mechanical: commutator, tachometer Electromechanical : magnetostrictive, tuning fork, piezo electric crystal Relaxation oscillator: gas tube, multivibrator Beat frequency oscillator Blocking oscillator Positive grid oscillator: Barkhausen-Kurz Velocity modulated : klystron, magnetron

Wave form

Sine wave Time base (sawtooth) Pulse Square

brator, and this circuit can produce square waves or sawtooth waves. A pulsed o u t p u t can be obtained with a blocking oscillator, which is an oscillator circuit so arranged t h a t the occurrence of one or more cycles causes the grid to build u p a negative charge sufficient to cut off the tube, and oscillations then cease until the passage of a definite pe riod of time, during which the charge leaks away. T h u s , the out p u t consists of pulses of one or more waves, separated by quiescent pe riods. If the outputs of two oscillators of different frequencies are suitably combined, the resultant frequency corresponds to both the sum and difference of the original signal fre quencies. I n a beat frequency os cillator, the signal desired is the difference, or beat signal of the two source oscillations. I n most vacuum tube oscillators, the control grid has an average po tential which is negative with re spect to the plate. A positive grid oscillator is arranged to have its grid a t a high positive potential rel ative to the plate, so t h a t electrons which pass through the grid tend to be repelled by the plate and cir cle around to return toward the grid. T h e electron stream m a y thus

pass through the grid m a n y times before the electrons are finally cap tured. In this t y p e of circuit, the oscillations are produced by the varying mirror charges induced as the electrons approach to and re cede from the grid. The frequency stability of an os cillator in t h e audio or radio fre quency range m a y be improved by the incorporation in the design of a component which is mechanically constrained to vibrate with a defi nite frequency, the vibrations of which can be coupled to the elec trical oscillations occurring in the circuit. This is the principle of the electromechanical oscillator, the commonest example of which is the crystal-controlled oscillator, based upon the piezoelectric properties of certain crystals such as quartz. T h e terminology employed in de scribing oscillators consists of sev eral overlapping classifications which emphasize different aspects of the device. Thus, oscillators m a y be characterized either in terms of the frequency range for which they have been designed, the type of circuit design employed to produce the oscillations, or the wave form of the output signal. These classification schemes are summarized in Table I. The mean-

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VOL 34, NO. 8, JULY 1962

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29 A

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REPORT FOR ANALYTICAL CHEMISTS

Application Conductometry

Table II. Recent Analytical ANAL. CHEM., Ref. Fischer and Fisher, 24, 1458

Frequency 50-1000 cycles/ second

Creamer and Chambers, 26, 1098 Sarkanen and Schuerch, 27,1245 Quiram, 27, 274 Gaslini and Nahum, 31, 989 32, 1027 Avizonis, et al., 34, 58 Polarography, a.c.

Square wave polarography

19-77 cycles/second (2nd harmonic measured) 60-320 (1st harSmith and Reinmuth, monic measured) Walker, Adams, and 84 308 38-620 Smith and Reinmuth, Walker, Adams, and 60 1526

33, 482 Alden, 33, 32, 1892 Juliard, 32,

20

Laitinen and Hall, 29, 1390, 1893

230 85 20 225

Hamm, 30, 350 Koyama, 32, 523 Hall and Flanigan, 33, 1495 Hamm and Furse, 34, 219

Stepped voltage polarography

147-3500

Mann, 33, 1484

Precise timing

90 kilocycles/ second

Biermann and Weber, 25, 1284

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Incremental-derivative polarography

460 kilocycles/second (for triggering of relay circuits)

Auerbach, et al., 33, 1480

Acoustic gas analyzer

20-20,000 cycles/ second

Weller, 26, 488

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High frequency conductometry

2 megacycles/second 18.5 1-30 15-20 2.54, 5.98 20 4.89 30 5-30 9.45 4.89

Hall, 24, 1244

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Milner, 24, 1247 Reilley and McCurdy, 25, 86 Flom and Elving, 25, 541 Fujiwara and Hayashi, 26, 239 Hall, et al., 26, 835 Bien, 26, 909 Blaedel and Knight, 26, 743 Hall, et al., 26, 1539 Jensen, et al., 26, 1716 Dean and Cain, 27, 212

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ANALYTICAL CHEMISTRY

ings of most of the terms employed in Table I should be clear from the foregoing discussion; other specialized terms will be treated below. Analytical Applications of Oscillators T h e presence of oscillator circuitry in chemical instrumentation

is far more ubiquitous t h a n m a y be evident to the uninitiated. The stability of modern sensing, amplifying, and measuring instruments is in large measure achieved by the use of amplifiers and recorders which are designed for the manipulation of a.c. signals. Thus, if a d.c. signal is to be measured, as in the case of a recording potentiome-


Applications of Oscillators

4.28 4.89 4.89 12-130 130 0.1-3 120 4.89 2.1 10-30

ANAL. CHEM., Ref. Baumann and Blaedel, 28, 2 Weaver, et al., 28, 329 Grant and Haendler, 28, 415 Johnson and Timnick, 28, 889 Lippincott and Timnick, 28, 1690 Kupka and Slabaugh, 29, 845 Johnson and Timnick, 30, 1324 Mukherji and Sant, 31, 608 Ishii, et al., 31, 1586 Walker, et al, 32, 9

Nuclear magnetic resonance spectrometry

12.3 16.2 30 27 6.88

Galles, et al., 28, 269 Guff y and Miller, 31, 1895 Chamberlain, 31, 56 Fujiwara and Wainai, 33, 1085 Rubin and Swarbrick, 33, 217

Dielectric constant

1 5 0.4-0.6 0.01-75 70

Axtmann, 24, 783 Monaghan, et al., 24, 193 Bvrne and Brockett, 28, 1207 Callinan, et al., 28, 1911 Winefordner, et ni., 33, 515

Time-of-flight mass spectrometry

2000-20,000 cycles/ second

Gohlkc, 31, 535

Electrodcless discharge for emission spectrography

40.68 megacycles/ second

Keller and Smith, 24, 796

R F spark source for mass spectrometry

ca. 10 megacycles/ second

Baun and Fischer, 34, 294

Electron absorption detector for gas chromatography

1 0.5-0.7

Lovelock and Zlatkis, 33, 1958 Landowne and Lipsky, 34, 726

pH-Stat, position sensor in control section

1

Wood, 32, 537

Electron magnetic resonance spectrometry

9150 megacycles/ Roberts, et al., 33, 1879 second modulated Saraceno, et al., 33, 500 at 100 kilocycles/ Yen, et al, 34, 694 second

Microwave absorptiometry

10,000 megacycles/ second

Application

Frequency

ter, it is generally "chopped" by an electromechanical device, and the resulting oscillation is " s h a p e d " into a wave form appropriate for subsequent circuitry. I n a spectrophotometer, the chopping m a y be accomplished by means of a rotating sectored shutter or mirror, which periodically interrupts or redirects the light beam ; a set of com-

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31 A


Figure 1. A negative-going pulse on the grid of a vacuum tube produces a negative-going effect on the tube current, and a positive-going effect on the plate voltage. The plate and grid potentials are 180° out-of-phase

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put of a high-frequency, crystalcontrolled oscillator is subjected to frequency division, a constant, lowfrequency signal can be obtained which is useful for the operation of very precise timers and stopclocks. Relaxation oscillators provide the time-base (sawtooth) signals that are employed for the horizontal sweep of the electron beam in cath ode ray oscilloscopes. A square wave oscillation is often used as an electronic switch ; if the voltage step is synchronized with the alternate switching from one input signal to another, independent signal, these two signals are offset from each other by a voltage equal to the square wave step, and the two sig nals can be handled by a single am plifier without losing their separate identities. A pulse oscillator is em ployed to apply a voltage to an electrode some definite time inter val after a certain event has oc curred, in order to differentiate be tween processes having different re laxation times. Examples of this type of application are the pulsed, incremental, or "tast" polarograph, and the pulsed electron absorption gas detector, Table II. High frequency oscillations are also useful as a means of transfer ring energy into chemical systems, as in induction or dielectric heating, or as an agency for the production of ionization and excitation, as in the gas discharge tube. Because of the sensitivity of a high-frequency oscillator circuit to the effects of distributed capaci

tance, such a circuit can be em ployed as a position sensor. If the distributed capacity is altered— e.g., by a change in the relative po sition of component parts of the circuit, or of nearby grounded ob jects—the amplitude and/or fre quency of the oscillation may change. In addition to the use of an os cillator for timing purposes, for en ergy transfer, for position sensing, or as a tool for the more effective manipulation of an input signal that is basically nonperiodic in na ture, there are many applications in which the primary information comes from the specific interaction of the output of an oscillator with a chemical system. In a.c. conductometry, the rela tionship between an impressed al ternating voltage and the resulting alternating current is investigated. The frequency of the signal may be as low as 20 cycles per second, or as high as 150 megacycles per sec ond. At low frequencies, the elec trolytic conductance of the sample is of predominant importance; as the frequency of the exciting signal increases, capacitive effects become increasingly important. In a.c. polarography, a small al ternating potential is superimposed upon the conventional d.c. voltage program, and the alternating com ponent of the cell current is meas ured. The frequency of the oscil lating signal is generally low, in the range from 20 to 600 cycles per sec ond, but a recent development is


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SPECTRA identify 4,000 compounds Selective spectra from the more than 19,000 samples in the Sadtler collection enable fast, positive identification of unknowns in Ultra Violet spectrophotometry according to chemical analysis demands. E a c h spectrum identifies: Figure 2. The principle of positive feedback involves taking a portion of the plate signal, shifting the phase by 180°, and adding it to the grid signal

based upon t h e use of radio frequency oscillations. In dielectric constant measurement, a n alternating potential is employed to measure t h e capacitance of a condenser containing t h e sample between its plates. T h e frequencies are generally in t h e range of tens of kilocycles per second to hundreds of megacycles per second. I n dielectric loss measurement, an a.c. signal is utilized to determine t h e ratio of t h e energy dissipated in t h e dielectric medium to the total energy stored by t h e a p plied voltage. T h a t is, in a pure capacitance, all of the energy stored in the dielectric during t h e increase of the applied voltage to its peak value during one q u a r t e r cycle, is returned to t h e oscillator when t h e voltage signal decreases to zero in the next q u a r t e r cycle. However, in a leaky, or lossy condenser, some energy is dissipated as PR heat in the system. In microwave absorptiometry, t h e diminution in intensity of an a.c. signal upon interaction with a

chemical system is measured. T h e system absorbs energy in proportion to t h e capacity of molecules in the sample to be excited into rotational motion b y t h e microwave frequencies employed. I n nuclear magnetic resonance spectrometry, nuclei precessing a t constant frequencies in the field of a p e r m a n e n t m a g n e t are caused to interact with a t r a n s m i t t e d short wave radio or ultrahigh frequency signal in such a w a y as t o induce a signal in a receiver coil. In electron magnetic resonance spectrometry, unpaired electrons aligned with a magnetic field are flipped over into t h e unaligned orientation b y means of a microwave signal beamed into the sample. T h e role of oscillators in modern analytical work is further illust r a t e d by t h e references summarized in T a b l e I I . These represent papers published during t h e p a s t few

years

in

ANALYTICAL

CHEM-

ISTRY in which there is reported an instrumental method based upon a n oscillator as an essential feature, or in which d a t a o b -

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33 A


Co Figure 3. The Armstrong or tickler-feedback oscillator achieves positve feedback through the inductive coupling between a plate coil, LP, and a grid coil La. During the positive portions of the grid voltage swings, grid current flows through the grid leak resistor, Re, producing an 7R-drop that diminishes the effective positive potential at the grid

tained involve the interaction of a sample with an alternating signal. The V a c u u m T u b e O s c i l l a t o r

Continuous, free-running oscilla tions can be produced if there is positive or regenerative feedback between the plate and grid circuits of a triode. The principle of this type of feedback is illustrated by the diagrams, Figures 1 and 2. If a negative-going signal is im pressed on the grid of a triode in which a current is flowing from cathode to plate, this plate current is depressed (Figure 1). Since the potential of the plate is less posi tive t h a n t h a t of the power supply, Efj, by an a m o u n t equal to the IRdrop across RL (a potential differ ence is generated across any resis tor through which current is flow ing) , the decrease in tube current causes the plate voltage to rise, a p proaching more closely the poten tial of the positive side of the bat tery. (If iv were reduced to zero, Εj, would then be equal to Eb.) Hence, a negative-going effect in 34 A

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Figure 4 . The Hartley oscillator employs a single, tapped coil in the fashion of an autotransformer to couple the plate-to-cathode variations to the cathode-to-grid circuit. Plate voltage changes affect the magnetic field about LK; the expanding or contracting flux emanating from LK cuts across the turns of La, thereby inducing voltages in the latter

the grid voltage produces a posi tive-going effect on the plate volt age—i.e., the grid and plate signals are 180° out-of-phase—as shown in Figure 2 for a sinusoidal variation. If, now, the phase of the plate voltage oscillation can be shifted by 180° and fed back to the grid, the feedback signal would reinforce the grid signal, and hence reinforce the tube current variation, thereby reinforcing the plate voltage oscil lation, etc. The result would be a regenerative chain reaction, result ing in sustained oscillations contin uing indefinitely. T h e generation of these free-run ning oscillations can be understood in greater detail on the basis of the following considerations. Consider t h a t the triode circuit with positive feedback from plate to grid has been assembled, and the power sup plies are connected to the tube. As the cathode begins to emit electrons and a plate current starts up, the plate voltage begins to fall. This negative-going effect is shifted in

phase by 180 L , and is fed back to the grid as a positive-going effect. As the grid becomes more positive, it causes the tube current to in crease, which in turn causes the plate voltage to decrease further, and it is fed back as an additional increase in grid voltage, etc. Thus, the tube current increases until it is as large as it can be. There is a definite upper limit to the tube cur rent, for as it increases, the plate Λ-oltage falls; also, as the grid volt age becomes more positive it picks up increasing amounts of current from the cathode-to-plate stream. The decrease in plate voltage di minishes the attractive force pull ing electrons across the interelectrode space to the plate; the grid current limits the magnitude of the positive grid potential by its biasing effect (because of the 7i2-drop it produces the grid leak resistor). When the tube current has reached its peak value, the plate potential has reached its lowest value, and is not changing. As will

REPORT FOR A N A L Y T I C A L CHEMISTS

MICROMETER SPINDLE

Figure 5. A practical example of the Hartley oscillator circuit in a form intended for dielectric constant measurements [Axtmann, R. C , ANAL. CHEM. 2 4 , 783 ( 1 9 5 2 ) ] . Units of capacitances are picofarads

be shown below, the phase-shifted feedback grid voltage is produced only in response to a changing plate voltage. Hence, the feedback signal starts to decay; this causes the grid potential to begin to be less positive t h a n its peak value. This negative-going effect on t h e grid causes the tube current to decrease; the decrease in tube current causes the plate voltage to rise; the positive-going effect at the plate is shifted in phase and fed back to the grid as a negative-going effect; this further decreases the t u b e current, etc. A regenerative chain reaction is once more produced, but this time it causes the tube current to fall to a minimum value. When the tube current is a t its "minimum value, and no longer decreasing, the negative potential of the grid due to the feedback begins to decay away. This is equivalent to a positive-going effect and a new regeneration sets in, producing another current peak. T h u s , t h e tube current oscillates continuously

from a maximum to a minimum value. T h e frequency of this oscillation in t u b e current is determined by the time required for the regeneration to cause the current to build up to its peak and to decrease to its minim u m value. T h i s t i m e is determined by the capacitances, inductances, and resistances present in the circuit, and their configurations relative to each other. For any given oscillator circuit, these p a r a m e t e r s combine to produce a definite time constant, and the oscillations therefore have a definite frequency, called the resonant frequency. Forms of V a c u u m Tube Oscillators

T h e various types of triode oscillators used in chemical instrumentation differ only with respect to the methods employed to feed a portion of the plate output back to the grid. Tickler Oscillator. Figure 3

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35 A


Figure 6. The Colpitts oscillator couples the plate of the tube to the cathode and grid through condensers C l r C2 C3, and C4. The amount of feedback is determined by the ratio of C1 to C2

shows the principle of the Armstrong or tickler-feedback oscillator circuit. As the tube current begins to flow, it causes a magnetic field to begin to build u p about the inductance coil, LP, in the plate circuit. T h e expanding magnetic flux lines from this coil cut across the turns of the coil, Le, in the grid circuit, and induce a voltage in the latter. T h e polarity of this induced voltage depends upon the directions of the windings of the respective coils ; these are arranged so t h a t an expanding magnetic field in LP causes the induced potential on the grid to be positive (and, hence, a contracting flux induces a negative grid pot e n t i a l ) . Thus, the starting up of current through the tube induces a positive-going effect on the grid, which causes the t u b e current to increase further, etc., and the regenerative cycle causes this circuit to produce sustained oscillations. I t will be noted t h a t a voltage is induced in the grid current only as long as the magnetic flux lines from the plate inductor are either expanding or contracting, and the induction ceases when the magnetic fields become stationary. The frequency of the oscillations 36 A

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Figure 7. A condenser connected between plate and cathode becomes charged up to the difference of potential existing between these electrodes. A negative-going effect at the plate means a pile-up of electrons at that electrode; this neutralizes some of the charge of the positive plate of the condenser, and releases some of the negative charge from the opposing plate. Thus, a negativegoing effect is felt at the cathode

is determined predominantly by the parallel inductor-capacitor combination in the grid circuit, viz., LQCQ. T h e grid leak resistor, RQ, limits the extent to which the grid can become positive, since the grid current picked up during the positive half-cycle flows through it, producing an Ziu-drop with the polarity shown in the diagram. T h e condenser, CiyVaSs, provides a low impedance p a t h for the high frequency oscillations around the high d.c. impedance of the grid leak resistor, while tending to maintain the grid bias voltage across the resistor by virtue of the charge built up on the condenser plates during the positive half-cycles.

discussion, a variable height mercury column is employed as an adjustable resistor in the plate circuit to compensate for changes occurring in the sample resistance. Thus, a null-balance approach to measurement is utilized, and the oscillator output is maintained constant. This has the advantage t h a t small changes in the variable resistance can be measured with better precision t h a n can the corresponding changes in the oscillator current. A straight column of mercury is employed as the variable resistor so t h a t the resistance adjustment does not introduce significant changes in the inductance of the plate circuit.

T h e amplitude of the oscillations also depends upon the circuit parameters, and is particularly sensitive to the conductance of the plate circuit. A tickler-feedback oscillator has been used by N a k a n o , et al., for high frequency titrimetry [ N a k a n o , K., H a r a , R., Yashiro, K., ANAL.

Hartley Oscillator. The circuit described above makes use of the mutual inductance between two separate windings; the principle is essentially the same as the one involved in a transformer when an a.c. excitation of the primary induces an a.c. signal in the secondary. The autotransformer is a modification of a transformer in which a single coil with a tapped connection is employed in place of the two coils. If the autotransformer principle is applied to a

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Figure 8. A practical example of the Colpitts oscillator designed to produce oscillations in the range of 120 megacycles per second, for application to high frequency conductometric titrations [Johnson, A. H., Timnick, Α., ANAL. CHEM. 30, 1324 (1958)]

triode oscillator, one obtains t h e H a r t l e y oscillator, shown in t h e dia gram of Figure 4. In this case, current starting up in the tube flows through the lower part of the inductance coil, LKT h e expanding magnetic field about this portion of the coil cuts across the turns of the remainder of the coil, La, and induces a voltage therein of such polarity as t o pro duce a positive-going effect on t h e grid. From this point on, t h e a c tion of the circuit is identical with t h a t which has already been de scribed. T h e H a r t l e y oscillator r e quires simpler components t h a n t h e tickler oscillator, and is more easily adjustable over a wide range of frequencies. An example of the practical a p plication of t h e H a r t l e y oscillator is given in Figure 5 [Axtmann. R. C,

A N A L . C H E M . 24, 783

(1952)].

I n this case, the circuit is designed for t h e measurement of dielectric constants. T h e frequency of t h e oscillations produced by t h e vac uum tube depends upon t h e induct ance and capacitance in the grid circuit. T h e dielectric constant cell contributes to the capacitance, as does the micrometer-adjusted vari able capacitor. For very sensitive measurements of dielectric constant changes, as might be of interest in following the kinetics of a reacting system, t h e frequency of t h e H a r t ley oscillator, with the sample in the cell, is adjusted to equal t h a t

of a fixed-frequency, crystal-con trolled oscillator (see discussion of t h a t circuit below) by suitable positioning of t h e micrometer spindle. T h e difference, or beat frequency, is then observed, and its variations reflect changes occurring in the dielectric constant of t h e sample. The sensitivity of this a p paratus was observed to be 0.0007% for a sample of dielectric constant equal to 2. T h e beat fre quency measuring principle is treated in more detail later.

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A H a r t l e y oscillator has been a p —

32, 537 ( I 9 6 0 ) ] to the operation of a sensitive, automatic control de vice. T h e capacitance in the grid circuit is composed, in part, of a pair of metal vanes, one of which is attached t o t h e movable pointer of a p H meter. When the p H departs from the desired control value, the meter needle deflects, changing the, capacitance of the grid circuit. If the change is appreciable, t h e a m plitude a n d frequency of the oscil lator are markedly altered, and a motor is activated as a consequence; the motor adds enough reagent to the system to bring t h e p H back t o the desired value.

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Colpitts Oscillator. A circuit in which the positive feedback b e tween plate and grid is obtained by means of capacitors is t h e Colpitts oscillator, illustrated in Figure 6. An increasing tube current causes the potential a t t h e plate to fall,

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Figure 9. A practical example of the Pierce oscillator in the form of a high-frequency conductance—dielectric constant meter. The vacuum tube is of the " m a g i c eye" type, and onset of oscillation is indicated by the " c l o s i n g " of the eye. The capacity substitution method of measurement is used. [Hall, J . L , ANAL. CHEM. 2 4 , 1244 ( 1 9 5 2 ) ]

and this negative-going effect is transmitted through the platecathode capacitor as a negatwe-going effect on the cathode potential. T h e mechanism of this process is illustrated in Figure 7, which shows t h a t t h e buildup of negative charges at t h e plate results in a partial neutralization of t h e positive charge on t h e plate side of t h e condenser, and hence, releases some of the negative charge on the cathode side. A negative-going effect a t t h e cathode means t h a t t h e cathode is becoming more negative t h a n it was relative to t h e grid, which is equivalent to a positive-going effect a t the grid. Thus, an increasing current through the tube r e sults in a positive-going effect a t the grid, which causes t h e tube current to increase still more, etc., and regenerative oscillations can occur. An illustration of the use of t h e Colpitts oscillator is t h e high-frequency titration a p p a r a t u s of A. H . Johnson a n d A. T i m n i c k [ A N A L . C H E M . 30, 1324 (1958)], shown in

Figure 8. T h e feedback between plate a n d grid is achieved through 38 A

.


Figure 1 0 . The tuned plate, tuned grid (TPTG) oscillator has tank circuits in both plate and grid sections. When both tanks are turned to the same resonant frequency, the coupling between the two takes place largely through the interelectrode capacitance, C0P

the capacitors C-> and C\. T h e single loop of wire in parallel with these capacitors provides t h e inductance, Li, t h a t is necessary to cause the circuit to produce oscillations in t h e desired range of 120 megacycles per second. T h e titration vessel is placed within this loop, a n d variations in t h e conductivity of t h e solution produce changes in t h e amplitude of t h e oscillations. I n this case, the amplitude is measured by determining the magnitude of the grid current (the grid picks up current during the positive half-cycles of its voltage swings), in terms of the IRdrop across resistor R>. Pierce Oscillator. If the capacitor between t h e cathode and grid in t h e Colpitts circuit is replaced by a piezoelectric crystal, the resulting circuit is called a Pierce, or crystal-controlled oscillator. T h e crystal is capable of vibrating only a t a fixed, characteristic frequency, determined b y t h e elastic constants of the substance, and b y t h e thickness a n d the orientation of t h e slice of crystal relative to its crystallographic axes. T h e vibration of a

piezoelectric crystal is accompanied by the generation of a charge separation across the crystal; hence, an alternating voltage is produced t h a t has the same frequency as the mechanical vibration. In a crystal-controlled oscillator, the alternation in the grid potential is constrained to occur at t h e frequency of the crystal, since the latter permits this frequency to pass with much less impedance than any other frequency. One advantage of a fixed-frequency oscillator in high frequency titrimetry is the possibility it offers for employing a precision capacitance substitution method in place of the amplitude measurements described in several of the illustrations given above. This is the basis of the simple titrimeter designed bv J. L. H a l l [ A N A L . C H E M . 24, 1244

(1952)], and illustrated in Figure 9. The vacuum tube (a "magic e y e " tube) serves both as the oscillator and t h e detector of the oscillations. Oscillations can occur only when the capacitance and inductance in the plate circuit correspond to a frequency t h a t is equal to the


/ Figure 11. A practical example of the TPTG oscillator designed for continuous monitoring of solution conductivity. Sample is contained in a vessel inserted in L·,. [Flom, D. G., Elving, P. J., ANAL. CHEM. 25, 541 (1953)]

n a t u r a l frequency of the crystal. When oscillations are being produced, the plate current creates an I B - d r o p across resistor R\, and this causes the shadow cast on a fluorescent screen by the secondary plate electrode to disappear—i.e., the " e y e " closes. If the variable capacitances in the plate circuit are adjusted until oscillations just s t a r t with no specimen in the cell, the addition of a sample increases the capacity and the oscillations are quenched. T h e amount of capacity t h a t must be dialed out in order to regain the oscillations is a measure of the capacity introduced by the sample, and hence, of its dielectric constant. TPTG Oscillator. One of the common types of oscillators used in chemical instrumentation is the 40 A

·


Figure 12. Acoustic gas analyzer in which electroacoustic transducers in the plate and grid tank circuits permit pronounced oscillations to occur only when the sound path length in the acoustic chamber has the proper value to let both transducers vibrate at same resonant frequency [Weller, E. F., ANAL. CHEM. 26, 488 (1954)]

tuned plate, tuned grid ( T P T G ) oscillator, Figure 10. I t differs from the oscillator circuits previously described in t h a t the LC combinations in the plate and grid circuits are independently adjusted, and the interaction between them is in large measure through the interelectrode capacitance which exists between grid and plate in the vacuum tube. The LC combinations in the plate and grid circuits are called the t a n k circuits. If either t a n k circuit is turned off resonance, the oscillations will cease. W h e n both are close to resonance, the frequency of the oscillations is controlled principally by the plate t a n k circuit, and the amplitude is controlled largely by the grid t a n k circuit. An example of the application of

this t y p e of oscillator to the monitoring of conductivity changes has been described by D . G. Flom and P . ,1. E l v i n g [ A N A L . C H E M . 25,

541

(1953)]; the circuit is shown in Figure 11. T h e sample cell is inserted in the plate t a n k inductance, Lt. T h e plate current, which is sensitive to the "loading" of the t a n k circuit by the sample—i.e., the conductivity of the sample decreases the impedance of the plate circuit, hence decreases the magnitude of the plate voltage swings, and results in reduced feedback to the grid circuit—is measured by means of a meter or recorder across dropping resistors Ri or Rs. Resistor iuii and tube T2 are employed to increase the stability of the circuit, which has been used for the continuous recording of


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Figure 13. The heterodyne principle applied to the design of a chromatographic zone detector. A variable radio frequency oscillator produces a signal that is mixed with the constant frequency from a crystal oscillator. The beat frequency is measured by comparison with a variable audio frequency oscillator. The comparison may be made with great sensitivity through the observation of Lissajous figures produced when the two signals are introduced onto the hori zontal and vertical plates of an oscilloscope, respectively. [Monaghan, P. H., et al., ANAL. CHEM. 24, 193 (1952)] 3. Chromatograph Scanner, Linear-Log Count Rate Meter, Linear Amplifier, High Voltage Power Supply.

conductivity changes as a function of time. An interesting example of an os cillator in which both t h e plate a n d grid circuits are tuned, b u t in which electrostrictive elements impose t h e resonant frequency, is the acoustic gas analyzer of E . F . Weller [ A N A L . C H E M . 26, 488 ( 1 9 5 4 ) ] , shown in

p a r t in Figure 12. An acoustically resonant chamber is used, with electroaeoustical transducers lo cated a t each end. T h e output from one transducer is transformercoupled to the grid of a vacuum tube, t h e plate ouput of which is similarly coupled to t h e other transducer. Since the velocity of sound in a gas is a function of the average molecular weight of t h e gas mixture, t h e two transducers can both resonate a t t h e same fre quency only if their distance a p a r t is an integral multiple or fraction of the sound p a t h length. Thus, t h e cell spacing is adjusted until sus tained oscillations are obtained; measurement of this distance can

then be utilized as an index of t h e composition of t h e gas. Heterodyne Oscillator. Fre quent use is made in chemical work of t h e heterodyne principle, which involves t h e mixing of oscillations from two separate sources a n d t h e measurement of the resulting beat frequency. An illustration of this approach is given in Figure 13, which is a block diagram of a chromatographic zone detector [Monaghan, P . H., Moseley, P . B . , Burkhaltcr, T. S., Nance, Ο. Α., A N A L . C H E M . 24, 193 ( 1 9 5 2 ) ] .

The

output of a fixed frequency, crystalcontrolled oscillator is mixed with t h a t of a radio frequency oscillator (in this case, a Clapp oscillator which is t h e Colpitis circuit p r e vioxisly described with an additional capacitor in scries with t h e plate inductance for increased frequency stability). If, for example, t h e crystal frequency is 5.000 mega cycles per second, and the R F os cillator frequency is 5.001 mega cycles p e r second, the mixer output

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will be a signal of 0.001 megacycle per second = 1000 cycles per second, which is in the audio range. This beat, or heterodyne frequency, can be measured with a precision of one cycle per second or better by comparing it with the output of a variable audio frequency oscillator. Thus, heterodyning permits a high frequency signal to be greatly re duced in frequency by a precisely known amount, with a consequent improvement in the sensitivity with which small changes in the original frequency can be detected. Sen sitivities of 1 p.p.m. are common with this technique, and parts per billion and better have been ob tained. The frequency comparison of the beat signal with the audio generator is very conveniently obtained by observing the Lissajous figures pro duced on an oscilloscope when one of these signals is introduced on the horizontal deflection plates, while the other is introduced on the vertical deflection plates.

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Oscillators

The preceding summary is not in tended as a comprehensive treat ment of the subject of oscillators. Many useful circuits have not been mentioned, and much important de tail has been left out of the dis cussion of those few circuits which have been included. The principal objective has been to show the role played by oscilla tors in present-day analytical work, to make clear some of the basic principles involved in the generation of sustained oscillations, and to call attention to a few of the unique potentialities of these devices for the solution of experimental prob lems. Bibliography

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Evans, R. N., Porter, C. B., "Experi mental Basic Electronics," McKnight and McKnight Publ. Co., Bloomington, 111., 1958. Ladd, M. F. C , Lee, W. H., Talanta 4, 274 (1960). Miiller, R. H., Gannan, R. L., Droz, M . E., "Experimental Electronics," Pren tice-Hall, New York, 1942. Rider, J. F., "The Oscillator at Work," John F . Rider Publ., Inc., New York, 1940.

The Electrical and Electronic Tools of the Analytical Chemist

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