A Modulated-Frequency System of

Telemetering

HAROLD E. RENFRO ARTHUR PERRY PETERSON NONMEMBER AIEE ASSOCIATE AIEE

WITH today's centralized control and supervision of power systems, it is

essential that the man at the controls knows what is going on at all outlying stations and interconnection points. Telemetering equipment keeps him in­formed of load changes, transfer of power from one system to another, variations in voltage, and all other values that are necessary for efficient operation.

No one system of telemetering can meet all requirements. The speed of in­dication, accuracy, number of separate channels available, dependability, ease of totalizing, freedom from interference, the availability and need of repair parts, and the initial cost, as well as maintenance cost, are factors that determine the de­sign of any system of remote metering. A resume of t i e telemetering practice up to 1941 will be found in the report of the joint committee of the AIEE committee on automatic stations and the AIEE com­mittee on instruments and measurements, published in October 1941. That report also contains an excellent bibliography on telemetering systems and methods.

When the Department of Lighting of the City of Seattle wanted a telemetering system which would operate over a car­rier-current channel they were unable to locate such a system that would- give them continuous and instantaneous read­ings. They felt that, for their purposes, an impulse system which provided read­ings at regular intervals was not the an­swer to their problem. Furthermore, they wanted a system that would permit several readings simultaneously over a single set of conductors. It was this need that led to the development of a modu-lated-frequency system of carrier tele­metering.

The telemetering system herein de­scribed was designed to give continuous and almost instantaneous readings which are independent of supply voltage and are not affected by wide variations in remote-line characteristics. I t was designed to operate with a high degree of depend­ability during emergencies, to permit a large number of channels on one pair of conductors, to give very fast response,

Paper 45-19, recommended by the AIEE committee on instruments and measurements for presentation at the AIEE winter technical meeting, New York, N. Y., January 22-26, 1945. Manuscript sub­mitted May 8, 1944; made available for printing December 1, 1944. HAROLD E. RENFRO'is an engineer in the Depart­ment of Lighting of the City of Seattle, Wash., and ARTHUR PERRY PETERSON is president of the Control Corporation, Minneapolis, Minn.

FEBRUARY 1945, VOLUME 64

and to incorporate certain other desirable features.

The transmitter generates its own car­rier frequency making it unnecessary to provide a separate carrier-current system. Standard radio receiver tubes are used throughout and all parts subject to wear or breakdown are easily replaceable stand­ard radio parts.

General

The block diagram, Figure 4, illustrates the principle of operation and this will be described briefly. In the telemeter trans­mitter is incorporated the primary in­strument, such as a wattmeter, which determines the values to be transmitted. This primary instrument controls the frequencies of two high-frequency oscilla­tors whose outputs are converted into a single beat frequency. The beat fre­quency, which varies with the magnitude of the quantity being measured, is ampli­fied and transmitted over the line, whether it be a transmission line or a tele­phone line. Normally, capacitive cou­pling is used with a transmission line whereas inductive coupling, with a band­pass filter to separate the carrier fre­quencies from the voice frequencies, is used with a telephone line. Only tele­phone-line applications will be described here.

At the receiving point the incoming signal from the line is amplified and then mixed with the output of a local oscillator of constant frequency. This converter stage is followed by a low-pass filter which attenuates all but the beat fre­quency. The current at this beat fre­quency is then amplified and fed into a frequency meter, of which the final indi­cating instrument is a part.

Transmitter

The schematic diagram of the tele­meter transmitter is shown in Figure 6. The primary instrument actuates two variable capacitors, C\ and C2, which, in turn, simultaneously control the fre­quencies of the two oscillators. The variable capacitors form part of the ca­pacitance of the frequency-determining resonant circuit of each oscillator.

The greatest difficulty with instru­ments of this type that have been tried in the past has been that a variable capaci­tor on the shaft of the primary instrument develops a torque of its own that varies

Renfro, Peterson—Telemetering

with the potential applied to it. A vari­able inductance develops a somewhat similar torque due, to current through it. This destroys the calibration of the in­strument and makes a reliable system im­possible, unless counter measures are taken.

An analysis of the torque developed by a variable capacitor with a voltage across its plates shows the following to be true: two conductors separated by an insulator tend to move toward each other when a difference of potential exists between them. With parallel-plane conductors of infinite area, this force is perpendicular

Figure 1 . Front view of the assembled tele­meter transmitter

The two wrousht-iron tanks at the bottom house the oscillatory circuits

Figure 2. Telemeter receiver

The indicating instrument is not shown

TRANSACTIONS 45

to the planes of the conductors. In a ro­tary variable air capacitor having two identical semicircular parallel fixed plates with a single semicircular rotor midway between them, a difference of potential causes several forces which are listed be­low, Figure 5 illustrates these forces and also shows the manner in which the two capacitors are formed by the stationary plates and damping disk of the primary instrument.

1. Forces between the rotor and upper stator which are balanced by forces between the rotor and lower stator. These forces, which are not shown on Figure 5, are parallel to the shaft and do not tend to rotate it. 2. Forces between the stator plates and the extended parts of the rotor, shaft, and so forth, which are symmetrical about the axis. These forces, shown as Au A2, Az, and AA, are radial and do not tend to produce rotation. 3. Forces between the diametrical edge of the rotor and the stators when the rotor is not fully meshed. These forces, shown as JBi, Bz, Bs, and B*, are tangential and tend to turn the rotor in a direction that will mesh it deeper between the plates of the stator.

Thus, the only rotary electrostatic force is due to edge effect so that, with plates so shaped that the edge exposure is constant with any angle of rotation, the torque produced is dependent only on difference of potential and is entirely independent of angular position except near zero mesh and full mesh. Through­out practically the whole angle of rota­tion, the torque may be kept constant with constant applied potential. Cur­rents flow radially and so do not produce any torque.

Two variable capacitors having a com­mon rotor are incorporated in the trans­mitting instrument in such a manner that increasing the mesh of one capacitor de-

damping disk passes between them. The four plates are identical and have the same voltage applied between each plate and the rotor, thus neutralizing any torque which is produced.

Figure 4. Block diagram of the modu-lated-frequency sys­tem of telemetering

maximum mesh and 60,500 cycles per second at minimum mesh. At the time one capacitor is at maximum mesh the other is at minimum mesh so that the two oscillator frequencies approach or re-

TRANSMITTER

PRIMARY INSTRU­

MENT

OSCILLATOR 49,500 TO 50,00p_CPS

r'ELECTROSTATIC \ff COUPLING VERTER

CON- u p-™ 14

OSCILLATOR 60,000 TO 60,500 CPS 1

FILTER

\

AMPLI­FIER

COUP-HLING

UNIT LINE

10,000 TO 11,000 CPS OUTPUT

RECEIVER

LINE COUP LING H UNIT

[AMPLI­FIER

Since the two capacitors formed by the plates and damping disk are of small ca­pacitance, the oscillator frequencies must be high in order to obtain the desired frequency change with a given angle of rotation of the instrument shaft. A fast response to changes in measured values is obtained since no mechanical connec­tion is made to the shaft and all torques due to electrostatic forces are eliminated through cancellation.

Various oscillator frequencies can be used and, in the case of several metering sets operating over a common line, the frequencies must be separated sufficiently in order to provide the required number of channels. A remote-metering arrange­ment using typical oscillator frequencies will be described.

One of the variable capacitors de­

creases the mesh of the other. At the time that one capacitor is at maximum mesh the other is at minimum mesh so that the two oscillator frequencies ap­proach or recede from each other with ro­tation of the common rotor. The two capacitors have a common rotor which is formed by the damping, disk of the watt­meter. Each of the two stators consists of two parallel aluminum plates spaced 0.083 inch apart and mounted so the

Figure 3. Indicating instruments at the receiv­ing end of six telemetering systems

scribed herein is in an oscillator circuit arranged to generate 50,000 cycles per second at minimum mesh of the capacitor plates and 49,500 cycles per second at maximum mesh. The other variable ca­pacitor is in an oscillator circuit which generates 60,000 cycles per second at

LOW HCONVERTER W PASS

FILTER AMPLI­

FIER FREQUENCY

METER

100 TO 1100 CPS OUTPUT

OSCILLATOR 9900 CPS

VOLTAGE [REGULATOR I

STATOR PLATES

Figure 5. Electrostatic forces in two rotary variable air capacitors, each having two iden­

tical semicircular parallel fixed plates

A single semicircular rotor midway between the fixed plates is common to both capacitors

cede from each other with rotation of the transmitting instrument shaft. The os­cillator tubes are A and B in Figure 6.

In order to insure an accuracy of two per cent, the maximum frequency varia­tion of each of the three oscillators used in the system cannot vary more than two thirds of one per cent, assuming zero error in other components. Investigation showed that electron-coupled oscillators, tuned by a combination of positive- and negative-temperature-coefficient ceramic fixed capacitors plus a very small variable capacitor, have excellent stability over long periods of time. All oscillator cir­cuits used in this system of telemetering are stabilized in this manner. The amount of negative-temperature-coefficient capaci­tor capacitance necessary to compensate for the coil and positive-temperature-coefficient capacitors in the set as de­scribed here was 10.71 per cent of the to­tal capacity. Coils and mountings were developed that repeated their tempera­ture-frequency characteristics faithfully.

The two voltages which are generated by the oscillators are fed into a converter

46 TRANSACTIONS Renfro, Peterson—Telemetering ELECTRICAL ENGINEERING

tube (tube C in Figure 6) where the dif­ference or beat frequency, which varies between 10,000 and 11,000 cycles per second, is produced.

The output of the converter is fed into a low-pass filter which attenuates the high frequencies and all harmonics and

amplified by the audio amplifier (tube C) which feeds the frequency meter. In­corporated as a part of the frequency meter is the final indicating instrument, Its pointer is at the left end of the scale with 100-cycle input and at the right end of the scale with 1,100-cycle input.

ci^S^Tn

Figure 6. Schematic diagram of the tele­meter transmitter

passes only the beat frequency to an amplifier which is arranged in push-pull. In Figure 6 the amplifier tubes are D and E.

The amplifier feeds the line through an inductive coupling unit (7\) which has laminated phenolic-sheet insulation be­tween the primary and secondary wind­ings and is capable of withstanding the highest voltage exposure of the line. A single-stage low-pass filter with a cutoff frequency.of 4,500 cycles per second pre­vents the carrier power from entering the telephone instrument circuit where it would be wasted. This filter is not shown in Figure 6.

Receiver

The schematic diagram of the telemeter receiver is shown in Figure 7. At the re­ceiving point the telephone line connects to a coupling unit similar to that at the transmitter. Voice frequencies are passed on to the telephone unit and the carrier frequency goes to the metering receiver.

The receiver consists of one 10,000 to 11,000-cycle amplifier stage, local os­cillator, detector, low-pass filter, audio amplifier, frequency meter with indicat­ing instrument, and a voltage regulator. The high-frequency amplifier stage (tube A and associated circuits) is fixed in fre­quency and is adjusted to 10,490 cycles per second. This frequency is approxi­mately the geometric mean between 10,000 and 11,000 cycles per second. The local oscillator (tube F) has a constant output frequency of 9,900 cycles per second. The oscillator output and the high-frequency output are fed into the converter stage (tube B) where a beat frequency is produced. This beat fre­quency varies between 100 and 1,100 cycles per second. The high frequencies and all harmonics are eliminated by the low-pass filter which follows the converter stage. The output of the low-pass filter (100 to 1,100 cycles per second) is then

The frequency meter consists of two triode tubes, D and E, connected in se­ries; that is, the plate of E is connected to the positive line, its cathode is con­nected to the plate of the other triode, D, and the other cathode is connected to the negative line. A fixed capacitor is shunted across the plate and cathode of each tube. These capacitors are shown as Cz and C4. The grids in normal opera­tion are biased beyond cutoff so that no plate current flows except that due to a-c grid excitation. At the time one grid is made more positive, the other is made more negative so that both tubes cannot conduct at the same time. Both tubes may be nonconducting at once but con­duction is alternate. When either tube becomes conducting, it discharges the ca­pacitor across its plate-cathode circuit and charges the capacitor which is con­nected across the nonconducting tube. When the grid polarities change, the op-

voltage and wave form within wide limits* The frequency meter and oscillator in

the receiver are supplied from an elec­tronic voltage regulator which is of a type frequently used and will not be described here. The neon-glow tube, / , and vacuum tubes G and H with their associated cir­cuits comprise the voltage regulator. The actual output potential remains con­stant at 80 volts with the line voltage between 100 and 130.

Zero current in the indicating instru­ment puts the pointer off scale to the left. Such a condition indicates that the d-c power supply is off at the receiver or that the frequency-meter tubes have failed. If the transmitter power is off, or if the line is in trouble so that no carrier cur­rent is received, the bias is removed from the frequency-meter tubes and the indi­cating instrument goes off scale on the upper end. In this case the current is limited by degenerative resistors in the cathodes of the frequency-meter tubes. If the indicating instrument pointer is on its scale, the observer- knows that none of the troubles just mentioned are being experienced.

Totalizing at the transmitter may be accomplished by a multiple-wound cur­rent transformer in the a-c circuits to the wattmeter. Totalizing at the receivers may be done in the frequency-meter out­put circuits by adding the direct current in a lead common to several instruments. Strip chart recording d-c milliammeters are available for continuous records at the receiving point.

Six complete sets of this system of tele­metering are now in continuous service and have been for the past few years. The experience thus gained definitely

posite capacitors charge and discharge. A d-c milliammeter in series with the d-c supply shows the average current used to charge the capacitors. With a constant direct voltage each charge requires a definite current for a definite time. The time of discharge and charge is shorter than the period of one input alternation at the highest frequency used and there­fore the d-c milliammeter reading is a definite indication of the number of charges per second. The number of charges is controlled by the input fre­quency and is independent of the input

Figure 7. Schematic diagram of the telemeter

indicates that the equipment has met the requirements', expected of it. It provides almost instantaneous indications with standard accuracy. It permits a large number of readings to be transmitted over a single circuit and it is a simple matter to totalize readings from several transmitters. It is dependable and free from interference. It has proved easy to maintain at a minimum of expense.

FEBRUARY 1945, VOLUME 64 Renfro, Peterson—Telemetering TRANSACTIONS 47