Oscillograph

HE absence of any appreciable inertia in a beam of fast moving electrons gives the cathode-ray oscillograph an inherent advantage over all other types for the study of waveforms involving a wide range of frequencies. However, linear frequency response does not, in itself, guarantee a useful oscillograph, and it is only recently that tube limitations have been overcome and satisfactory auxiliary equipment developed to make possible exacting study of periodic phenomena up to 50,000 or 100,000 cycles per second.

The General Radio cathode-ray oscillograph tube is of the low-voltage type, employing anode potentials of 500 to 2000 volts to accelerate the electrons emitted by the alternating-current heated filament. The arrangement of the tube elements is shown in Figures 1 and 2.

There are two pairs of electro-static deflecting plates; one to produce hori-' ^ontal deflection, and the other to produce vertical deflection. If an alternating voltage is applied to either pair, the beam will be deflected rapidly so that a straight line appears upon the screen.

Obviously, some means of providing a time axis so that phenomena may be seen in their true amplitude-time relationship is most desirable. The rotating mirror has been used with vibrating-element oscillographs, and is also useful with the cathode-ray instrument. When the line produced by voltage across one pair of plates is viewed in a rotating mirror arranged with its axis parallel to the line on the fluorescent scrccn, the waveform may be seen, if the mirror is turning at a suitable speed.

The use of the rotating mirror with the cathode-ray oscillograph is somewhat limited in its applications since careful observation of the higher frequency phenomena to which the cathode-ray tube will respond would involve very high mirror speeds.

But probably the most serious disadvantage of the rotating mirror is its inability to keep in synchronism when the frequency shifts. Although the mirror speed may be adjusted so that a stationary pattern is obtained of the

Figure 1. The General Radio cathode-ray oscillograph tube. The inside of the glass at the large end is coated with a fluorescent substance on which the moving beam of electrons traces patterns m constant-frequency phenomena, any change in the frequency will cause the pattern to move. If the frequency changes appreciably, the pattern will probably move too quickly to be of any value.

Another method of obtaining a lime axis is the moving-film camera. Where a photographic record of a non-recurrent phenomenon is desired, this undoubtedly is the most satisfactory equipment. The General Radio Company has perfected moving-film cameras for various uses which will operate satisfactorily at film speeds up to 15 feet per second, giving a reasonably clear representation of any phenomenon involving frequencies in the audible spectrum.

The camera is, of course, limited by mechanical and photographic factors, including maximum velocity at which the film can be driven without tearing, and the maximum film "speed" at which proper photographic records can be obtained. The latter depends to a great extent upon the optical system and the type of film or sensitized paper, as well as upon the brilliancy of the cathode-ray spot and the speed with which it moves. In this connection, it is obvious that higher frequencies will produce fainter records than lower frequencies, since the actual length of the record will be considerably greater for a given length of film, consequently reducing the amount of light to which any particular spot of the film is exposed. 1

I t is evident that some other means of visual waveform examination is desirable. Since the oscillograph is arranged with deflecting plates so that two dimensional figures may be seen upon the screen, the possibility of being able to

1 The General Radio cathode-ray oscillograph tube is characterized by the unusual brilliance of the spot. With plate voltages of the order of 1500 to 2000 volts, the patterns on the fluorescent screen may easily be seen by a large group of persons in a lighted room. The fluorescence is unusually actinic, thus facilitating photography.

four deflecting plates four deflecting plates

ANODE

Figure 2. Klectrode structure of the oscillograph tube. The negatively charged cylinder concentrates the electrons emitted by the filr ment so that practically all pass through thv small hole in the anode. The beam passes between each pair of deflecting plates

ANODE

Figure 2. Klectrode structure of the oscillograph tube. The negatively charged cylinder concentrates the electrons emitted by the filr ment so that practically all pass through thv small hole in the anode. The beam passes between each pair of deflecting plates

Figure 3. A record of speech made with sensitized paper traveling at feet per second. Each of the dots in the line above the trace represents one millisecond. The reproduction suffers from the halftone screen required for the preparation of a printing plate, hut the original is more than satisfactory for any purpose of analysis see a waveform without the use of external mechanical equipment suggests itself.

If alternating voltages are applied simultaneously to both pairs of deflecting plates in the cathode-ray tube, Lissajou's figures will be formed, remaining stationary when one applied frequency is an exact multiple of the other. By proper interpretation of these figures, frequency comparisons can be made, but except to a skilled observer, little knowledge as to any deviation in the waveforms from a pure sinusoidal form can be gained.

This type of pattern can frequently

Figure 4. Characteristic Lissajou figure from applying a voltage £/, across horizontal plates having exactly Moth the frequency and twice the amplitude of the voltage Ev applied to the vertical pair. Note that the pattern near the center is an approximation to the true shape of Ev since the spot velocity due to Eh is nearly constant in this region. The "back trace" coincides with the forward sweep for this particular phase difference (90°)

Figure 4. Characteristic Lissajou figure from applying a voltage £/, across horizontal plates having exactly Moth the frequency and twice the amplitude of the voltage Ev applied to the vertical pair. Note that the pattern near the center is an approximation to the true shape of Ev since the spot velocity due to Eh is nearly constant in this region. The "back trace" coincides with the forward sweep for this particular phase difference (90°)

be made more useful when the wave being observed has a high frequency compared with the other or timing wave. If, for instance, a low-frequency timing wave, say 60 cycles, is impressed across the horizontal deflecting plates, and another recurring 600 times a second is impressed on the vertical plates, a pattern will be formed upon the screen which, with a little imagination, can be visualized as the 600-cycle wave.

If some system is used whereby the cathode-ray beam can be deflected across the screen at a constant velocity, an actual representation of any wave may be seen in linear relation with respect to time. Furthermore, if the beam can be made to traverse the screen at the desired speed, in one direction only, and then return instantaneously to its starting position, only a single representation of the waveform will be seen, whereas, with the sinusoidal timing wave previously mentioned, two views of the wave are seen, one going in each direction. The frequency at which the cathode-ray beam sweeps across the screen must, of course, coincide with the frequency of the observed wave or some submultiple of it, or the pattern will appear to move.

To provide a source of a controlled linear timing wave or "sweep," the

Figure 5. Output waveform of one Type 506-A Sweep Circuit as shown on the cathode-ray oscillograph using another to supply the linear time axis. Note the close approach of each trace to the ideal straight line

Figure 5. Output waveform of one Type 506-A Sweep Circuit as shown on the cathode-ray oscillograph using another to supply the linear time axis. Note the close approach of each trace to the ideal straight line

General Radio Company has developed the new Type 506-A Sweep Circuit, which was announced in last month's issue of the Experimenter. The sweep circuit provides a timing wave having a saw-tooth form, as shown in Figure 5 by means of a circuit which is shown diagrammatically in Figure 6.

The condenser C and the current lim-iter tube are connected in series across a source of 500 volts, d.c. Current flows in the circuit, charging the condenser, but since the current is limited to a certain maximum value, the voltage rises at a constant rate rather than exponentially, as would otherwise be the case.

Across the condenser is connected a mercury-vapor discharge tube (Type 506-P1). This tube is provided with a control grid so that it can be arranged to break down at any predetermined value of plate voltage. When this voltage is reached, the discharge tube Hashes, discharging C, and reducing the voltage across its terminals to practi cally zero. The flash in the discharge tube is then extinguished and the condenser charges again, going through the same cycle as before.

The voltage across the condenser terminals, accordingly, lias the waveform shown in Figure 5, and if the horizontal deflecting plates of the cathode-ray oscillograph are connected across the terminals of C, the fluorescent spot will have a periodic horizontal movement, crossing the screen at a constant velocity and then returning quickly to its original position.

The amplitude of the saw-tooth wave, that is, the horizontal length of the path traversed by the fluorescent spot, is determined by the voltage at which the discharge tube operates, which is controlled by the d.-c. bias on the grid of this tube. This bias is adjustable, so that the sweep may be long or short, as desired, and may be kept within the limits of the fluorescent screen, regardless of the anode voltage used on cathode-ray oscillograph tube.

By varying C and the maximum current passed by the current limiter tube, the speed at which the voltage across the condenser rises may be controlled. These two adjustments are used in the General Radio sweep circuit to adjust its natural frequency.

Since it is desirable to be able to cen-

OUTPUT

OUTPUT

Figure 0. Schematic diagram of ihe Type 506-A Sweep Circuit

ter the pattern on the fluorescent screen of the oscillograph, an auxiliary position control is provided on l he sweep circuit which consists of the voltage divider K. It will he seen from Figure 6 that the horizontal deflecting plates of the oscillograph are so connected to the sweep circuit through K and the C-> that a variable direct voltage bias is impressed upon the deflecting plates in addition to the saw-tooth wave. This makes it possible to move the entire pattern horizontally in either direction on the fluorescent screen.

Probably the most important feature of the General Radio sweep circuit is the manner in which the instrument is made to synchronize w ith any observed recurrent phenomena. A voltage of the frequency of the observed wave, usually obtained by direct connection to the vertical deflecting plates, is impressed across the terminals marked control. A shielded transformer transmits this voltage to the grid circuit of the discharge tube. If, without the control voltage connected, the circuit is adjusted to operate at approximately the desired frequency, introduction of the source of control voltage will cause the sweeping action to synchronize exactly with the observed wave. Not only is the transformer shielded, but it is so designed that, in connection with a resistance network, it reduces to a negligible value any interference which might be transmitted from the discharge tube back through the control circuits. A volume control is also included for varying the amount of control voltage applied to the discharge tube grid so that the best operating point may be secured without the use of external equipment.

The sweep circuit is completely shielded to minimize interference, and designed so that the mercury-vapor discharge tube operates at the correct temperature. The instrument includes power-supply equipment, so that it operates entirely from the 115-volt, 60-cycle lines.

Figure 7. The Type 506-A Sweep Circuit is shown at the right of the mounted oscillograph tube and its a-e operated power supply equipment

Figure 7. The Type 506-A Sweep Circuit is shown at the right of the mounted oscillograph tube and its a-e operated power supply equipment

Because of the automatic control feature of the sweep circuit, this equip- ■>

graphic examination of all types of recurrent phenomena occurring at audio frequencies, as well as certain types of transients and recurring waveforms involving frequencies up to approximately 100 kc. Since the sweep circuit may be controlled by the waveform under ex-animation, it may be made to lock in a A

step, not only on absolutely recurrent v '

phenomena, but on many types of phenomena involving shifts in frequency and amplitude. For instance, complex audio-frequency waves, such as are emitted by musical instruments or an orchestra, may be observed while music is being played, since a station-

ary pattern will be obtained on any h f tone which is sustained long enough for observation. * 4 *

The photographs of Figure 8 were , -r taken with a small pocket-type of camera (Ansco Memo) using 16-millimeter sensitized paper. The camera was equipped with an f/3.5 anastigmat lens, and I/10th second exposure was allowed for each oscillogram. & ¿L i'

Because of the linear frequency response of the oscillograph, the detail with which the various harmonic com-

only upon the excellence of the micro- • ^ * v phone and amplifiers employed. The amplifying system used had an excellent frequency characteristic and no y

noticeable harmonic distortion. * ^A/ **'

Figure 8. Sustained notes from a Bt> clarinet (left) and a C-melody saxophone (right) as they appear on the oscillograph screen using the Type 506-A Sweep Circuit. Exposure: 0.1 second for each record

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