Oscillograph

In the past, use of the cathode-ray oscillograph has been considerably restricted due to the lack of an adequate means for obtaining the linear time axis that is necessary if wave shapes are to be observed. Although Lissajous figures are convenient in making frequency comparisons, they are in many cases entirely useless for examining the actual form of a wave.

Rotating mirrors and moving-film cameras have been used with varying degrees of success. The former, however, are limited to rather slow mirror speeds and the latter to photographic records.

The other common method for obtaining a linear time axis is the so-called sweep circuit. This consists of a means for generating a "saw-tooth" wave

The General Radio cathode-ray oscillograph. At the right is the sweep circuit and behind it the power-supply unit for the oscillograph tube

The General Radio cathode-ray oscillograph. At the right is the sweep circuit and behind it the power-supply unit for the oscillograph tube

Here is what the sweep circuit does. Alternating voltages may he applied to the vertical deflecting plates (as in A) or to the horizontal plates (as in a). Pattern C results when Eā€ž and Eh are applied simultaneously, the frequencies fv and ft, being the same. If Eh is a "saw-tooth" wave derived from a sweep circuit and Ev is an alternating current wave to he examined, the wave shape of Eā€ž appears in Pattern T)

which, when applied to one pair of the oscillograph deflecting plates, produces a linear time axis. Commercial types of sweep circuits have, however, been restricted to operation over narrow rangesof frequency and amplitude. Furthermore, the problem of controlling the sweep circuit to keep in step with the waveform being observed, providing a stationary pattern on the oscillograph screen, has caused considerable trouble.

In order to overcome this limitation in the use of the cathode-ray oscillograph anil provide a convenient means for waveform examination, the General Radio Company has developed the new Type 506 Sweep Circuit. While it is intended primarily for use with the General Radio cathode-ray oscillograph, it can also be used with similar oscillographs of other manufacture. It provides a practically linear time axis so that the waveform being observed is seen directly upon the fluorescent screen of the oscillograph.

A great advantage of this new sweep circuit is that its action may be easily controlled, that is, the periodic movement of the fluorescent spot may be made to coincide exactly with the frequency, or some submultiple of the fre quency, of the wave being observed, thereby causing the pattern on the fluorescent screen to remain stationary. This makes careful examination of recurrent waveforms possible, and if desired, photographs may be taken of the patterns as they appear upon the screen.

The Type 506 Sweep Circuit has been carefully designed to provide a very high degree of control over the sweeping action. Separate adjustments are provided for changing the sweep frequency and length of sweep, as well as for regulating: the amount of control

voltage applied and adjusting the position of the pattern on the fluorescent screen.

Extreme care has been taken in the design of the control system so that no interference is transmitted from the sweep circuit itself back through the control circuits. Therefore, it is possible to obtain the control voltage from the same source as the observed wave without having the sweep circuit itself affect the form of the observed wave as seen upon the fluorescent screen.

The sweep circuit controls so readily that it may be used for examination of many types of transient phenomena. If a voice or music wave is impressed upon it, it will lock into control at some low frequency or its submultiple in the voice or music wave. It is, accordingly, possible to study the waveform of any sustained note or sound, as well as to see many of the transients involved.

The principle of operation of the General Radio sweep circuit is very simple, but considerable care was required in constructing suitable equipment to carry out this principle satisfactorily. The circuit is, in effect, a modified form of relaxation oscillator. A condenser is charged at a constant current until the voltage across its terminals reaches a certain predetermined value. At this point, a special mercury-vapor tube breaks down and discharges the condenser practically instantaneously. The voltage across the terminals of the condenser is applied to the horizontal deflecting plates in the catliode-ray oscillograph, thus causing the fluorescent spot to travel in one direction at a practically constant velocity and then return almost immediately to its original position.

The voltage at which the discharge tube operates is controlled by a grid in the tube. This controls the amplitude of the sweep. The rate at which the condenser charges is controlled by the current limiting tube. This controls the velocity of the sweep. By introducing a small alternating voltage in series with the direct bias on the grid of the discharge tube, the sweep circuit may be made to lock in step with an alternating voltage near its natural frequency.

The General Radio Type 506 Sweep Circuit is completely a-c operated and carefully shielded. It is mounted in a small walnut cabinet of the same size and shape as the Type 496-B Power Supply Unit for the cathode-ray oscillograph.

The price of the Type 506 Sweep Circuit is $160.00.ā€” H. H. Scott

A STANDARD-SIGNAL GENERATOR FOR THE MEDIUM PRICE FIELD

The development of radio receivers for operation at high frequencies has had its corollary in a demand for a satisfactory standard-signal generator to go to very high frequencies. A careful study of the problem has indicated that generators similar in general construction to those now employed can be used with suitable design modification at frequencies as high as 15 megacycles. It is believed work at higher frequencies will require a rather different design of signal generator and attenuator system from that now used.

The General Radio Company has had under development for the last year a new signal generator designed to meet the need for dependable behavior at high frequencies. The Type 603-A Standard-Signal Generator which will be shown at the Chicago Trade Show is the result of this development. This signal generator may be used over a frequency range extending from 100 kilocycles to 15 megacycles.

Over this range its performance is well within the limits of accuracy which we have become accustomed to

The main tuning control for the Type 603-A Standard-Signal Generator carries a dial with 600 divisions. The microscope at its right is an aid in making close settings; the triangular index at the left is for use with calibrations filled in with pencil by the user

expect from signal generators in the broadcast-frequency range. It is capable of modulation up to 90% at broadcast and higher frequencies. Internal modulation at 400 cycles is provided and provision is made for external modulation as well.

The new standard-signal generator will be used for the usual fidelity, sensitivity, and selectivity tests on receivers throughout the very wide frequency range for which it is adapted. It is also suitable for field-strength measurements throughout this wide range, since it is semi-portable and can easily be transferred in an automobile.

The new standard-signal generator has a number of interesting design features. The shield has been so modified without increase of leakage that it is not necessary to remove any screws in order to change coils. Immediate access to the coil compartment is obtained by raising the lid on the cabinet.

Space for the extension coils is also provided inside of the cabinet. Leakage around the lid is avoided by a refrigerator-door type of construction in the shielding.

An interesting new type of attenuator and shield has been evolved. The usual resistance type of attenuator has been used with a modified construction as made necessary by the much higher frequencies involved. The attenuator is enclosed in a sectionized shield which makes possible very large attenuations even at frequencies at 15 megacycles without serious errors.

The controls are shown on the front of the panel. Those at the right govern the radio-frequency circuit, those at the left the modulation circuit. The carrier frequency is controlled by a large dial with slow-motion adjustment. This dial carries an accurately engraved scale of 600 divisions, spread around 270 degrees of its circurnfer-

ence. The use of this dial in conjunction with a coil spread of approximately 2 to 1 in frequency makes possible direct calibration of the main frequency scale for use in selectivity and bandwidth determinations. Calibration charts are provided and they are of such size as to be read to the same accuracy as the dial scale.

Two additional convenience features will be noted on this dial: The magnifying glass over the main index greatly assists in setting and reading the scale. The secondary index, together with the space on the dial rim for extra scales, permits calibration of the instrument at special points to suit the user's requirements.

The carrier-frequency output voltage is controlled by the three adjustments in the lower right section of the panel. The carrier amplitude is adjusted, by means of the middle control, to a reference line on the right-hand meter. Maintaining this adjustment constant, the output is adjusted by means of the slide wire labeled microvolts and the multiplier. Continuous variation from one volt to 3^2 microvolt is provided. The output is taken off from the shielded plug terminals in the lower right edge of the cabinet.

The modulation-control system is shown at the left. The meter indicates the modulation voltage and is set by means of the modulation amplitude control to a reference line. With the per cent modulation dial set at the desired modulation percentage, external modulation may be connected at the terminals indicated and controlled in the same manner.

In condensed form the performance characteristics, in the new signal generator, are as follows:

Frequency range: 100 kilocycles to 15 megacycles.

Accuracy of the output voltage at 1 microvolt: 3% in the broadcast range and below, 10% at 10 megacycles, and 12% at 15 megacycles.

The output voltage range permits continuous adjustment from

The attenuation system for the new standard-signal generator viewed from the hack. The casting at the left contains the slide wire; the one at the right is the "mouse-trap-type" step-

by-step attenuator

The attenuation system for the new standard-signal generator viewed from the hack. The casting at the left contains the slide wire; the one at the right is the "mouse-trap-type" step-

by-step attenuator microvolt to one volt. Four hundred-cycle internal modulation is provided and the apparatus can be modulated with either internal or external modulation up to 90% at broadcast and higher frequencies.

The frequency modulation total swing does not exceed 200 cycles under the worst conditions in the broadcast band. At the RMA test frequencies it amounts to about 50 cycles.

The reaction of the attenuator on the carrier frequency is less than 150 cycles under all conditions.

The input impedance at the external modulation terminals is approximately 5000 ohms and a power of about 100 milliwatts is necessary to modulale the instrument to 30% throughout its range.

The radio-frequency output impedance is 10 ohms up to 10,000 microvolt setting.

The price of the Type 603 Standard-Signal Generator is $600.00. This includes the two calibrated inductors necessary for covering a frequency range of 420 to 1900 kilocycles, which includes the broadcast band.

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