Alignment Of Tuned Circuits

The alignment of a tuned circuit, so as to establish the correct resonant state by visual observation, is commonly understood to be the most valuable application of the cathode-ray oscillograph. Having read through this volume up to this point, you no doubt appreciate our statement when we say that observation of resonant circuits is but one more application of the cathode-ray instrument we are discussing. It is true, however, that, from the viewpoint of the servicing industry, it is an extremely important application. In fact, many service technicians have displayed infinitely more interest in its uses for the purpose of tuned circuit alignment than in its myriad other applications.

Just as a casual comment, let it be known, that visual curve tracing devices, such as are formed by the cathode-ray oscillograph when used for alignment operation, have been in use for some time. They are new in the radio servicing field, but have been used for a long time in manufacturing plants, in connection with production testing. While it is true that all of these commercial visual curve tracing devices were not of the cathode-ray tube variety, they were oscillographic. Incidentally, as far back as 1912, Marx and Banneitz used the cathode-ray oscillograph, with a motor driven condenser and a synchronized potentiometer-battery type of sweep, for the tracing of resonance curves. Deflection coils were used in the sweep circuit instead of deflection plates.

The production lines of many receiver manufacturers have been using the string galvanometer type of oscillograph, modernized, of course, for the alignment and testing of the various tuned circuits, which were part of the receivers being produced. Further historical background is not necessary, for after all is said and done, we are interested in that which now is available for use in the servicing field.

If we analyze the basic underlying principles of the curve tracing or visual alignment processes employed in connection with cathode-ray tubes, we find them very much like those which apply to other applications of the cathode-ray oscillograph. It is true that there are various ways of accomplishing some of the actions which enable operation of the complete visual aligning system, but the basis of the system has already been described, although it may not have appeared as such.

If we remember that the image which appears upon the cathode-ray tube viewing screen, irrespective of its shape, is the result of a deflection of the beam by means of a varying voltage applied to the deflection plates, we have the basis for visual alignment system. This, as you know, is also the basis for the normal operations of the cathode-ray oscillograph.

There are, however, two additional considerations found in alignment systems, which are not involved in the waveform observation operations with the cathode-ray oscillograph. One of these is that instead of a fixed frequency voltage input to the device under test, as is the case when observing voltage waveforms, the frequency of the voltage input varies continuously over a predetermined band. The second consideration is that relating to the frequency of the linear sweep circuit, which is used in conjunction with alignment operations. Wherea", for normal waveform observation, the frequency of the linear sweep circuit is adjusted in a certain ratio with the frequency of the voltage being observed, so as to place a certain number of cycles upon the screen, for alignment work the frequency of the linear sweep in the oscillograph is adjusted in accordance with the rate of frequency variation over the aforementioned band. The actual frequencies being produced, by whatever device is used to vary the band covered, have no bearing upon the frequency setting of the oscillograph sweep circuit. Bear these facts in mind as this discussion progresses.

A resonance curve is a graph of the voltage output of a tuned stage or tuned system with respect to frequency. The voltage output develops the vertical deflection and the variation in frequency is plotted along the horizontal axis. By synchronizing the sweep with the rate of speed of the frequency variation, time, as shown along the horizontal or "X" axis, is interpreted in frequency, because, theoretically, the rate of change in frequency is linear over whatever operating band is used. The result is that the base of the resonance curve represents frequency in equally spaced divisions, and makes possible the use of calibrated scales, graduated in kilocycles.

The above can be said to be the briefest of explantions of what goes on during visual alignment with the cathode-ray oscillograph.

Naturally, there is much more to the subject and in order to present most satisfactorily the facts of the matter, we are going to divide the subject into two parts: the frequency modulator, which is the means or device whereby the voltage fed into the tuned stage or system is made to vary continuously over the band; and second, the synchronization of the sweep circuit, so as to develop the resonance curve image.

The Motor Driven Frequency Modulated Oscillator

Since a resonance curve is an indication of the manner in which a tuned stage or system responds to an impressed voltage over a band of frequencies, one of which is the resonant frequency of the circuit, it is necessary that voltage be available at frequencies above and below the resonant frequency. The limits covered by this band, that is the number of kilocycles below the resonant frequency and the number of kilocycles above the resonant frequency, are spoken of as the "bandwidth." This test voltage, at a radio frequency or an intermediate frequency, is secured in the following manner: A constant voltage is secured from an oscillator at a fixed frequency, determined by the setting of the variable tuning condenser. This fixed frequency is really variable, since the tuning condenser is a variable, but, for the sake of clarity, we will classify it as a fixed frequency, because at any one time it is fixed as the setting of the tuning condenser remains untouched. This signal voltage at a certain frequency may be secured from a single oscillator, or may be the difference frequency or beat frequency between two oscillators, one of which is fixed tuned. Expressed in another manner, the test voltage may be the output voltage from a single oscillating system, or may be the voltage at the beat frequency resulting from the heterodyning of one oscillator with another.

Bear in mind that in both of these systems the output voltage is at a single frequency determined by the tuning. The design of these oscillators is such that the output voltage, while not substantially the same at all frequencies, is practically constant for a small band of frequencies, each side of any one setting. To secure such a test signal voltage over a band of frequencies a device known as a frequency modulator is incorporated into the system. This frequency modulator may be a mechanically operating device or may be an electrical device. In the mechanical device, a small variable condenser is connected or arranged in the tuned circuit. This condenser is continuously rotated by a small, fractional horse-power motor. As it rotates, the capacity naturally varies from minimum to maximum and back from maximum to minimum. Since this condenser is electrically connected to the tuning condenser in the oscillator circuit, each variation in capacity varies the total tuning capacity, hence the frequency of the circuit. While the frequency modulator condenser rotates, the frequency of the oscillator is being continually varied. The width of the frequency band thus covered is a function of the circuit capacity change due to the rotation of the frequency modulator condenser and the change in frequency of the oscillator output voltage, as a result of the change in the L-C circuit.

Fig. 251. Schematic diagram of a motor driven frequency modulator con' nected to an oscillator. The oscillator is tuned to the mean frequency of the circuit under test and the motor driven condenser varies the output frequency over a predetermined band, say 10 kc. each side of the mean frequency.

With suitable calibration it is possible to adjust the oscillator condenser to generate a fixed frequency of say 260 kc. and to cause, as a result of the capacity change in the frequency modulator condenser, a continuous variation in frequency from 250 kc. to 270 kc., thus providing a band width of 20 kc., or a 10 kilocycle variation each side of the mean or periodic frequency. (The mean frequency, which is the setting of the oscillator and which also is the resonant frequency of the circuit under test, often is referred to as the periodic frequency.) As shown in figure 251, which is a simple version of what has been said, the continuously rotating variable condenser is that contained in the frequency modulator. The condenser in the oscillator, which is of the continuously variable type, is not geared to a motor or any other form of drive. It is tuned by hand to whatever periodic frequency is desired and is operated in exactly the same manner as if there were no frequency modulator unit and a single frequency signal was desired.

Referring to figure 251, the motor and condenser shown within the dash line, comprise the frequency modulator unit. It is possible to arrange the frequency modulator condenser with several sections, so that two ranges of capacity are available, thereby providing two band widths for any one periodic frequency. The connection between the frequency modulator condenser and the tuning condenser within the oscillator may be made in any manner which is convenient and which will remain constant. The oscillator circuit shown is by no means an example of any one commercial oscillator system. It is a simple oscillator shown solely for illustrative purposes.

The exact band width desired depends upon the nature of the tuned circuit or stage being investigated. A controlling influence is the frequency band width rating of the circuit under investigation. Fortunately, the majority of tuned circuits or stages associated with such devices, are rated at from 5 kilocycles each side of the periodic or resonant frequency to about 10 kilocycles each side of the resonant frequency, so that by arranging a system whereby these two band widths are available, complete coverage is secured, with possibly two adjustments of the frequency modulator capacity range. Whatever the band width of the frequency modulated system, it should always be at least 25 percent greater than the rated band width or band pass of the tuned circuit or system being investigated. This is desired so that the base of the resonance curve will be established as a reference point, even when circuit conditions are such that tuning is broader than normal.

We made reference to the fact that the band width achieved was a function of the relation between the total circuit capacity at any one periodic frequency setting of the tuning condenser and the change in total circuit capacity as a result of the rotation of the frequency modulator condenser. It, therefore, stands to reason that the higher the periodic frequency, or the higher the frequency setting of the oscillator circuit, the greater the band width when the frequency modulator condenser is applied. This is so because a uniform percentage change in frequency, as a result of a uniform change in circuit capacity, is greater in actual number of kilocycles, the higher the mean frequency.

It further stands to reason that the lower the basic circuit capacity, the greater the band width, when the frequency modulator condenser is set into operation. The reverse is also true. This is so, because, with a fixed maximum value for the frequency modulator condenser, the change in total circuit capacity is greater, the smaller the basic circuit capacity and naturally smaller, the greater the basic circuit capacity. It is important to remember this variation in band width experienced with single oscillator equipment of this type, because other arrangements are also used, wherein constant band width exists upon all frequencies.

An idea of what is meant by an increase in band width with frequency is as follows: Suppose that operation of the frequency modulator condenser results in a 6-percent change in frequency at 260 kc. and at 1000 kc. In the former instance, the total band width will be 15.6 kc. or the frequency limits for a mean frequency of 260 kc., will be 252.2 kc. to 267.8 kc. When set to 1000 kc., the total band width is 60 kc. and the frequency limits would be 970 kc. and 1030 kc. Thus, the single oscillator used with a motor driven frequency modulator condenser, can be classified, for the want of a better name, as a variable band width frequency modulator.


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