Also shows the presence of another carrier voltage that is beating

against the frequency modulated signal. A signal of the mean frequency found its way into the circuit being tested with the frequency modulated oscillator. Although not very clear in the photograph, the undesired signal is at zero beat at the mean frequency. An example of a similar condition, but wherein the beating signal is at zero beat several kilocycles away from the mean frequency, is shown in figure 295. Note the zero beat, indicated at both sides of the pattern. Incidentally, it is possible to determine the band width or the position of any frequency within the frequency modulated band, by feeding a known variable frequency unmodulated signal into the circuit being tested with the frequency modulated signal and cause these

Fig. 295, left, Fig. 295-A, right. On the lower right side of the curve in Fig. 295 may be seen a large peak, which indicates the presence of some undesired frequency that is at zero beat several kilocycles away from the mean frequency. The "ghost" patterns behind the main curpe in Fig. 295-A are due to the presence of a harmonic of the frequency modulated i-f. signal in the r-f. amplifier.

Fig. 295, left, Fig. 295-A, right. On the lower right side of the curve in Fig. 295 may be seen a large peak, which indicates the presence of some undesired frequency that is at zero beat several kilocycles away from the mean frequency. The "ghost" patterns behind the main curpe in Fig. 295-A are due to the presence of a harmonic of the frequency modulated i-f. signal in the r-f. amplifier.

two to beat against each other. Since the beating oscillator frequency is known, the zero beat position upon the image spots that frequency upon the resonance curve.

A condition, which may arise during alignment of i-f. amplifiers and which interferes greatly with proper interpretation of the pattern, is indicated in figure 295-A. The "ghost" alignment patterns in the background are due to a harmonic of the frequency modulated i-f. signal feeding into the r-f. amplifier. The remedy is either to detune the receiver so that it does not respond to the harmonic or to short the oscillator section of the variable condenser while aligning the i-f. stages.

Excessively Strong Frequency Modulated Signal

The image, which appears when the frequency modulated signal fed into the circuit being tested is too strong and overloads the tubes, is shown in figures 296, 297 and 298. Figure 296 is the correct curve with the proper amount of signal input. Figure 297 shows the effect of tube overload as stated. Note that the base of the curve now shows a change in resonance, which is more aggravated when

Figs. 296, left, 297, middle, 298, right. The resonance curve in Fig. 296 was made when the frequency modulated signal input to the circuit under test was the correct strength. The distortion in Fig. 297 was due to too strong a signal with resultant overloading of the tubes. Compare the spreading of the curve, indicating change in resonance when the signal is increased still further, as shown in Fig. 298.

Figs. 296, left, 297, middle, 298, right. The resonance curve in Fig. 296 was made when the frequency modulated signal input to the circuit under test was the correct strength. The distortion in Fig. 297 was due to too strong a signal with resultant overloading of the tubes. Compare the spreading of the curve, indicating change in resonance when the signal is increased still further, as shown in Fig. 298.

the degree of overload is increased still more, as shown in figure 298. This is to be expected, because the presence of grid current creates a condition which is the equivalent of a load placed upon the circuit and causes a change in the resonance condition. It is interesting to note the relative amplitudes of these three images, bearing in mind that AVC was present in the circuit.

Conditions of Alignment

The condition of alignment is established by interpretation of the pattern. In the double image system, the figures of 299, 300 and 301

Figs. 299, left, 300, middle, 301, right. These resonance curves show correct alignment of a symmetrical circuit as to frequency and band width (Fig, 299), alignment at the wrong frequency, (Fig. 300), and alignment at the correct peak but improper response over the band, (Fig. 301).

indicate correct alignment of a symmetrical response circuit as to frequency and band width, alignment at the wrong frequency, and alignment at the correct peak, but improper response over the band, respectively. Additional adjustment of the circuit under test, without any change of the frequency modulator setting, resulted in the correct curve of figure 299. The correct mean frequency is indicated in figure 300, by the point where the two curves, representative of frequency modulation in the two directions, cross each other. It is evident from figure 301 that it is possible for the peaks to coincide, but not the bases. Under certain conditions, as for example the presence of regeneration, it may be impossible to get the two bases to coincide, although the two peaks will fall atop each other. Minimization or elimination of the regeneration in the circuit is the only means of securing the required single pattern.

Under certain conditions of regeneration, the circuit may go into oscillation only when the signal frequency is being swept through in one direction, so that the pattern indicative of regeneration appears upon one side of the curve. When this is the case, the peak and one side of the two bases will coincide, whereas the other side will not coincide. This is shown in figure 302. The corrected curve with

Fig. 302, left, 303, right. The lack of symmetry in the resonance curve in Fig. 302 is due to regeneration. See Fig. 303 for the corrected curve.

minimum regeneration is shown in figure 303. Note the difference in frequency band pass of the two adjustments, bearing in mind that both were made with the same frequency modulated sweep. The two amplitudes are slightly out of proportion. Actually, that of figure 303 was smaller than that shown for 302. In order to present a satisfactory picture, we increased the vertical amplitude. A slight discrepancy in coincidence between the two bases on the right side of the curve of figure 303 is still evident.

A different effect, caused by regeneration, is shown in figure 305. The correct curve, showing the absence of regeneration, is shown in figure 304. The same circuit with a definite amount of regeneration, established the curve above in figure 305. Note that the bases coincide, whereas the peaks do not, so that in this instance the condition of alignment was changed in such manner that the resonant frequency was changed.

Another example of incorrect alignment, using the double image system and working upon closely coupled circuits productive of double peaked curves, is shown in figure 306. The bases coincide perfectly, but the peaks in the curve are not correctly established. It is necessary to equalize the two peaks so that they are of the same

amplitude. This is done by adjusting the trimmers. Regeneration in a circuit will make such equalization virtually impossible. When the bases of asymmetrical response curves coincide, the mean fre-

Figs. 304, left, 305, right. The correct resonance curve is shown in Fig. 304. In Fig. 305 the bases coincide, but the peaks do not, this being due to regeneration.

quency is indicated or spotted upon the pattern or image by the point where the two curves cross each other. A pattern of this type indicates that the response of the circuit is better to frequencies one side of the mean frequency than to frequencies on the other side of the mean frequency.

If the frequency band, which may be passed through a system, is greater than the frequency sweep, the pattern appearing upon the screen will not be complete. The base of the curve will be absent. Examine figures 307 and 308. In figure 307, the modulated frequency band sweep is about 6 kc. each side of the mean frequency,

Figs. 306, left, 307, middle, 308, right. The resonance curve of Fig. 306 indicates incorrect alignment. Note that the bases of the two patterns coincide, but not the peaks. The incomplete curve in Fig. 307 was caused by the frequency band of the circuit being greater than the signal frequency sweep.

' The corrected condition is shown in Fig. 308.

Figs. 306, left, 307, middle, 308, right. The resonance curve of Fig. 306 indicates incorrect alignment. Note that the bases of the two patterns coincide, but not the peaks. The incomplete curve in Fig. 307 was caused by the frequency band of the circuit being greater than the signal frequency sweep.

' The corrected condition is shown in Fig. 308.

whereas the circuit being checked, is capable of passing about 9 or 10 kc. each side of the mean frequency. The result is an uncompleted resonance curve, because at no time within the modulated frequency signal band does thé circuit reject the frequency. The same system, but fed by a frequency modulated voltage which covered about 15 kc. each side of the mean frequency, developed a completed pattern. This is shown in figure 308.

The absence of a completed pattern due to insufficient signal frequency sweep is just as readily applicable to the single image system, as to the double image system. This also applies to those units which furnish constant band width frequency modulated signals, because the completion of the pattern depends upon the operating characteristics of the resonant circuit or system, rather than upon the characteristics of the oscillator which supplies the frequency modulated signal.

Synchronization of Double Image Pattern

The degree of synchronization has a great effect upon the appearance of the finished pattern or image. This is so irrespective of the state of alignment of the circuit under test. By this we mean that the shape of the pattern is influenced by the adjustment of the synchronizing voltage control for both symmetrical and asymmetrical circuits. Figure 309 indicates the pattern for an asymmetrical response circuit.

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