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Electronics-World-1959-05 M An assortment of crystals of random values can be combined to yield an amazing range of accurate signal frequencies. QUARTZ CRYSTAL oscillators are quite familiar to technicians in all branches of electronics. Two limitations, however, have operated against their more general use as signal generators, either for the service shop or in the laboratory. These limitations are the relatively high cost of individual crystals ground to desired frequencies and, as a result, the inconvenience of being limited to just a few fundamental frequencies and their harmonics. Thus, despite such desirable features as constancy of frequency and stability of amplitude, crystal oscillator applications have been restricted. Since World War II, the surplus market has offered "grab-bag" lots of surplus crystals at unusually low prices. Unfortunately, few of these are ground to frequencies directly usable in either radio and TV service or in laboratory apparatus. The method described here permits many of these inexpensive crystals to be used for any service function whatsoever: as markers for calibration of variable -frequency generators or even directly for circuit alignment. The idea of this apparatus was conceived while using a conventional crystal oscillator with a number of surplus crystals to check the dial calibration of a new signal generator. This procedure worked well, as far as it could be applied. None of the crystals in the assortment had fundamentals be- Generating Test Signals with Crystals THOMAS DEANE HERRIMAN low 3000 kc. and the ranges of the generator extended from 75 kc. to 150 mc. Of course, the higher ranges could be checked with harmonics, but what about the broadcast and i.f. bands? While searching the crystal bank, a pair was found having marked frequencies of 4490 kc. and 4495 kc. Curiosity prompted the construction of a second crystal oscillator in order to determine whether the audio beat would be heard. A crystal -diode probe and a pair of headphones revealed the presence of the 5000 -cps tone with considerable amplitude. This incident suggested the solution to the problem of calibrating the low- frequency ranges. Heterodynes were obtainable from pairs of crystals at fairly close intervals from below 75 kc., through the i.f. and broadcast bands, and well above! Most of the broadcast band could be checked at intervals of 20 kc. or even closer. Twin -Crystal Oscillator Fig. 1 shows the very simple circuit ultimately adopted for the twin oscillator. A separate mixer proved to be unnecessary; the common 7500 -ohm cathode resistor provides adequate mixing. The optional output amplifier, V,, is helpful only in case audio or very low radio- frequency heterodynes are to be used. It will contribute little gain at high frequencies and the wide range to be covered precludes the use 66 ELECTRONICS WORLD of tuned amplifiers if the apparatus is to be kept simple and flexible. The circuit is uncritical as to tube types, component values, operating voltages, etc. Component values shown in Fig. 1 are suitable for medium -mu triodes like the 6J5. However. a v.t.v.m. can be used to check output - signal amplitude while varying component values to secure optimum performance. For best frequency stability the "B" supply should be regulated (100 to 150 volts). Because of the lack of critical features and components in the circuit and recognition of differences in personal preference, explicit construction details have been deliberately omitted. The crystals should, of course, he mounted where they will not be subjected to heating. Other variations are possible: pentodes might be used in place of triodes, ultra -compactness could be achieved with miniature tubes and a selenium- rectifier d.c. supply or by transistorization, and so on. An alternative method of signal take -off is illustrated in Fig. 2. As to potential signal availability. consider the following: A single- crystal oscillator can give but one fundamental, plus its harmonics. Yet two crystals, used as described will give four frequencies, without counting any harmonics. These frequencies are F. and F., the frequencies of the individual crystals; F, plus F,; and F; minus F,. Three crystals. from which three distinct pairs can he made up. will give a total of nine separate signals. In fact, the number of frequencies obtainable from any number, N, of individual fundamentals, combined in different pairs, is N'. Also available are the harmonics of the individual units, harmonics of the heterodynes, and heterodynes of the harmonics, such as 2F, minus 3F,, etc. Avoiding Confusion This multiplicity of simultaneously available frequencies may seem confusing and may cause some apprehension lest an undesired signal be mistaken for the desired in use. Actually, it is not difficult to avoid errors. The untuned Pierce circuit, although giving an output rich in harmonics, produces a much greater fundamental amplitude even when using "harmonic" crystals. Consequently, the heterodynes of the fundamentals will have higher amplitudes than the harmonics of the heterodynes or the heterodynes of the harmonics. The operator will usually know the approximate frequency of operation of the circuit being measured, whether it be a receiver or a generator. Hence. a quick pencil- and -paper calculation will reveal whether any confusing difference frequencies will be generated that are close enough to the desired signal to be mistaken for it. (That is, subtract the fundamental of each crystal from the harmonic of the other; also subtract the second harmonic of each from the third and fourth of the other, etc.) If any spurious frequencies are May, 1959 too close, a different pair of fundamental crystals may be selected to avoid errors. The use of harmonics of the heterodynes and heterodynes of harmonics is not recommended unless calculation shows that they may be used without danger of confusion with other simultaneously produced signals. Although the harmonics of sum heterodynes are spread somewhat farther apart, their use is subject (to a lesser degree) to the same limitation as that of the difference heterodynes. As an example, 18 mc. could be produced by the second harmonics of the sum of a 4 -mc. crystal and a 5 -mc. crystal. However, if the crystals gave considerable 3rd -harmonic amplitude, the fundamental of each could beat with the 3rd harmonic of the other to produce 17 mc. and 19 mc. simultaneously. 14+ 15 =19; 5 +12 =17.1 Such a possibility should be checked and investigated before use. There are also certain combinations of fundamentals that should be avoid- ed. These occur where one of the fundamentals would he close to the desired heterodyne. For example, a 9 -mc. heterodyne could be obtained with a 10 -mc. and a 1 -mc. crystal. However, the output would also include 10 and 11 mc., either of which might be confused with the desired signal. A much better combination for producing 9 mc. might he 4 mc. plus 5 mc., y d 20K Fig. 1. A simp e twin oscillator. Output capacitor may be .001 yid. Fig. 2. An alternate method for taking off heterodyne oscillator output. s Fig. 3. Using the circuit to determine frequency of an unmarked crystal. vARIABLE- FREOVENCY GENERATOR CRYSTAL HE TERODYNE GENERATOR CR VS TAL DIODE PHONES OR A F AMPLIFIER or any other comparab'e combination of available crystals. With the latter, no fundamental (or other strong beat) would he close to the desired frequency. When use of a "close" combination is unavoidable. a careful search should be made to find all output frequencies. The desired one may be identified by its location relative to the others in their fixed sequence. Using harmonics of crystal heterodynes is not especially recommended. However, the harmonics of a single crystal in conjunction with a generator is useful, to a point. Most crystals have useful harmonics at least to the 5th or 7th, often much higher. (Note that some are cut for better output on odd than even harmonics.) It is questionable whether harmonics beyond the 10th should be used, since it becomes increasingly easy to skip one harmonic after that point and such an error will result in incorrect identification of all subsequent overtones. Frequency Errors A word of caution is in order: when the plate voltage in a crystal oscillator is much higher than necessary, some crystals may have a tendency to produce a spurious fundamental several hundred kilocycles from the true fundamental. While this phenomenon can destroy the desired accuracy, it is fortunately easy to detect and correct. A slight reduction of plate voltage will render this spurious output very weak or eliminate it altogether. The true fundamental, when reached, will remain constant with further changes in voltage. The obvious remedy is using no more "B +" than is necessary. Some readers may be wondering how accuracy of the heterodyned signals compares with output from single crystals. This accuracy may he as good as, poorer than, or better than that obtained with individual units. Since the user probably does not have facilities for measuring error, his starting point must be the rated tolerance of the crystals themselves. The possibility of the greatest percent of error exists when two units, relatively close together and high in frequency, are used to generate a relatively low (audio or low r.f.) difference signal. Consider the very first case discussed in this article, in which a 5000 -cps tone was developed from crystals rated at 4490 and 4495 kc. If both had rated tolerances of .01''',, their individual errors could have been as much as 449 cps and 449.5 cps, respectively. If these errors should occur in opposite directions, the error frequency output would be the sum of the original error frequencies. This would be 898.5 cps- nearly 18'4, of 5000 cps! Such an error would, fortunately, be fairly easy to detect. Also, if the errors occurred in the same direction I both crystals above or both below rated frequency), they would cancel out in the heterodyned output resulting in the original per -cent of error -.01 %, or .5 cps at 5000 cps. (Continued on page 111) 67

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