Frequency Control with Quartz Crystals
|
Engineering Bulletin E-6: Frequency Control with Quartz Crystals |
The multivibrator is an oscillating system having special advantages applicable in frequency measuring equipment. It is, essentially, a two stage resistance-coupled amplifier with the output circuit coupled back to the input such that the feedback causes the amplifier to oscillate (motorboat) at a frequency determined by the time constants of the resistance-capacity combinations in the circuit. Because the oscillations are brought about by the charging and discharging of condensers through resistance, the waveform of the oscillatory currents is irregular and distorted as compared to a sine wave. This means that the output voltage must be rich in harmonics.
Operating by itself, the multivibrator possesses no particular advantages because the circuit action is unstable and the output contains no usable frequencies. The performance changes, however, when a small voltage from a stable oscillator is injected into the circuit. If the frequency of the oscillator is made approximately equal to the natural frequency of the multivibrator, the injected voltage assumes control and brings about stable performance. The multivibrator frequency is then dependent on the controlling voltage and is independent of small changes in circuit values. When so stabilized, the multivibrator becomes a useful instrument in that it serves as an excellent harmonic generator.
The most important property of the multivibrator is that synchronization can be brought about when the frequency of the controlling voltage is harmonically related to the natural circuit frequency. In this way, the device may be employed for frequency division or multiplication. Its application is, however, usually limited to frequency division and harmonic generation since there are preferable multiplying arrangements.
Because of its stability when synchronized, and the fact that the frequency is determined by the controlling voltage, the multivibrator is widely used to produce a series of standard frequencies from a single crystal controlled oscillator. Frequency division can be carried out to a ratio of about forty to one but, for assured stability, it is best limited to a factor of 10. Where a total division factor greater than 10 is required, a number of multivibrators can be operated in cascade. If the total division ratio is large, the factor per stage should best be limited to 5, or less. This is to insure positive locking-in of each stage every time the device is placed in operation; the necessity for a frequent stage-by-stage checkup to trace incorrect locking is inconvenient as well as undesirable.
Figure 22 illustrates a representative multivibrator circuit. The tubes can be standard triodes such as the 27, 56, 37 or 6C5 or, for simplicity, twin-triodes such as the 6N7, 53 or 6A6. The grid resistors R1, R2, and the coupling condensers C1, C2, are the major frequency determining elements. There is no simple formula which will give the exact values but the approximation, F = 1000/(R1C1 + R2C2) is sufficient for practical purposes. F is the frequency in kilocycles per second, R the resistance in ohms and C the capacity in microfarads. For purposes of simplification, it is Usual practice to choose R1 = R2 and C1 = C2.
R1, in the diagram, is shown as a potentiometer and a fixed resistor in series. The potentiometer, which has a value of 5000 ohms, offers a simple method of injecting the controlling voltage and regulating its value. In the formula given R1 should be the total value of the two resistances in series.
Other than grid circuit injection as illustrated, the controlling voltage can be inserted in the plate circuit. A common method for accomplishing this is to include a portion of either, or both, plate resistors in the output circuit of the driving source. The resistor, common to both the driver and the multivibrator, provides the necessary coupling. Naturally, a bypass condenser should not appear at the coupling point for such would decrease, or possibly destroy, the function of the coupling resistor. An equivalent alternate method for injection is to magnetically couple a coil in the driver output to a coil in series with either one, or both, of the multivibrator plate resistors.
| 50kc. | 10kc. | |
| R1 | 10,000 ohms (total) | 25,000 ohms (total) |
| R2 | 10,000 ohms | 25,000 ohms |
| C1 | 750-1500 mmf. | 1000-3000 mmf. |
| C2 | 750-1500 mmf. | 1000-3000 mmf. |
| R3 | 20,00025,000 ohms | 20,00025,000 ohms |
| R4 | 200,000250,000 ohms | 200,000250,000 ohms |
There is a difference in the controlling action of the injected voltage depending on whether the frequency ratio is an odd or even number. This is due to the phase relationship between the injected voltage and the multivibrator oscillations. In general, a multivibrator, which has symmetrical components and is symmetrically fed (control voltage applied equally to both tubes), has the strongest tendency for operation at even ratios. If the symmetry is destroyed, the circuit functions better at odd ratios. The symmetry can be disturbed by choosing unequal circuit values for the two tubes or by the simple expedient of injecting the control voltage into the circuit of only one tube. This suggests the use of a snap switch for connecting the control voltage optionally to one or both of the tubes where a single multivibrator is intended to work at more than one frequency. For practical purposes, however, satisfactory control at either odd or even ratios can be obtained by choosing the value of one plate resistor 10 to 50 times greater than the other.
Either the grid-coupling condensers or the grid-coupling resistors should be made variable so that the multivibrator can be adjusted to the correct frequency. The use of variable condensers is most practical and these are shown in the diagram. Should it be inconvenient to obtain adjustable mica condensers of the proper capacities, the largest available sizes may be used in parallel with appropriate fixed condensers.
To adjust the multivibrator, the input controlling voltage is reduced to zero and the condensers C1 and C2 simultaneously varied until the fundamental frequency is very close to the desired value. As the capacities are increased the frequency will decrease, while a decrease in capacity will cause the frequency to increase. The most convenient method to check the frequency is to couple the output to a radio receiver and estimate the frequency difference between the harmonics (the harmonics will be quite "rough" but discernible). As an aid in rapidly determining the frequency, the dial settings for two adjacent harmonics of the standard oscillator may be used as marker points. The multivibrator frequency can then be determined by counting the number of harmonics which appear between these points. For instance, if the crystal oscillator is at 100 kc. and the desired multivibrator frequency is 10 kc., there should be 9 multivibrator harmonics between any two adjacent 100 kc. harmonics. If there are less than 9, the frequency is too high whereas more than 9 indicates that the frequency is too low. When the multivibrator is operating at 10 kc., there will be a harmonic at each 100 kc. point and 9 in between.
After the multivibrator has been adjusted to the correct frequency, a small voltage from the crystal oscillator should be injected into the circuit. As the voltage input is increased, a point will be noticed at which the multivibrator becomes stable and the output voltage resolves into definite frequencies. For best performance, the input should be increased slightly beyond that point. An excessive increase, however, will cause the multivibrator to jump to another frequency; when the crystal oscillator is assuming full control, the variable circuit elements can be varied appreciably without loss of synchronization. The frequency hop which can occur when the controlling voltage is excessively raised is the result of the fact that anything Which is done to retard or accelerate the condenser 1 charge or discharge time has the same effect as altering the value of the condensers.
The design of a multivibrator is not complicated and, as far as results are concerned, requires less engineering than an equivalent resistance-coupled amplifier. In construction, only high quality stable condensers and resistors should be employed. To keep stray capacities and inductances at low unvarying values, all interconnections should be short, rigid and isolated from ground. The use of number 12 radio buswire is recommended. The power supply should have good regulation inasmuch as the multivibrator frequency can be caused to change by varying the applied d.c. voltage just as it can be altered by varying the injected voltage or the values of the resistance-capacity combinations.
Although not a necessity, it is best practice to employ a resistance-coupled input and output amplifier stage with each multivibrator. The input amplifier serves to decouple the oscillator and prevents circuit reactions from influencing the frequency. The output amplifier protects the multivibrator in the same manner and also increases the output. Radio-frequency chokes should be connected in series with the grid and plate-coupling resistors of the output amplifier to bring about accentuation of the higher harmonics. These are shown in figure 21 and the circuit is discussed in SECONDARY STANDARDS OF FREQUENCY.
