Frequency Control with Quartz Crystals
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Engineering Bulletin E-6: Frequency Control with Quartz Crystals |
The term "activity" is usually employed in describing, or comparing, the oscillating qualities of crystals. The general interpretation of the term is somewhat vague, however, because there has been no specific definition commonly adopted for it.
Activity is, in the broad sense, the ability of a crystal to oscillate. It is controlled by the type of cut, the frequency, the precision of grinding, and the method of mounting. For a given cut, frequency, and holder of good design, the ability to oscillate is dependent on proper grinding. As would be expected, the power output of a given test oscillator will vary widely between crystals of the same frequency unless special efforts are made to grind the crystals with respect to some standard. Originally, crystal activity was determined by comparing the power output, or the oscillator d.c. grid current, of various crystals in a test oscillator. Crystals showing relatively high power outputs had, on this basis, a high activity.
A power output or d.c. grid current test is not wholly sufficient; an important consideration is whether the crystals will be positive in starting under load. If a group of "active" crystals of approximately the same frequency is checked in a loaded keyed oscillator, some of the crystals may accurately follow the keying while others may lag behind or refuse to follow at all. The activity of a crystal is most closely associated with its ability to start rapidly and Bliley Engineers, therefore, have adopted a definition which includes both power output and keying ability. That definition is: Activity is the ability of a crystal to start rapidly and to accurately follow keying in a loaded test oscillator at a given degree of loading.
Activity, when comparing crystals of essentially identical frequencies, is a measure of the effective crystal Q; the higher the activity, the higher the Q. To the engineer and amateur, high activity means high frequency stability.
It is impossible to express activity as an exact mathematical quantity because it is only a comparative quality. Of course, activity could be specified by a statement of the minimum keying speed at which a crystal will accurately follow the characters in a definite test oscillator with a given loading. This, however, is significant only in that particular test circuit since the characteristics of oscillator circuits vary. As a manufacturing standard, however, the keyed loaded test oscillator is a valuable instrument for maintaining high standards of uniformity and activity. Such instruments are used regularly in the manufacture of Bliley Crystal Units.
The proper operating conditions for a crystal controlled oscillator are determined by the relative activity of the crystals to be used. A crystal, having a low activity for its particular frequency, Can be made to oscillate by adjusting the oscillator voltages, the grid bias, and the circuit feedback for conditions of maintained oscillation. The frequency stability will, however, be relatively poor and the crystal may be sluggish in starting and following characters when the oscillator is keyed. Should a highly active crystal of approximately the same frequency be substituted, without any circuit changes, the chances are that the crystal would oscillate so vigorously as to shatter itself. This is simply due to the fact that the active crystal is more easily excited.
Obviously, a relatively inactive crystal will withstand considerably more abuse than a highly active crystal. This, on the surface, might seem to indicate that low activity is desirable. Such a premise is most incorrect. With proper operating conditions, the active crystal will follow keying more faithfully, it will provide much better frequency stability and will give equal, or better, power output at a higher circuit efficiency.
The relative activity of quartz crystals varies with frequency over the practical frequency range from 16kc. to 30,000kc. At 16kc. the activity is lowest while maximum activity occurs at about 3000 kc. Bar-type crystals, which are used in the frequency range from 16kc. to 150kc., are relatively sluggish in starting and can be used only in low powered oscillator circuits. This is largely due to the mass of the crystals because their Q remains high (6000 to 18,000). At about 6000 kc. the apparent activity starts to fall off due partly to the characteristics of the crystals themselves and partly to the increasing circuit and tube losses as the frequency is raised.
It is always best practice to take precautions when first connecting a crystal, known to have a high activity, into a circuit which might cause excessive excitation. This is particularly true where new or experimental circuits are being tested.
Under such conditions, the comments given in the section GENERAL OPERATING NOTES should be followed.
