Frequency Control with Quartz Crystals
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Engineering Bulletin E-6: Frequency Control with Quartz Crystals |
Any previously calibrated frequency determining instrument is a secondary standard of frequency. Through common usage, however, secondary standards are considered to be crystal controlled oscillators of high stability employed for frequency measurements.
Secondary standards are used where the extreme precision and flexibility of the primary standard is not required and more simplified equipment is adequate. They have no provision for directly determining frequency and must be both calibrated and checked against some primary standard. When the fundamental frequency is appropriate, secondary standards can be checked directly against the transmissions of stations offering standard frequency services. The outstanding station of this type is WWV, the U. S. Bureau of Standards, which transmits on frequencies of 5000kc., 10,000kc. and 20,000kc. with an accuracy of better than 1 part in 5 million.
The primary standard, less the timing equipment, is a secondary standard. If the frequency stability need not be extremely high the constant temperature oven can be simplified or dispensed with entirely. The associated frequency measuring equipment can be complete for all types of measurements or abbreviated for specific applications. A simple 100kc. or 1000kc. crystal controlled oscillator in conjunction with a calibrated receiver or a calibrated frequency meter is often adequate and gives better accuracy than could be obtained with precision wavemeters. Whether the equipment is complete or reduced to essentials, measurements are wholly or partially made in accordance with one of the methods outlined in PRIMARY STANDARDS OF FREQUENCY.
The frequency monitors used in transmitting stations to check the operating frequency or frequencies are secondary standards. They are designed for one, or a group of, particular frequencies and the measurements are, therefore, considerably simplified. Some frequency monitors, especially those for use in broadcasting stations, are direct reading in terms of cycles per second deviation from the assigned value.
Secondary standards are useful in any application dealing with radio frequencies. With the increasing complexity of modern radio receiving equipment, radio servicemen find that the usual type of calibrated service oscillator is not sufficiently accurate for precision alignments. Through the use of a standard frequency oscillator in conjunction with the service oscillator, frequency accuracy of alignments can be greatly increased and better receiver performance assured. Harmonics of the standard can be directly employed for accurately checking dial calibrations since there will be a series of harmonics over each band at a frequency spacing equal to the fundamental frequency of the oscillator. Up to about 4000kc. a fundamental frequency of 100kc. is excellent while a 1000kc. fundamental is to be preferred for the higher frequencies.
For alignment of the intermediate frequency stages, the service oscillator is set to the intermediate frequency by interpolation between 100kc. harmonics. This is best performed by picking up a harmonic of the service oscillator in the broadcast band of any suitable receiver. Suppose the intermediate frequency is 460kc. Set the service oscillator to 460kc. by its calibrated dial and pick up the second harmonic (920kc.) in a receiver. Note the dial settings for two adjacent 100kc. harmonics (900kc. and 1000kc.) and, by interpolation, determine the correct dial setting for 920kc. Then set the oscillator such that its second harmonic falls at the dial setting corresponding to 920kc. Or, beat the output (2nd harmonic) of the service oscillator with the 900kc. and 1000kc. standard harmonics in a receiver and note the oscillator dial settings. By interpolation, the correct oscillator dial setting for 460kc. can be calculated. For better accuracy, a 10kc. multivibrator may be employed. This, however, is generally unnecessary. If the intermediate frequency is such that one of its harmonics falls at an even 100kc. (such as 450kc. X 2 = 900kc.), interpolation will be unnecessary because the oscillator harmonic can then be set to zero beat with the proper standard frequency.
The Bliley type SMC100 crystal unit was designed especially for service work. It contains a specially ground crystal which will oscillate at either 100kc. or 1000kc. and, in a simple inexpensive circuit, gives dependable accuracy. The circuit and recommended values are shown in figure 20. (Footnote 9)
Footnote 9:
Refer to Engineering Bulletin E-7 for complete details on construction and application.
Amateurs will find a 100kc. secondary standard to be a most valuable instrument for locating the edges of the bands and subdividing them into 100kc. points. Any amateur expecting to operate close to the edge of a frequency band should, by all means, have a method of accurately checking frequency to make certain that operation is within the legal requirements. The secondary standard can be easily and economically constructed with a Bliley SOC100, SOC10OX or SMC100 (Footnote 9) 100kc. Standard Frequency Crystal Unit.
The type SOC100 crystal unit is well suited for primary or secondary standards of frequency for it incorporates a low temperature-coefficient bar-type crystal mounted between knife edges. The crystal is calibrated for use in the Colpitt's Circuit shown in figure 16 and discussed in LOW FREQUENCY OSCILLATORS. To insure best performance and accuracy, a correctly designed tank coil (L) is an integral part of the unit. The capacity C should be a dual 350 mmf. tuning condenser. The output at 100kc. is approximately 1.5 volts R.M.S. and the harmonics will be usable up to the 30th or greater, depending on the sensitivity of the receiving equipment employed.
For greater output and higher harmonics from the secondary standard, one or two untuned amplifier stages should follow the oscillator. These, as shown in figure 21, are simply resistance-coupled amplifiers with r.f. chokes in series with the plate and grid-coupling resistors, and biased to give a distorted output. The r.f. chokes cause the amplifier gain to increase somewhat with frequency thereby accentuating the higher harmonics. Either triode or pentode tubes May be used although pentodes provide the greatest gain and harmonic output. The circuit values are not critical but are best adjusted by trial for greatest output at the highest harmonic desired. For increasing the output at any given harmonic, or harmonics, the plate circuit of the amplifier can be tuned.
The following references are suggested as possible sources of helpful information covering the construction of secondary standards:
