ECA-CB6-A-1987.pdf
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1、EIA COMPONENTS BULLETIN Guide for the Use of Quartz Crystal Units for Frequency CntroI CB6-A (Revision of CB8) OCTOBER 1987 ELECTRONIC INDUSTRIES ASSOCIATION ENGINEERING DEPARTMENT Copyright Electronic Components, Assemblies i.e., are said to be “coupled“. the motional parameters which are no longer
2、 independent of frequency and Figure 1 does not accurately represent the crystal unit. In this case the equations and measuring methods normally used do not apply. The validity of the circuit representation can be determined by measuring and plotting the impedance or admittance of the crystal unit a
3、s a function of frequency. of some The presence of a coupled mode causes large variations in Copyright Electronic Components, Assemblies i.e., its frequency changes little with variations of the input impedance, and vice versa. Co/Ci. Some crystal units exhibit large increases in the value of R. at
4、1 Copyright Electronic Components, Assemblies The increase of Ri is usually accompanied by perturbations of the frequency. The phenomenon is termed “an activity dip“. It is caused by coupling between two modes of vibration which happen to have the same frequency at a certain the unit may even fail t
5、o operate. temperature. The problem is especially acute in units in which the ratio of the lateral dimensions of the blank to its thickness is too low, Since the thickness is determined by the frequency and the lateral dimension by the holder dimensions, it is sometimes difficult, or even impossible
6、, to fabricate low frequency units in small holders. The customer should consult the crystal unit supplier before designing around units in which the diameter/thickness ratio is less than about 20. If low frequency is required one should consider going to the Tuning Fork or Strip AT design to avoid
7、the above problem. It will be readily understood in view of the comments made so far, that it is in the interests of manufacturers and users equally, to decide upon a suitable design of crystal unit in terms of the users requirements, using data sheets as a basis. Questions of availability of crysta
8、l units to the chosen specifications may modify, to some extent, the users original requirements. 2. - CRYSTAL HOLDER CLASSIFICATION Some or all of the following information will normally be given Ort the crystal unit: - Frequency - Manufacturers name and c3de - EIA code and/or manufactuiers catalog
9、 or type number - Any other information necessary to obtain a coqdete definition of the crystal unit - At the option of the manufacturer or by special request by the purchaser, a serial number and date of manufacture may be added - Date code (week and year) - Users part number Copyright Electronic C
10、omponents, Assemblies e.g., limited production of specialized types, non-standard holders have to be used. In these cases, individual agreement will have t o be reached between customer and manufacturer. Copyright Electronic Components, Assemblies there will generally, of course, be the normal range
11、 of harmonics generated in the oscillator circuit at 96, 144 MHz, etc. d) The properties of overtone crystal units as regards frequency vs. temperature and other characteristics can be suited to most requirements by correct design; however, the properties of a given crystal for a given order of over
12、tone are quite different from those for its fundamental Copyright Electronic Components, Assemblies therefore, when specifying the temperature range within which the crystal unit must maintain a certain frequency tolerance, due allowance must be made for internal equipment heating. The crystal unit
13、manufacturer prefers a frequency specification of form the I+/- 100 ppm over the temperature range -40 OC to +70 stated load capacitance and drive level conditions. In the interests of customer and manufacturer alike, standard temperature ranges, frequency tolerance and input conditions should be us
14、ed in all cases. Standard temperature ranges, frequency tolerances, etc, may be found in IEC Publications 122-1. It should be clearly understood that it is not practical to combine wide temperature ranges with very small frequency tolerances. under 5. _ FREQIJEBCY STABILITY RELATED TO THE CONDITIONS
15、 OF USING THE CRYSTAL UNIT 5.1 FACTORS AFFECTING THE FREQUENCY-STABILITY In an oscillator whose frequency is determined principally by a it must be understood that the circuit and conditions quartz crystal unit, of operation have effects on the oscillation frequency. Although a crystal unit is adjus
16、ted during manuiacture to a nominal frequency, the conditions under which this frequency is.actually obtained Copyright Electronic Components, Assemblies e.g. under these conditions the circuit will oscillate when the denominator A = Ab/(l-Ao(Bp - Bn) reaches zero, and the gain becomes infinite. Thi
17、s occurs when Ao( - Bn) = 1 Referring to Figure 12, the positive feedback is produced by the voltage divider comprised of C2and Ci while negative feedback is due to the divider comprising RI and the crystal resistance Ri This leads to the definition for Bp and Bn. Ri and Bn = cf g P = The frequency
18、at which the phase shift in the system is zero, defines the frequency of oscillation, This can be modified by the use of loading capacitor CL in the same manner as described previously, but also by Cx which has the opposite effect of rec!ucing the operating frequency. Copyright Electronic Components
19、, Assemblies i.e., with only one FET on at a time. The average current will then be proportional to the operating frequency and the transit time ratings. Since the resonator, and its associated capacitors, as shown in Figure constitute a narrow bandpass, positive reactance circuit, the inclusion 13,
20、 of resistor RI allows the square wave output from the inverter to be converter into a since wave at the capacitors terminals. RI also controls the loop gain and should be adjusted to as high a value as possible, consistent with power consumption and frequency stability. The circuit of Figure 14 is
21、much more stable due to its bridge configuration. It requires an amplifier with a differential input. These are available in bipolar IC packages intended for RF and video amplifier applications. The series-resonant mode of operation was described in section 5.2.4 where an L-C tank was used to provid
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- ECA CB6 1987
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