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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Quartz generators. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Knots of amateur radio equipment. Generators, heterodynes The relative instability of the frequency of self-oscillators, performed on resonators in the form of LC circuits, is usually not lower than 10-3... 10-4. The frequency stability of the generator depends significantly on the quality factor and the stability of the oscillatory system. The quality factor of an LC circuit is usually not higher than 200 ... 300. Modern radio transmitters and receivers are subject to higher requirements for frequency stability. Usually a long-term relative frequency instability of at least 10-6... 10-8, which can be achieved by using quartz resonators. The quality factor of quartz resonators is many times higher than the quality factor of resonators on LC circuits and is 104... 106. There are many circuits of quartz oscillators. Therefore, it became necessary to consider the most commonly used schemes in practice. The generally accepted equivalent circuit of a quartz resonator is shown in Fig. 1. Dynamic inductance Ls, dynamic capacitance Cs and loss resistance Rs are due to the presence of direct and inverse piezoelectric effect and resonant properties of the piezoelectric element. The parallel capacitance Cp is due to the interelectrode capacitance of the piezoelectric, the capacitance of the case and mounting. The resonant frequency of the dynamic branch is called the series resonance frequency of the quartz resonator Fs.
The quality factor of a quartz resonator Q is determined by the dynamic branch in accordance with the formula for a series oscillatory circuit Q=(2pFsLs)/Rs The frequency of the parallel resonance Fp is somewhat higher than Fs, which is due to the parallel resonance of Cp, Cs and Ls. An important parameter of a quartz resonator is the ratio of its parallel to dynamic capacitance, denoted by r and called the capacitive coefficient r=Cc/Cs According to various literary sources, the capacitive coefficient for the AT-cut of quartz is 220...250. Considering that Cs/Cp<0,1, one can use an approximate expression for the parallel resonance frequency Fp=Fs(1+(Cs/2Cp)). For a capacitive coefficient r > 25, the resonant interval, defined as the difference between the frequencies of parallel and series resonances of a quartz resonator, can be written as dF=Fs/2r. At the mechanical harmonics of a quartz resonator, the resonant interval decreases and is determined by the expression dFn=Fs/(2rn2) where n is the harmonic number. The capacitive coefficient determines the size of the resonant gap of the resonator, therefore, the frequency deviation of the controlled quartz oscillator, the frequency stability when the circuit parameters change, the conditions for the occurrence and maintenance of oscillations in the quartz self-oscillator circuit. To evaluate the ability of a quartz resonator to be excited, some crystal oscillator circuits use a parameter called quality factor. It is defined as the ratio of the quality factor of the resonator to its capacitive coefficient m=Q/r. For quartz resonators, the values of M lie in the range from 1 to 10000. At M<2, the reactance of the resonator turns out to be positive (capacitive) and does not have an inductive reaction region. Consequently, the excitation of such a resonator in the circuits of quartz oscillators that require an inductive reaction becomes impossible. When M>2, the resonator has an area of inductive reaction, and the greater the value of M, the wider this area. In practice, two types of quartz oscillators are most widely used: a) generators in which a quartz resonator is part of an oscillatory circuit and is equivalent to an inductance; b) generators in which a quartz resonator is included in the feedback circuit is used as a narrow-band filter and is equivalent to active resistance. Crystal oscillators, in which a quartz resonator is used as an element of a circuit with inductive reactions, are called oscillator, and oscillators in which a quartz resonator is included in the feedback circuit are called series resonance generators. The oscillator circuit of a quartz oscillator with quartz between the collector and the base, made according to the circuit with a grounded emitter (capacitive three-point) is shown in Fig. 2.
At present, the capacitive three-point is widely used in the frequency range up to 22 MHz when the resonator operates at the fundamental frequency, and up to 66 MHz when excited at the third mechanical harmonic (Fig. 3). The self-oscillator with a quartz resonator between the collector and the base in a circuit with a high-frequency grounded emitter, is not prone to parasitic oscillations on anharmonic overtones, has excellent frequency stability with changes in supply voltage and ambient temperature.
The influence of changes in the reactive parameters of the transistor, depending on the supply voltage and time, weakens with an increase in capacitances C1, C3 (Fig. 2), i.e. as the operating frequency of the oscillator approaches Fg. However, an excessive increase in capacitance leads to a deterioration in the conditions of self-excitation. On the other hand, with an increase in capacitances, the power dissipated in the resonator increases, which leads to an increase in the instability of the generated frequency. According to the specifications, the dissipated power on quartz is limited to 1 ... 2 mW. However, in the frequency range of 1 ... 22 MHz with such dissipated power, the series resonance frequency depends on the dissipated power, and the proportionality factor is (0,5 ... 2) • 10-9 Hz / μW, therefore, for highly stable generators, the dissipated power is resonator should be limited to 0,1 ... 0,2 mW. In practice, it is recommended to choose capacitances C1, C3 so that the generation frequency is no more than a quarter of the resonant interval from Fs. When the quartz resonator is excited on the odd mechanical harmonics of quartz, instead of the resistor R3, an inductor Lk is switched on (Fig. 3). At the generation frequency, the Lk-C4 circuit must have a capacitance, i.e. its resonant frequency must be below the generation frequency. The circuit parameters should be chosen so that its natural frequency is 0,7 ... 0,8 of the generation frequency. As a result, the circuit has capacitive conduction at the frequency of the required harmonic, which eliminates the possibility of generation at lower harmonics and the fundamental frequency. In oscillator generators operating at frequencies above 22 MHz, the resonator is usually excited at the 3rd or 5th harmonic, but not at higher ones, since the effect of parallel capacitance is strong. More often than shown in Fig. 2, a capacitive three-point circuit of a quartz oscillator with a quartz resonator between the collector and base is used in the circuit for switching on a transistor with a grounded collector (Fig. 4). This circuit is especially useful for electronically tunable oscillators (when connected in series with the varicap quartz), and has fewer blocking elements than the grounded emitter circuit. Many experts in the field of crystal oscillators consider the capacitive three-point to be the best of all crystal oscillator circuits operating on the fundamental or 3rd mechanical harmonic of the resonator. It should be noted that there is a capacitive three-point circuit that does not contain inductance, which is excited at the 3rd and 5th harmonics.
Auto-oscillator with quartz in the circuit. If the inductor L4 is connected in series with quartz in the circuit in Fig. 1, this will lead to the appearance of new properties, i.e. in the generator (Fig. 5), self-oscillations are possible that are not stabilized by a quartz resonator.
At high frequencies, where the reactance of the parallel capacitance of the resonator is less than the reactance of the dynamic branch of the quartz resonator, self-excitation through the parallel capacitance Cp is possible. The presence of inductance L1 means the possibility of performing phase balance at the frequency of the series resonance, as well as in a certain region of detunings below the frequency of the series resonance. The inductance L1 ensures the implementation of the phase balance in conditions where M<2, and the equivalent reactance of quartz cannot be inductive. This means that an oscillator with a quartz in the circuit can operate at higher frequencies and higher numbers of mechanical harmonics of a quartz resonator. To exclude parasitic self-excitation through a parallel capacitance Cp, which is most likely at high frequencies and higher mechanical harmonics, a resistor R1 is connected in parallel with the resonator, which introduces losses into the parasitic self-excitation circuit. It is possible to reduce the requirements for the activity of a quartz resonator at mechanical harmonics by using serial resonance generator circuits. Since with an increase in the frequency and harmonic number, the activity of the quartz resonator decreases due to an increase in its equivalent resistance and an increase in the shunting effect of the static (parallel) capacitance Ср, it is necessary to neutralize or compensate for it. Neutralization can be carried out in a bridge circuit, where quartz is placed in one of the arms of a balanced bridge. Bridge self-oscillator of series resonance. In the circuit shown in Fig. 6, with the exact balance of the bridge (Cp=C2, XL1-2=XL2-3), feedback is carried out only through the dynamic branch of the resonator. At the mechanical harmonic of the quartz resonator, the conductivity of the series branch of the resonator sharply increases, the bridge is unbalanced, and with an appropriate choice of circuit elements, the generator is excited. The L1-C3 loop must be tuned to the desired harmonic frequency.
In this scheme, it is possible to excite quartz resonators at the 5th or 7th harmonics. Schemes with neutralization of the static capacitance of the resonator are very critical to the operating mode and difficult to adjust, although they can be used at frequencies up to 100 MHz. The upper frequency limit of the oscillator with neutralization is due to the difficulty of obtaining a large equivalent loop resistance with increasing frequency, since the initial capacitance of the loop cannot be made small due to parasitic capacitances. The Butler scheme (Fig. 7) is characterized by the greatest resistance to destabilizing factors in the range up to 100 MHz. The upper limit of the generated frequencies is due to the deterioration of the properties of the emitter follower. In the Butler circuit, a quartz resonator is included in the feedback circuit between the emitters of the transistors. Transistor VT1 is connected according to the scheme with a common collector, and transistor VT2 - with a common base. The disadvantage of this circuit is the tendency to parasitic self-excitation due to the connection of the output with the input through a parallel quartz capacitance Cp. To eliminate this phenomenon, an inductor is connected in parallel to the quartz, forming, together with the parallel capacitance of the quartz, a resonant circuit tuned to the frequency of the parasitic oscillation.
Self-oscillator according to the Butler scheme on one transistor with compensation Avg. At frequencies up to 300 MHz, it is advisable to use single-stage filter circuits, for example, a common-base filter circuit (Fig. 8). In essence, such an oscillator is a single-stage amplifier in which the circuit is connected to the emitter of a bipolar transistor through a quartz resonator, which acts as a narrow-band filter. The circuit formed by the parallel quartz capacitance Cp and the coil L2 is tuned to the frequency of the harmonic used. With an increase in the operating frequency, the equivalent conductivities of the transistor increase, i.e. fulfillment of the self-excitation conditions worsens. However, despite this, the conditions for self-excitation of this oscillator at high frequencies are met more easily than oscillators with quartz between the collector and base and quartz in the circuit, which determines its advantage.
In conclusion, it should be noted that the considered circuits of quartz oscillators do not exhaust the entire variety of oscillator circuits stabilized by a quartz resonator, and the choice of a circuit is decisively influenced by the presence of quartz resonators with the necessary equivalent parameters, requirements for output power, power dissipated in the resonator, long-term stability frequencies, etc. A little about resonators. When choosing a resonator for the generator, special attention should be paid to the quality factor of the resonator - the higher it is, the more stable the frequency. Vacuum resonators have the highest quality factor. But the better the resonator, the more expensive it is. Often there are resonators with a high level of side resonances. In the USSR, in addition to quartz resonators, resonators were produced from lithium niobate (marked as PH or RM), lithium tantalate (marked as RT), and from other piezoelectrics. Since the equivalent parameters of such resonators differ from those of quartz resonators, they may not be driven in circuits in which quartz works perfectly, although the frequency marked on the case may be the same. They may have worse frequency stability and tuning accuracy. The enterprises of the USSR, as a rule, produced quartz resonators with a fundamental frequency up to 20 ... 22 MHz, and higher - on mechanical harmonics. This is due to the outdated processing technology of quartz plates. Foreign enterprises produce quartz with a fundamental frequency of 35 MHz. Leading foreign firms produce resonators in the form of the so-called inverse mesa structure, operating on bulk vibrations of shear in thickness, in which the first harmonic frequency reaches 250 MHz! Using such quartz resonators in oscillator circuits, in which systems with distributed inductance and capacitance parameters are used as oscillatory systems, it is possible to obtain highly stable oscillations up to a frequency of 750 MHz without frequency multiplication! Author: O. Belousov, Vatutino, Cherkasy region; Publication: N. Bolshakov, rf.atnn.ru
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