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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Theory: generators of sinusoidal oscillations. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur One of the types of sinusoidal oscillation generators is used to set the frequency of RC elements. Such generators are quite complex, require special measures to stabilize the amplitude of oscillations and do not have high frequency stability. Generators with a parallel oscillatory circuit as a frequency setting element work more reliably and better - they are often called LC generators. Recall that a parallel oscillatory circuit contains a capacitor and an inductor. If a charged capacitor is connected to a coil, then damped oscillations will occur in the resulting circuit (Fig. 47). Their frequency is determined by the Thomson formula: fo = 1/2π(LC)1/2.
The oscillations would continue indefinitely if there were no energy losses in the circuit, for example, on the active resistance of the coil wire. Besides, some. even if a small part of the energy must be given to the load of the generator! The lower the energy loss, the higher the quality factor of the circuit, which is equal to the number of oscillations until their amplitude decreases by about 10 times. This fact is little known. Losses in a loop capacitor are usually small compared to losses in the coil, so the quality factor of the circuit is almost equal to the quality factor of the coil, defined as the ratio of the reactance of the coil to the active. The quality factor of radio frequency coils in the DV, SV and KB ranges usually lies in the range of 30 ... 300, depending on the size and quality of workmanship. Large coils wound for the DV and SV ranges with a special stranded wire (LZSHO - litz wire) or thick silver-plated wire for the KB range usually have a high quality factor. Significantly reduce the size of the coils while maintaining a high quality factor allows magnetic circuits (cores) made of high-frequency ferrite or other magnetodielectric (magnetite, oxyfer, carbonyl iron). However, when using such coils in generators, it is necessary to pay attention to the temperature dependence of the properties of the magnetic circuit so as not to worsen the stability of the generator frequency. The quality factor of the circuit also determines the width of its resonant curve. It characterizes the dependence of the amplitude of oscillations in the circuit on the frequency when it is excited from an external source of sinusoidal oscillations. The connection of the source with the circuit in order to obtain correct results must be very weak; when the oscillation frequency of the source coincides with the resonant frequency of the circuit, the oscillation amplitude in it is maximum, and when detuned, it decreases. The width of the resonance curve at the points where the amplitude drops to 0,7 (by 3 dB) is inversely proportional to the quality factor: 2Δf=f/Q (Fig. 47). The main idea of building generators with an LC circuit is as follows: the loss of energy in the circuit during the oscillation process must be replenished by an amplifying element excited from the same circuit, in full accordance with Fig. 44. In this case, two conditions must be met: the balance of amplitudes and the balance of phases. The first condition requires that the energy supplied to the circuit from the amplifying element is exactly equal to the energy losses in the circuit itself and in the communication circuits with the load. With a weaker feedback, the oscillations die out and generation stops, with a stronger one, the amplitude increases and the amplifying element either enters the limiting mode or is closed by the voltage generated by the amplitude stabilization circuit. In both cases, the gain is reduced, restoring the amplitude balance. The phase balance condition is that the oscillations from the amplifying element are supplied to the circuit in phase with its own. Therefore, the total phase shift in the feedback loop must be zero. However, a small phase shift introduced by the amplifier can be compensated by the circuit. The phase shift of the oscillations in the circuit (relative to the exciting ones) is 0 at the resonant frequency and reaches ±π/4 when the frequency is detuned by ±Δf in accordance with the phase characteristic of the circuit. In the presence of a phase shift in the amplifying element, oscillations will be excited not at the resonant frequency, but somewhere to the side of it, which, of course, is undesirable. Historically, the first LC oscillator was invented by Meissner in 1913 (German Society for Wireless Telegraph) and then improved by Round (British firm Marconi). It used inductive feedback (Fig. 48).
Oscillations from the L2C2 circuit are fed to the grid of the VL1 lamp. Its anode current, which changes in time with the oscillations in the circuit, flows through the coupling coil and, and the energy of the amplified oscillations flows back into the circuit. For correct phasing, both coils must be switched on as shown in the figure (the beginnings of windings wound in one direction are indicated by dots). You can adjust the feedback by changing the distance between the coils. To stabilize the oscillation amplitude, a gridlick is used - the C3R1 chain (by the way, it was not yet in the very first Meissner generator). It works like this: during positive half-periods of oscillations on the grid, part of the electrons is attracted to it and charges the right side of the capacitor C3 according to the scheme with a negative voltage. It shifts the operating point to a less steep section of the characteristic (the tube closes a little), and the gain is reduced. The "grid leakage" resistor R1 allows the accumulated charge to drain to the cathode, otherwise the lamp would close completely. Capacitor C1 serves to close high-frequency currents to a common wire ("ground") - after all, it is not at all necessary that they flow through the power source, creating interference and interference with other elements of the device in which the generator is used. Later, the American company Western Electric developed simpler and more advanced generators - the inductive "three-point" Hartley (1915) and the capacitive "three-point" Colpitz (1918). We deliberately cited the names of the inventors, since the circuits of their generators have remained practically unchanged for more than three quarters of a century, and the names "Meissner circuit" or "Colpitz circuit" are still found in the technical literature without explaining what it is. The element base, however, has changed significantly, and as an example, consider a generator made according to the inductive three-point (Hartley) scheme on a modern field-effect transistor with an insulated gate (Fig. 49).
According to the principle of operation, such a transistor is in many ways similar to a three-electrode radio tube - a triode, but the current in it does not flow in a vacuum, but in the thickness of a semiconductor, where a conductive channel has been technologically created between the drain (upper output according to the circuit) and source (lower output). The conductance of the channel is controlled by the gate voltage - an electrode located very close to the channel, but isolated from it. When a negative voltage is applied to the gate, its field "squeezes" the channel, as it were, and the drain current decreases. If a positive voltage is applied and increases, the channel conductivity increases and the drain current increases. In any case, there is no gate current, and this made it necessary to supplement the C2R1 grid - the amplitude stabilization circuit - with diodes VD1, which detect oscillations entering the gate and create a negative bias as their amplitude increases. Oscillations are supplied to the gate from the L1C1 circuit, which determines the frequency of the generator. The advantage of a field-effect transistor is that its input impedance at radio frequencies is very high, and it practically does not shunt the circuit without introducing additional losses into it. Feedback is created by connecting the source of the transistor to a portion of the turns of coil L1 (usually 1/3 to 1/10 of the total number of turns). The generator works like this: with a positive half-wave of oscillations, the current of the transistor increases at the top output of the circuit, which “throws” another portion of energy into the circuit. In fact, the transistor in this generator is turned on by a source follower, and the phase of the oscillations at the source coincides with the phase of the oscillations at the gate, which ensures the phase balance. The voltage transfer coefficient of the follower is less than unity, however, the coil with respect to the source is included as a step-up autotransformer. As a result, the total feedback loop gain becomes greater than unity, providing amplitude balance. As another example, consider a generator made according to the capacitive "three-point" scheme on a bipolar transistor (Fig. 50). Actually the generator is assembled on the transistor VT1. Its DC mode is set by the divider in the base circuit R1R2 and the resistance of the emitter resistor R3 (we have already considered such circuits in the section on amplifiers). The oscillatory circuit of the generator is formed by an inductor L1 and a chain of three capacitors C1-C3 connected in series. Not only the emitter, but also the base of the transistor are connected to the taps of the resulting capacitive divider. This is dictated by the desire to reduce the shunting of the circuit by the transistor - after all, the input resistance of a bipolar transistor is relatively small.
In practice, the capacitances of the capacitors C2 and C3, shunting the transitions of the transistor, are trying to choose more, and the capacitance C1 is the minimum necessary for the occurrence of oscillations. This improves frequency stability. The rest of the generator works the same way. like the previous one. The cascade on the transistor VT2 - the so-called buffer cascade - serves to weaken the influence of subsequent cascades on the generator. The transistor is turned on by an emitter follower and receives a bias directly from the emitter of the generator transistor VT1. Additionally, the connection is weakened by resistor R4. All the measures taken make it possible to bring the relative frequency instability of the described generator to such a small value as 0,001%, while it is an order of magnitude worse for conventional LC generators. In broadcasting and television receivers, simpler three-point capacitive generators are used, a typical circuit of one of which is shown in Fig. 51.
Here, the L1C3 circuit is included in the collector circuit of the transistor, the base is connected at high frequency to the common wire through the capacitor C2, and the feedback is fed to the emitter through the capacitive divider C4C5. Turning on a transistor according to a common-base circuit makes it possible to obtain especially high generation frequencies close to the limit for this type of transistor. The generator signal is taken from the coupling coil L2. Author: V.Polyakov, Moscow
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