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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING GPA frequency stabilization. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Knots of amateur radio equipment. Generators, heterodynes Perhaps the most critical node in the transceiver is the VFO, which determines the frequency stability and noise characteristics. This article is an attempt to present in a popular form what is beautifully described in the textbook [1]. At the same time, the entire mathematical apparatus is omitted so as not to frighten unprepared readers with formulas and vector diagrams. Frequency instability of self-oscillators has many causes. It is conditionally possible to divide all the causes of instability into two directions:
The simplest reason for the first direction is the mechanical fragility of the structure. The next obvious reason for the same trend is temperature instability. Heating parts of the oscillator causes changes in inductance and capacitance. For example, heating a coil wound with copper wire on a ceramic frame causes the expansion of copper, an increase in the length of the wire, and an increase in the diameter of the winding. This entails an increase in inductance and a decrease in frequency. The same heating of a coil wound on a fluoroplastic frame causes an increase in the diameter of the turns, but due to the excessively large linear expansion of the fluoroplastic, the coil is stretched in length so much that it more than covers the increase in diameter, and as a result, the inductance does not increase, but decreases, and the frequency rises. For this reason, PTFE is completely unsuitable for highly stable circuits. The magnetic permeability of most ferromagnetic materials increases when heated. Increase with heating and capacity of varicaps. When heated, the capacitance of capacitors can either increase or decrease, depending on the materials of the plates and the dielectric. Sometimes (unfortunately, not always) the value of the temperature coefficient of capacitance (TKE) is written on capacitors, which shows how many parts per million the capacitance of the capacitor changes when it is heated by 1 ° C. The sign of the change (minus or plus) is indicated by the letters "M" or "P". The designation M750 means that when heated for each degree, the capacity decreases by 750x10-6. The designation P33 means an increase in heating for each degree by 33x10-6. If a capacitor with TKE M750 had a capacitance of 1500 pF at a nominal temperature, then when it is additionally heated by 20 ° C, the capacitance will become equal to 1500-1500x750xl0-6x20 \u1500d 22,5-1477,5 \u500d 3,79 pF. If the oscillator worked, for example, at a frequency of XNUMX kHz, and its frequency was determined only by this capacitor, then the frequency deviation would be XNUMX kHz, which is clearly a lot. Radical method in this case is thermostating. But simpler and cheaper - the choice of parts with the smallest temperature deviations. The so-called thermal compensation makes it possible to reduce temperature instability to some limits, but does not completely eliminate it. There are two reasons. Firstly, the GPA circuit is tunable, and the percentage of constant and variable capacitors changes during tuning. Therefore, compensation achieved at one frequency is violated at another frequency. Secondly, changes in capacitances and inductances during heating occur according to different laws. Therefore, the compensation achieved with heating by 10°C will be violated if we heat the generator by another 10°C. As parts for the GPA, we can recommend coils wound with silver-plated wire heated during winding on a ribbed ceramic frame. Capacitors can be used KM5 (five-layer, small-sized) with TKE M47 or M75. If varicaps are used to tune the GPA, then there should be even more TKE capacitors, because. The TKE of varicaps are positive and, depending on the bias (i.e., on the tuning frequency), they vary from 70 ... 80x10'6 at high voltages to 500x10 "6 at low voltages. Therefore, it is unacceptable to use varicaps at a bias voltage of less than 8 ... 9 V If the capacity of the varicaps is insufficient for a given circuit, either use varicaps with large capacities (for example KB 105) or put two or three varicaps in parallel.The author does not recommend using burnt silver coils.Yes, they have good temperature stability, but ... low quality factor, and the quality factor is more important. The next reason affecting the frequency of the circuit is the instability of parasitic capacitances of active elements that are connected to the circuit and serve as components of its capacity. During operation, these parasitic capacitances change, and directly drive away the frequency of the circuit. The previously considered temperature frequency drifts occur slowly, they can be corrected on a digital scale or compensated. The influence of the instability of parasitic capacitances occurs quickly, most often in time with the modulation, and is accompanied by characteristic signal distortions. Parasitic interelectrode capacitances in transistors are the usual barrier capacitances of pn junctions, which are rebuilt when the voltage applied to them changes. The influence of parasitic capacitances can be reduced to some extent, but not completely eliminated. To reduce their influence, it is necessary to ensure that the percentage of parasitic capacitances in the total capacitance of the circuit is as small as possible, so that against the background of a large total capacitance of the circuit, several picofarads of parasitic capacitances have less effect. There are, however, two limitations here. First, too much capacitance with low inductance leads to a decrease in the quality factor of the circuit. Secondly, too large a constant capacitance requires a proportional increase in the variable capacitance, otherwise the loop tuning limits will not be met. In any case, it is impossible to make a GPA on almost only parasitic capacitances, as was done in [2], where a KVS1,8 varicap with a low capacitance is used in the 7 ... 111 MHz circuit. And in order to get the tuning, the author applied a large inductance and a small constant capacitance. In this case, the parasitic input capacitance of the transistor was 20% (!!) of the total capacitance of the circuit. Parasitic capacitances would have little effect on the frequency if the supply voltages and the generator's operating mode were ideally stable, which is really unattainable. One of the methods that solve the problem to some extent is the use of decoupling cascades between the GPA circuit and the active element. Figure 1 shows the simplest circuit of an inductive three-point, and Figure 2 shows three-points with the addition of a decoupling source follower.
The voltage difference "between the gate and source is 10 times less than the input voltage itself. And if the voltage difference is small, then 10 times less alternating current flows through the input capacitance of the follower, which is equivalent to a decrease in input capacitance by a factor of 10.
But that's not all. The repeater (Fig. 2) has a deep DC feedback. When the supply voltage changes, the current in the transistor changes many times less than it would change without a source resistor, i.e. parasitic capacitances are more stable. In the first case (Fig. 1), the generating transistor takes current to create an automatic bias from the circuit, degrading its quality factor. In the second case (Fig. 2), this current is taken from the follower and does not affect the quality factor. Due to the large power gain, the source of the generating transistor is connected to a smaller part of the turns of the circuit (1/10 ... 1/20) and has less effect on the circuit. Best results are obtained if a left-hand FET is used as a follower, with no bias applied to the gate. We can recommend KP305I. The circuit parameters must be chosen so that the repeater transmits the oscillation amplitude either without distortion, or with a uniform limitation from above and below. There is another mechanism of frequency destabilization, which is not so obvious. The oscillator operates continuously due to the fact that its high-quality circuit "rings" and maintains oscillations. The energy in the circuit is replenished by shocks only at peaks of positive half-waves at the gate. For stable operation in the generator, it is necessary to maintain a balance of amplitudes and a balance of phases. The first requires that for each period of oscillation in the circuit, energy is replenished as much as it is consumed from the circuit (for gate currents, losses in capacitors and resistors, radiation into the surrounding space). This balance is maintained by automatic bias. As soon as the oscillation amplitude decreases slightly, the bias also decreases, the transistor opens a little more, and the portions of the pumping energy increase. And vice versa. The second requires that the boosting current pulses enter the circuit strictly in time with the existing oscillations - not earlier and not later. The phase balance is also maintained automatically, but this process is more difficult to understand. For simplicity, we describe it in the case of a self-oscillator based on a vacuum triode. When the lamp is opened, a bunch of electrons begins to move from the cathode to the anode. There is no current in the anode circuit at this time. The current pulse will go through the anode circuit only after the bundle of electrons reaches the anode. During this, in general, negligible time, the phase of the oscillation on the circuit will change, and the pushing current pulse will lag behind the voltage pulse on the grid. This lag is expressed in a phase angle of several degrees. This is the so-called slope angle (not to be confused with the slope of the current-voltage characteristic!). The slope angle, which shows the magnitude of the signal delay, depends on the distance between the electrodes and the speed of the electrons, which, in turn, depends on the magnitude of the anode voltage. So, the impulses enter the circuit late. How does the generator adapt to this? It turns out that it does not generate exactly at the frequency of the circuit, but just below this frequency. If an alternating current flows through an oscillatory circuit, then the voltage on the circuit is exactly in phase with the current in one case: when the current is exactly in resonance with the frequency of the circuit. In all other cases, the voltage on the circuit either leads the current or lags behind it. So, the oscillator automatically selects a frequency at which the voltage on the circuit is ahead of the boosting current pulses by exactly the same amount that the lamp then delays. It is known that a high-Q circuit reacts very sharply to frequency deviations. A very small frequency deviation causes large phase deviations. Accordingly, in order to compensate for the phase delay in the lamp, the generator only needs to slightly move away from the resonant frequency of the circuit. If the anode voltage has changed, then the delay in the lamp has also changed. The generator will switch to another frequency, at which the phase balance would again be observed. The frequency shift will be negligible if the quality factor of the circuit is high. With a low-Q circuit, the generator must change the frequency much more to compensate for the same delay. Signal delays exist not only in lamps, but also in transistors and microcircuits. Only there their physics is not so obvious. Thus, by changing the operating mode of a lamp or a transistor, we can change the generation frequency, this is even used for frequency modulation. But what to do if not only we can't, but we don't want to - and the frequency "floats"! Firstly, if possible, stabilize the power supply, and secondly, use an oscillatory circuit of the highest possible quality factor, for which the coil is wound with a sufficiently thick silver-plated wire on a ribbed frame made of radio porcelain or polystyrene. If the frame does not have a forced notch, then it is necessary to wind it with heated wires from a step-down transformer. After cooling, the wire shrinks and tightly fits the frame, fixing the turns. Coating the coil for this purpose with varnishes, paints, etc. completely unacceptable. If the oscillator operates at frequencies above 10 MHz, then the circuit elements should not be soldered to the printed circuit board. Capacitors and varicaps used in the circuit should be soldered directly to the ends of the coil, without additional mounting wires. If the generation frequency is high - and the parasitic capacitances of the transistor inevitably make up a significant part of the capacitance of the circuit, then the transistor itself must be soldered to the coil by surface mounting. Thirdly, it is necessary to use transistors with minimal parasitic capacitances for GPA. Often, to prevent self-excitation of the oscillator on VHF, antiparasitic resistors are used in the gate or base circuit. Along with damping parasitic oscillations, they reduce the quality factor of the main circuit. Therefore, resistors, even if they are provided by the circuit, do not need to be installed first. If parasitic oscillations still occur, then it is necessary to look for other ways to eliminate them, and if this does not give an effect, then only put an anti-parasitic resistor of the minimum value, starting with a few ohms. Parasitic excitation on VHF not only creates additional channels for receiving and parasitic radiation, but also disrupts the stability of the main generation. The parasitic circuit may have a low quality factor, while parasitic oscillations have an unstable amplitude. The oscillator mode is constantly changing, causing changes in the fundamental frequency and perplexing its creators. Frequency instability can be caused by so-called "pulling". If the oscillator is poorly shielded, then during transmission, large pickups affect the circuit, which, adding up with the main oscillations, lead to a complete disorder of the phase at the input of the transistor. Accordingly, the generation frequency begins to "walk". Control measures - screening. power decoupling and compliance with the level diagram, in which the amplitude of natural oscillations would be many times greater than the amplitude of pickups. It may be objected to me that much of what has been said here is not so important. After all, transceivers work, in which the GPA is made contrary to many of the thoughts expressed here. Yes, they do. But how? Take this or that GPA, change the supply voltage by 10% and look at the frequency shift on the frequency meter. Of course, in real work, it does not change by 10%, but much less, but this is more convenient for greater clarity. Then you will see all your misses - what kind of frequency instability does coating the coil with varnish give, how much does soldering capacitors and varicaps onto a printed circuit board, etc. An oscillator with high electronic frequency stability has correspondingly low phase noise. This does not apply, however, to the case where stability is achieved with a digital scale and a CAFC, and not with a good design of the VPA itself. Literature
Author: G. Gonchar (EW3LB), Baranovichi; Publication: N. Bolshakov, rf.atnn.ru
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