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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING RF generators. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur So, the most important unit of any transmitter is the generator. It depends on how stable and accurate the generator works, whether someone can catch the transmitted signal and receive it normally. In our beloved Internet, there are just a lot of different bug schemes that use various generators. Now we are classifying this lot a little. The ratings of the details of all the above circuits are calculated taking into account the fact that the operating frequency of the circuit is 60 ... 110 MHz (that is, it covers our favorite VHF band). "Classics of the genre"
The transistor is connected according to the common base circuit. Resistor voltage divider R1-R2 creates an operating point offset on the base. Capacitor C3 shunts R2 at high frequency. R3 is included in the emitter circuit to limit the current flowing through the transistor. Capacitor C1 and coil L1 form a frequency-setting oscillatory circuit. Conder C2 provides the positive feedback (PFC) required for generation. Generation mechanism A simplified diagram can be represented as follows:
Instead of a transistor, we put a certain "element with negative resistance." Essentially, it is an amplifying element. That is, the current at its output is greater than the current at the input (so that's cunning). An oscillatory circuit is connected to the input of this element. From the output of the element, feedback is applied to the same oscillatory circuit (through conder C2). Thus, when the current at the input of the element increases (the loop capacitor is recharged), the current at the output also increases. Through the feedback, it is fed back to the oscillatory circuit - "feeding" occurs. As a result, undamped oscillations settle down in the circuit. Everything turned out to be easier than steamed turnips (as always). Varieties In the boundless Internet, you can still find such an implementation of the same generator:
The circuit is called "capacitive three-point". The principle of work is the same. In all of these circuits, the generated signal can be taken either directly from the VT 1 collector, or a coupling coil connected to the loop coil can be used for this. Inductive three point I choose this scheme, and I advise you.
R1 - limits the generator current, R2 - sets the base offset, C1, L1 - oscillatory circuit, C2 - Conder PIC Coil L1 has a tap to which the emitter of the transistor is connected. This tap should not be located exactly in the middle, but closer to the "cold" end of the coil (that is, the one that is connected to the power wire). In addition, you can not tap at all, but wind an additional coil, that is, make a transformer:
These schemes are identical. Generation mechanism: To understand how such a generator works, let's look at exactly second scheme. In this case, the left (according to the scheme) winding will be secondary, the right - primary. When the voltage on the upper plate C1 increases (that is, the current in the secondary winding flows "up"), then an opening pulse is applied to the base of the transistor through the feedback capacitor C2. This causes the transistor to supply current to the primary winding, this current causes an increase in current in the secondary winding. There is a supply of energy. In general, everything is also quite simple. Varieties My little know-how: you can put a diode between the common and the base:
This diode accelerates the recharge of C2, which leads to an increase in the power of the generated signal. However, at the same time, this introduces non-linear distortions into the signal, so you will have to install low-pass filters at the output to suppress parasitic harmonics. The signal in all these circuits is removed from the emitter of the transistor or through an additional coupling coil directly from the circuit. Two-stroke generator for the lazy The simplest generator circuit I have ever seen:
In this circuit, the similarity with a multivibrator is easily captured. I'll tell you more - this is the multivibrator. Only instead of capacitor and resistor delay circuits (RC circuits), inductors are used here. Resistor R1 sets the current through the transistors. In addition, without it, the generation simply will not work. Generation mechanism Let's say VT1 opens, collector current VT1 flows through L1. Accordingly, VT2 is closed, the opening base current VT2 flows through L1. But since the resistance of the coils is 100 ... 1000 times less than the resistance of the resistor R1, then by the time the transistor is fully opened, the voltage across them drops to a very small value, and the transistor closes. But! Since before closing the transistor, a large collector current flowed through L1, then at the moment of closing, a voltage surge (self-inductance emf) occurs, which is supplied to the base of VT2 opens it. Everything starts over again, only with a different generator arm. And so on… This generator has only one plus - ease of manufacture. The rest are cons. Since it does not have a clear time-setting link (oscillatory circuit or RC circuit), it is very difficult to calculate the frequency of such a generator. It will depend on the properties of the transistors used, on the supply voltage, on temperature, etc. In general, it is better not to use this generator in serious things. However, in the microwave range it is used quite often. Duple generator for the hardworking Another generator that we will consider is also a push-pull. However, it contains an oscillatory circuit, which makes its parameters more stable and predictable. Although, in fact, it is also quite simple. Here it is
What do we see here? We see the oscillatory circuit L1 C1, And then we see each creature in pairs: Two transistors: VT1, VT2 Two feedback capacitors: C2, C3 Two bias resistors: R1, R2 An experienced eye (and not a very experienced one) will also find similarities with a multivibrator in this circuit. Well, it is what it is! What is remarkable about this scheme? Yes, because due to the use of push-pull switching, it allows you to develop double power, compared with the circuits of 1-stroke generators, at the same supply voltage and subject to the use of the same transistors. How! Well, in general, she has almost no flaws :) Generation mechanism When the capacitor is recharged in one direction or the other, current flows through one of the feedback capacitors to the corresponding transistor. The transistor turns on and adds energy in the "right" direction. That's all wisdom. I have not seen any particularly sophisticated versions of this scheme ... Now for some creativity. Logic generator If the use of transistors in the generator seems to you outdated or cumbersome or unacceptable for religious reasons - there is a way out! You can use chips instead of transistors. Logic is usually used: elements NOT, AND-NOT, OR-NOT, less often - Exclusive OR. Generally speaking, only NOT elements are needed, the rest are excesses that only worsen the speed parameters of the generator. We look:
We see a terrible scheme. Squares with a hole in the right side are inverters. Well, or - "NOT elements". The hole just indicates that the signal is inverted. What is the NOT element from the point of view of banal erudition? Well, that is, from the point of view of analog technology? That's right, this is an amplifier with a reverse output. That is, at increase voltage at the input of the amplifier, the output voltage is proportional to decreases . The inverter circuit can be depicted something like this (simplified):
This is, of course, too easy. But there is some truth in this. However, for now it is not so important for us. So, we look at the generator circuit. We have: Two inverters ( DD1.1, DD1.2) Resistor R1 Oscillating circuit L1 C1 Note that the oscillatory circuit in this circuit is in series. That is, the capacitor and the coil are next to each other. But it is still an oscillatory circuit, it is calculated according to the same formulas, and is no worse (and no better) than its parallel counterpart. Start over. Why do we need a resistor? The resistor creates a negative feedback (OOS) between the output and input of the element DD1.1. This is necessary in order to keep the gain under control - this is one, and also - to create an initial offset at the input of the element - this is two. How it works, we will consider in detail somewhere in the tutorial on analog technology. For now, let's clarify that thanks to this resistor, at the output and input of the element, in the absence of an input signal, a voltage equal to half the supply voltage settles. More precisely - the arithmetic mean of the logical "zero" and "one" voltages. Let's not worry about it for now, we still have a lot to do ... So, on one element we got an inverting amplifier. That is, an amplifier that "turns" the signal upside down: if there is a lot at the input, there is little at the output, and vice versa. The second element serves to make this amplifier non-inverting. That is, it flips the signal again. And in this form, the amplified signal is fed to the output, to the oscillatory circuit. Well, let's look carefully at the oscillatory circuit? How is it enabled? Right! It is connected between the output and input of the amplifier. That is, it creates positive feedback (PFC). As we already know from reviewing previous generators, POS is needed for a generator, like valerian for a cat. Without a POS, no generator can do what? That's right - wake up. And start generating... Everyone probably knows this thing: if you connect a microphone to the input of the amplifier, and a speaker to the output, then when you bring the microphone to the speaker, a nasty "whistle" begins. This is nothing more than generation. We feed the signal from the output of the amplifier to the input. A POS occurs. As a result, the amplifier starts to generate. Well, in short, by means of the LC chain, a POS is created in our generator, leading to the excitation of the generator at the resonant frequency of the oscillatory circuit. Well, is it difficult? If (difficult) { we scratch (turnip); read again; } Now let's talk about the varieties of such generators. First, instead of an oscillatory circuit, you can turn on quartz. You get a stabilized oscillator operating at the frequency of quartz:
If you include an oscillatory circuit instead of a resistor in the OS circuit of the DD1.1 element, you can start a generator based on quartz harmonics. To obtain any harmonic, it is necessary that the resonant frequency of the circuit be close to the frequency of this harmonic:
If the generator is made of elements AND-NOT or OR-NOT, then the inputs of these elements must be paralleled and turned on like a conventional inverter. If we use XOR, then one of the inputs of each element is put on + power. A few words about microcircuits. It is preferable to use TTLS or fast CMOS logic. Series TTLSH: K555, K531, KR1533 For example, microchip K1533LN1 - 6 inverters. CMOS Series: KR1554, KR1564 (74 AC, 74 HC), for example - KR1554LN1 In extreme cases - the good old series K155 (TTL). But its frequency parameters leave much to be desired, so - I would not use this logic. The generators considered here are far from all that you may encounter in this difficult life. But knowing the basic principles of how these generators work, it will be much easier to understand the work of others, tame them and make them work for you :) Publication: radiokot.ru
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