Radio station - do it yourself

Technology for building and tuning a radio station at 27 MHz + 8 structures (modifications) with a range of 2–4 km

The documentation is intended for novice radio amateurs who independently design portable radio stations for individual use.

In the first part, the basics of building a radio station are given, the Functional blocks of the receiver and transmitter and their operation are described, the influence of circuit elements on the operation of the radio station is considered, recommendations are given for choosing the optimal modes. The emphasis is on the main principal circuit solutions.

The second part gives practical diagrams of radio stations and their description, as well as a tuning technique. Schemes of simple assistant devices for tuning and controlling radio stations are given.

When compiling the documentation, we proceeded from the fact that the vast majority of radio amateurs, especially beginners, do not have at their disposal such devices as an oscilloscope, frequency meter, etc., as well as the possibility of acquiring scarce radio components such as quartz resonators.

In the process of developing the documentation, many schemes were tested, from which the most suitable for repetition were selected, refined and tested. At the same time, it turned out that most of the schemes given in the literature contain inaccuracies, errors and fundamental flaws, and, as a result, are not repeatable at home.

We hope that the materials prepared by us will be useful for you and help you take your first steps into the fascinating world of radio communications.

1. Basics of building a radio station

1.1. The radio station consists of a receiver and a transmitter.

The radio transmitter converts sound vibrations (speech, music, etc.) into electromagnetic vibrations emitted by the antenna. These electromagnetic waves are received by the receiver and converted back into sound.

Days of amateur radio communication are allocated several bands. The radios described in this documentation are designed to operate on the 10 meter amateur band at 27.120 MHz. The type of modulation used in the transmitter is the simplest - amplitude modulation. The receivers are built according to a super-regenerative scheme.

1.2. General principles operation of a super-regenerative receiver.

This type of receiver is best suited for building simple radio stations:
- no scarce parts;
- a small number of circuit elements;
- simplicity of the scheme;
- sufficient sensitivity.

Many novice radio amateurs, collecting such receivers, were disappointed. The receiver either did not start at all, or was too "capricious" in tuning. This is largely due to the fact that in many publications circuit solutions are very critical to the ratings of the elements, especially the transistor.

The schemes given in this documentation are usually run immediately after assembly.

The super-regenerative receiver (Fig. 1) consists of three functional blocks:
- input circuit;
- super-regenerator;
- low frequency amplifier.

The input circuit consists of an antenna and a filter L1, C2, C3 and is designed to increase the selectivity of the receiver. The fact is that the super-regenerative receiver has a fairly wide band (250-500 kHz). Therefore, if the input circuit is excluded from the receiver, then along with the main signal, other radio stations operating in this range can be heard. In addition, with a sufficiently high sensitivity of the receiver, various electrical interferences can be induced. The input circuit itself does not amplify the main signal, on the contrary, it weakens somewhat, but it significantly suppresses the radio stations that operate at the nearest frequencies. The input circuit can be excluded, then the capacitor C1 is connected to the circuit L2C5C7 directly.

Rice. 1. Super regenerative receiver.

The task of the super-regenerator is to amplify and demodulate the received high-frequency signal. The super-regenerator is designed as a feedback amplifier. The circuit, when properly configured, has the maximum sensitivity that the VT1 transistor with good high-frequency parameters can provide. The most acceptable and simplest method for selecting "good" transistors, in the absence of devices, is a practical test of their operation according to the circuit. The circuit (Fig. 1) of the super-regenerator makes it possible to use almost any high-frequency transistors of low and medium power with reverse or forward conduction without changes.

In the latter case, it is necessary to change the polarity of the power supply.

There are three types of oscillations in the super-regenerator:
- high-frequency - equal to the received frequency (27.12 MHz);
- auxiliary - 30-50 kHz;
- low-frequency - corresponding to amplitude modulation.

For normal operation It is necessary for the receiver that the high-frequency oscillations of the super-regenerator coincide with the received frequency of the transmitter, and the frequency of the auxiliary oscillations be within 30-50 GHz.

To ensure the regeneration of high-frequency oscillations, the resonant frequency of the circuit L2-C5-C7 must match the frequency of the transmitter (set by capacitor C7), and with the help of C8, optimal feedback is obtained, i.e. the highest sensitivity of the super-regenerator just before the onset of self-excitation. With a decrease in capacitance C8 to a certain limit of 4-15 pF, the sensitivity of the receiver increases, and then generation disruption occurs.

In addition, the capacitance of the collector-emitter junction of the transistor VT1 also affects the generation process. The junction capacitance forms a kind of capacitor connected in parallel with C8. If the capacitance of the VT1 junction is large enough (20-30 pF), then by adjusting the capacitor C8 it is not possible to achieve a high sensitivity of the receiver. It is possible, in this case, to exclude the capacitor C8 altogether and the feedback will be carried out only due to the capacitance of the "collector-emitter" junction of the transistor VT1. The frequency of auxiliary oscillations is determined mainly by the R4C9 chain.

The emitter current of the transistor VT1, flowing through the resistor R4, simultaneously charges the capacitor C9. The emitter becomes more negative and a lower bias voltage is applied to the base than to the emitter. The transistor current decreases and the transistor turns off. Further, the capacitor C9 begins to discharge through R4, the emitter voltage drops, and the process resumes. With the given ratings R4-C9, the frequency is from 30 to 50 kHz.

Inductor Dr1 (20-60 MKGN) filters out high-frequency oscillations, and the remains are closed to ground through C9. Therefore, if you change the values \u200b\u200bof the R4-C9 chain, you should not select C9 less than 1000 pF so that the resistance to RF residues is minimal.

Transistor VT1 is connected according to the scheme with a common base. Resistors R1 R2 set the operating point of the transistor. This point must be chosen in such a way that it oscillates between amplifying and self-exciting modes.

The super-regenerator circuit (Fig. 1) provides maximum receiver sensitivity by simple regulation due to capacitors C7, C8. If you are using other types of transistors, then you may need to select the resistor R2 to increase the sensitivity.

When choosing a transistor VT1 with good characteristics, the sensitivity of the receiver is brought to 1-2 microvolts.

The R5-C10-C11 chain serves to separate the low and auxiliary frequencies. The low-frequency signal with the remainder of the auxiliary frequency is fed to R5.

The low frequency amplifier is simple, does not require tuning, and provides sufficient output power. In addition, the R5-C10-C11 chain is a filter that attenuates the passage of the auxiliary frequency C10 to the ULF, should not be set to more than 2 microfarads.

1.3. General principles for the design of transmitters.

The radio transmitter consists of a high frequency generator (HHF), a high frequency power amplifier (UMHF), an end stage and a modulator.

1.3.1 High frequency generator (HFG).

The basis of any transmitter is the GHF (Fig. 2). The main task of the GHF is the generation of high-frequency oscillations, the main characteristic is frequency stability. Stability is understood as a deviation, a change in the frequency of the MHF from a given one. For our case, a satisfactory stability of 0.01 - 0.001% deviation, i.e. deviation from the frequency of 27.120 MHz by no more than 27.12 kHz is allowed. Moreover, such stability should be maintained with changes in temperature, supply voltage, humidity and other adverse factors. The operating point of the transistor VT1 is set by resistors R1, R2. Capacitor C3 and oscillatory circuit L1-C2-C1 determine the carrier frequency of the generator. To ensure reliable operation of the transmitter, the MHF is tuned to the point of maximum oscillation stability by adjusting the oscillatory circuit. The temperature stabilization of the GHF is provided by the R3-C4 circuit, the feedback is C5.

Rice. 2. High frequency generator.

Let us consider the main reasons causing the instability of the MHF (Fig. 2).

one). Instability is caused by a change in the parameters of the transistor VT1, mainly due to fluctuations in temperature and supply voltage. Silicon transistors in this respect are more preferable than germanium ones. In addition, when choosing a transistor VT1, it is necessary, according to reference data, to select the transistor with a limiting frequency of 200 MHz or more, as well as with possibly smaller internal junction capacitances. The better these parameters, the more stable, with less distortion, the MHF works. During operation, the transistor heats up, and this, in turn, changes its parameters (transistor reverse currents, etc.) and can cause a significant frequency drift.

To prevent this process, the transistor must be selected in terms of power and collector current with a margin. In this case, VT1 will work in light mode - internal heating will be minimal, the collector current VT1 is optimal - 8-10 times less than the maximum reference, respectively, and in terms of power.

2). A very important element of the GHF, which affects the stability of the frequency, is an oscillatory circuit consisting of an inductor L1 and capacitors C1, C2.

The frequency stability is higher, the greater the quality factor of the oscillatory circuit, and this depends both on the inductor L1 and on the type and size of the capacitances C1, C2.

The quality factor of an inductor is determined by the resistance of the material (wire), the size and shape of the coil, and the type of core. Printed coils have high stability, mainly due to the minimum interturn capacitance. The inner diameter (smaller turn) of the printed coil is recommended to be at least 10 mm, the conductor width is at least 0.5 mm, the distance between the turns is at least 0.3 mm. A sufficiently stable coil can also be made from ordinary copper wire.

a). Do not strive to miniaturize the coil. The inner diameter must be at least 8 mm.

b). The intrinsic resistance of the conductor should be minimal, and hence, the diameter of the wire is within 1-1.5 mm. Material - copper (wire brand PEV, PYL).
If it is possible to use a silver-plated wire, or to apply a silver film on the wire yourself, for example, using a spent fixer, this will further increase the quality factor of the coil.

in). It is desirable to use frameless coils, and if a frame is used, then ceramic. With fluctuations in temperature, the frame can expand and accordingly change the geometry of the coil, and this, in turn, changes the inductance and frequency.

G). Single-layer coils with forced pitch are characterized by high stability. This is because the closer the turns are to each other, the greater their capacitance and interconnection. And this worsens the characteristics of the circuit.

e). When placing the coil on the board, it must be taken into account that other circuit elements located near (5-10 mm) of the coil can cause instability. It is especially not recommended to place such parts as electrolytic capacitors, metal transistors from the ends of the coil. Capacitor C1 is better to use ceramic with air dielectric(capacitance C1 - 4/20 pF, C2 - 10 pF), capacitor C2 is ceramic and serves to suppress harmonics.

e). To stabilize the frequency, the power of the GHF is chosen small (5-10 MW), and the load is maintained weak. The main power is obtained by a high frequency power amplifier. If you have at your disposal a quartz resonator at a frequency of 27.12 MHz, then it can be included in the circuit instead of C3 GHF (Fig. 2). This will provide excellent stability.
and). It is advisable to make the conductors connecting the circuit elements shorter, without overlapping the mounting wires.

1.3.2 High-frequency power amplifier (UMHF) and high-frequency filter.

The main purpose of UMHF is to amplify the power of high-frequency oscillations, and the filter is to match the antenna and transmitter for more efficient radiation electromagnetic oscillations and suppression of spurious emissions.

UHF and a filter can be combined in one unit, the use of modern silicon transistors allows using simple single-stage UHF to obtain radiation power at the antenna up to 600 MW, and this provides a communication range of up to 2-5 km. When building transmitters with UHF, careful filter adjustment is required to suppress spurious emissions (harmonics), otherwise the transmitter will interfere with household and other television and radio equipment. Consider the operation of the UMHF and the final cascade according to the scheme in Fig. 3.

Rice. 3. High frequency power amplifier.

High-frequency oscillations are fed to the base of the transistor VT1, the operating point of which is selected and fixed by a rigid divider R1, R2. The high-frequency signal is amplified by the transistor VT1 and is allocated to the inductor Dr1, which has a high resistance to high frequency. For more stable operation, instead of the Dr1 choke, it is necessary to turn on the LC oscillatory circuit, tuned to the main carrier frequency (27.120 MHz). To compensate for the influence of the temperature regime of the transistor VT1, the R3-C1 circuit is connected to the emitter. With a decrease in the resistor R3, the collector current VT1 increases and, consequently, the power of the UHMW. It must be remembered at the same time that too much collector current causes the transistor to heat up.

Therefore it is necessary:

one). Choose the power of the transistor VT1, which is 2-5 times higher than the actual one. This is determined by the maximum collector current, by the reference data of the transistor and actually measured.

2). To remove heat from the transistor, radiators must be used.

The amplified signal through the capacitor C2 is fed to the P-filter C3-L1-C4 and further, through the coil L2 to the antenna. The amplified high-frequency signal contains not only the fundamental frequency, but also its harmonics. The power of the harmonics is often comparable to the power of the fundamental frequency. To suppress them, you need to carefully select the ratings and adjust the P-filter. The circuit elements of the P-filter must be selected individually for each transmitter, because its characteristics depend on the transistor VT1, as well as on the resistance and capacitance of the antenna. Usually it is enough to adjust the cores of the coils L2, L1.

At home, the most rough estimate of the effectiveness of harmonic suppression with a P-filter can be your TV and radio equipment.

1.3.3. Modulation.

As already mentioned, these radios use amplitude modulation. High-frequency oscillations, their amplitude (value) change in proportion to low-frequency oscillations. Low-frequency oscillations from the microphone are amplified by the ULF and control the magnitude of high-frequency oscillations (Fig. 4).

Rice. 4. Amplitude-modulated high-frequency oscillation.

On fig. 4-a shows unmodulated high-frequency carrier oscillations of 27.12 MHz, and the amplitude is constant UHF (a-c). There is no superposition of low-frequency vibrations and no information is transmitted.

Amplitude-modulated oscillations (Fig. 4-c) of a high-frequency signal change in accordance with low-frequency oscillations (Fig. 4-b).

The amplitude of high-frequency oscillations (Fig. 4-c) changes by the value of UHF (a-c) and UHF (b-d), i.e. there is an unmodulated UHF (c-d) component that does not change. The value of the changing amplitude as a percentage is called the modulation depth. With amplitude modulation, it is very important to achieve the maximum (100%) modulation depth. Otherwise, even with powerful radiation of high-frequency oscillations, the range of the radio station will be significantly limited. It can be considered that the transmitter power, due to which the unmodulated component is provided, is simply lost. For example, if the transmitter power is 100 MW at a modulation depth of 30%, then this is equivalent to a transmitter power of 30 MW and a modulation depth of 100%.

Most in a simple way amplitude modulation is power modulation. If less power is supplied to the MHF, then the amplitude of the high-frequency oscillations generated by the MHF decreases accordingly. Therefore, the power supply of the MHF is changed in accordance with the change in the low-frequency signal, we can modulate the high-frequency oscillations.

Rice. 5. Modulator circuit.

The modulator circuit (Fig. 5) consists of a ULF on transistors VT1, VT2 and a modulated transistor VT3. Through the decoupling capacitor C4, the amplified low-frequency oscillations are fed to the base of the transistor VT3. Resistor R5 sets the mixing of the VT3 base so that the current at point (A) is equal to half the current if the minus GHF is connected directly to the minus supply. In this case, the magnitude of the amplitude of HF oscillations will also be equal to approximately half of the maximum. In this case, the positive half-waves of low-frequency oscillations will open VT3, and the negative ones, on the contrary, will close. Accordingly, the amplitude of the RF oscillations will increase and decrease proportionally. To achieve 100% modulation, it is necessary to select such a low-frequency signal power so that VT3 opens completely with a positive half-wave, and completely closes with a negative half-wave. If the power of the low-frequency signal is insufficient, then the positive half-wave will not fully open the transistor VT3, which means that the amplitude of the high-frequency signal will not reach its maximum. Accordingly, the negative half-wave will not completely close VT3 and the RF signal will not reach its minimum, then with insufficient power of the low-frequency signal, the range of the amplitude of the high-frequency oscillations is limited.

If the low-frequency signal, on the contrary, is too strong, then overmodulation occurs. In this case, the VT3 transistor is fully opened even before the low-frequency signal reaches its maximum. And with a further increase in the amplitude of the LF, the amplitude of the HF oscillations does not increase. This limits the amplitude from above. Accordingly, there is a limitation from below. The S1 button is used for an intermittent tone call.

2. Method of tuning the radio station.

2.1. Transmitter setup.

To test the transmitter's performance, configure and control it, it is necessary to make a simple detector receiver. At home, in the absence of devices and experience with them, the detector receiver will allow you to tune the transmitter to a frequency of 27.12 MHz with allowable deviations, evaluate the radiation power and modulation depth. The detector receiver (Fig. 6) must be tuned to a frequency of 27.120 MHz.

Rice. 6. Detector receiver.

It is desirable to tune the receiver using a standard signal generator (GSS). Having set the GSS frequency to 27.120 MHz, tune the receiver with capacitor C1 according to the maximum signal in the headphones. In this case, the receiver must be gradually moved further from the GSS, adjusting the receiver. After tuning, you can not change the antenna. Instead of the GSS, you can use a self-made GHF stabilized by a quartz resonator (Fig. 2). If this is not possible, then it is necessary to more carefully make the L1 coil and the antenna, and replace the capacitor C1 with a constant one, with a capacity of 30 pF. In this case, the deviation from the frequency of 27.12 MHz will be acceptable, i.e. in the amateur range, the L1 coil is frameless, with an inner diameter of 8 mm, the number of turns is 17, the pitch is 0.5 mm, the wire diameter is 1 mm. Antenna - wire with a diameter of 1 mm, length - 25 cm.

The transmitter is configured in the following order:
1. modulator setting.
2. setting the MHF to a frequency of 27.12 MHz.
3. UMHF setting for maximum gain and minimum harmonics.
4. setting the modulator to 100% modulation depth.
5. adjustment of the assembled transmitter.

To check the modulator, you need to connect headphones instead of the GHF (Fig. 5) and apply power to the 9 V modulator. In this case, the modulator should work like a regular ULF. The sensitivity is adjusted by selecting the resistor R1. The call is checked by closing the contacts of the switch S1, while an intermittent sound signal should be heard (the tone is changed by the capacitance C5).

To configure the GHF, you need to connect it to the modulator, fix (turn on) the S1 tone call button, and solder a piece of wire 5-7 cm long and 0.5-0.7 mm in diameter as an antenna to the GHF capacitor C6 (Fig. 2), turn on the power. Your HHF will operate as a transmitter with a carrier frequency of about 27 MHz and modulated with a tone signal.

Place the receiver close (10-20 cm) to the MHF. The GHF is tuned to a frequency of 27.12 MHz by capacitor C1 (Fig. 2). When tuned to 27.120 MHz, a dial tone should be heard.

After that, you can adjust the modulation depth, it is better to do it together: one speaks into the modulator microphone and changes the resistance R5 (Fig. 5), and the other controls the audibility through the receiver, the most intelligible audibility corresponds to deep modulation.

The next block is configured UMHF. For this you need to enable complete scheme transmitter with antenna.

An easy way to control the transmitter tuning on maximum power- maximum current consumption of the transmitter. Turn on the ammeter between the power supply and the transmitter, controlling the amount of current in the UMHF (Fig. 3). First, if you connected a circuit instead of a choke, tune the LC resonant circuit to resonance by adjusting the capacitor. Next, choose the optimal operating point of the transistor divider R1 R2. Tuning control is tentatively estimated according to the current consumption. Adjustment of the harmonic suppression filter is carried out by the cores of the coil L1 L2 with the antenna connected. The effectiveness of suppression is controlled by the absence of interference on all TV and radio channels. After filter adjustment, spurious emissions are usually well suppressed, but 100% suppression is not guaranteed. To do this, you need to check the transmitter on the curve tracer.

2.2. Receiver setup.

To tune the receiver, it is necessary to have a source of radiation of modulated high-frequency oscillations. It is better to use the GSS, in the absence of it, you can replace the GHF or a transmitter already tuned to a frequency of 27.12 MHz. Before setting up the receiver, make sure it is working. To do this, it is enough to apply power and, by adjusting the feedback value (capacitor C8 - Fig. 1), to achieve the appearance of noise in the headphones. After that, the receiver tuning is carried out together with the transmitter or GSS. The setup is simple. By adjusting the capacitors C7 and C8, it is necessary to achieve the maximum signal in the receiver's headphones, gradually moving away from the transmitter. Tuning must be carried out with the antenna that will be on your radio station. Changing the length and shape of the antenna will require a new tuning of the receiver. The receiver frequency is adjusted by capacitor C7, and the sensitivity is C8. If the receiver contains an input circuit, then the capacitor C2 adjusts the input circuit to a frequency of 27.120 MHz.

The range is determined by the following main functions:
- transmitter power;
- receiver sensitivity;
- environmental conditions.

The power of simple transmitters in a radio station (Fig. 7) can be increased up to 250-300 MW without significant alterations. This is achieved through:

a) replacing the transistor VT1 with a medium power transistor KT603, KT608, KT645, KT630 with the highest possible gain;

b) increasing the supply voltage to 12 V supplied to the transmitter (the power supply of the receiver should not be changed);

c) strengthening the connection of the oscillatory circuit L1-C2-C5 with the antenna (the closer the antenna is connected to the collector VT1, the stronger the connection and the radiated power on the antenna);

d) reducing the resistance of the resistor R3 (in this case, the collector current VT1 and the amplitude of the RF oscillations increase).

Making changes to the transmitter requires adjusting the carrier frequency with capacitor C5. Sometimes, when replacing VT1, it is necessary to adjust the divider R1 R2. With an increase in the transmitter power, the radiation power of harmonics increases, creating interference on the air. Partially, this can be eliminated by selecting a matched antenna length and increasing the capacitance of the capacitor from 2 to 30 pF.

If, nevertheless, it is not possible to get rid of interference, then it is necessary to additionally connect an II filter, i.e. turn on coils L1, L2 and capacitors C3, C4 (Fig. 3).

A more "harmless" means of increasing the range is to increase the sensitivity of the receiver. This is achieved:
1) more precise adjustment of sensitivity by capacitors C19, C20 (Fig. 7);
2) replacing the transistor VT5 with GT311Zh, KT311I, KT325V, KT3102, KT3102E, etc.;
3) more accurate selection of the value of the resistor R10.

The length and shape of the antenna greatly affects both the sensitivity of the receiver and the radiation power of the transmitter. When choosing whip antennas, the antenna length of 125 cm (1/8 wavelength) is considered the most acceptable.

2.4. Details and design.

In radio stations, the schemes of which are given below, mostly functionally similar parts are used.

Coils with an inductance of 0.8 MKH are performed as described in paragraph 3.1. for a detector receiver, plus power (in all circuits) is connected to the middle turn of the coil, and a high-frequency signal is taken from the 5th turn, counting from the collector of the transistor.

In UMHF (Fig. 3), the coils are made on a polystyrene frame with a diameter of 7 mm with a carbon iron trimmer. Coil L1 contains 9 turns, and L2 - 15 turns of copper wire with a diameter of 0.8 mm. The design of the transmitter coils (Fig. 9), including L2 with an inductance of 0.8 µH, is described above, and L4 is wound over L2 and consists of 4 turns of wire with a diameter of 0.8 mm, evenly distributed over the L2 coil. Similarly, coils L2, L1 are made in the transmitter (Fig. 8). Coil L3 (Fig. 9) is wound on a polystyrene frame with a diameter of 7 mm with a carbon iron trimmer, the number of turns is 10, the wire diameter is 0.5 mm.

A rod or flexible wire 50-150 cm long is used as an antenna.

TON-2M telephones are used as a microphone and telephone. When using a different microphone, you will need to adjust the first stage of the modulator. Other ULFs can be used in the receiver, including those designed for dynamic heads, but the 1st stage of the ULF receiver should not be changed.

Rice. 7.

Rice. eight.

Rice. 9.

Rice. ten.

Rice. eleven.

Rice. 12.

Rice. 13.

Rice. fourteen.

R11 - 75 ohms, 2 x 33 ohms inserted, should be connected in series.
C14 - 30 pf, invested 2 to 68 pf, should be included in series.
R16 R8 is selected during adjustment.

The antenna is connected to the lower contact of switch P1.2 (see assembly drawing).

Install jumpers 1-1, 2-2, 3-3, 4-4, 5-5 on the board. Installation according to the scheme and assembly drawing.

Setting up and adjusting the radio station is carried out according to the documentation.

Switch P1.1 and P1.2 are turned on at the same time to enter the transmission mode. Switch P3 in transmit mode enables tone call.

Switch P2 can be of any type, depending on the design of your case.

Resistors type MLT-0.125.

Capacitors type KD, KN, KPK, K50-6.

Assembly drawing printed circuit board 27 MHz radios

Compiled by: Patlakh V.V.

High-frequency generators are designed to produce electrical oscillations in the frequency range from tens of kHz to tens and even hundreds of MHz. Such generators, as a rule, are performed using LC-oscillatory circuits or quartz resonators, which are frequency-setting elements. Fundamentally, the circuits do not change significantly from this, therefore, high-frequency LC generators will be considered below. Note that, if necessary, the oscillatory circuits in some oscillator circuits (see, for example, Fig. 12.4, 12.5) can be easily replaced by quartz resonators.

(Fig. 12.1, 12.2) are made according to the traditional and well-proven in practice "inductive three-point" scheme. They differ in the presence of an emitter RC circuit that sets the operating mode of the transistor (Fig. 12.2) according to direct current. To create feedback in the generator, a tap is made from the inductor (Fig. 12.1, 12.2) (usually from its 1/3 ... 1/5 part, counting from the grounded output). The instability of the operation of high-frequency generators on bipolar transistors is due to the noticeable shunting effect of the transistor itself on the oscillatory circuit. When the temperature and / or supply voltage changes, the properties of the transistor change noticeably, so the generation frequency “floats”. To weaken the influence of the transistor on the operating frequency of generation, it is necessary to weaken the connection of the oscillatory circuit with the transistor as much as possible, reducing the transition capacitances to a minimum. In addition, the change in load resistance noticeably affects the generation frequency. Therefore, it is extremely necessary to switch off the emitter (source) follower between the generator and the load resistance.

Generators should be powered by stable power supplies with low voltage ripple.

Generators made on field-effect transistors (Fig. 12.3) have better characteristics.

Assembled according to the “capacitive three-point” scheme on bipolar and field-effect transistors, are shown in fig. 12.4 and 12.5. Fundamentally, in terms of their characteristics, the “inductive” and “capacitive” three-point circuits do not differ, however, in the “capacitive three-point” circuit, it is not necessary to draw an extra conclusion from the inductor.

In many generator circuits (Fig. 12.1 - 12.5 and other circuits), the output signal can be taken directly from the oscillatory circuit through a small capacitor or through a matching inductive coupling coil, as well as from ungrounded alternating current electrodes of the active element (transistor). In this case, it should be taken into account that the additional load of the oscillatory circuit changes its characteristics and operating frequency. Sometimes this property is used "for good" - for the purpose of measuring various physical and chemical quantities, controlling technological parameters.

On fig. 12.6 shows a diagram of a slightly modified version of the RF generator - a "capacitive three-point". The depth of positive feedback and the optimal conditions for excitation of the generator are selected using capacitive circuit elements.

The generator circuit shown in fig. 12.7, is operable in a wide range of values of the inductance of the coil of the oscillatory circuit (from 200 μGh to 2 H) [R 7 / 90-68]. Such a generator can be used as a wide-range high-frequency signal generator or as a measuring converter of electrical and non-electrical quantities into frequency, as well as in a circuit for measuring inductances.

Generators based on active elements with an N-shaped CVC (tunnel diodes, lambda diodes and their analogues) usually contain

current source, active element and frequency setting element (LC-circuit) with parallel or series connection. On fig. 12.8 shows a diagram of an RF generator on an element with a lambda-shaped current-voltage characteristic. Its frequency is controlled by changing the dynamic capacitance of the transistors when the current flowing through them changes.

LED HL1 stabilizes the operating point and indicates the on state of the generator.

A generator based on an analog of a lambda diode, made on field-effect transistors, and with stabilization of the operating point by an analog of a zener diode - an LED, is shown in fig. 12.9. The device operates up to a frequency of 1 MHz and higher when using the transistors indicated in the diagram.

Ma Fig. 12.10, in order to compare circuits according to their degree of complexity, a practical circuit of an RF generator based on a tunnel diode is given. A forward-biased junction of a high-frequency germanium diode was used as a semiconductor low-voltage voltage stabilizer. This generator is potentially capable of operating in the region of the highest frequencies - up to several GHz.

A high-frequency generator, according to the scheme very reminiscent of Fig. 12.7, but made using a field effect transistor, is shown in fig. 12.11 [RL 7/97-34].

The prototype of the RC oscillator shown in fig. 11.18 is the generator circuit in fig. 12.12.

The note generator is distinguished by high frequency stability, the ability to work in a wide range of changes in the parameters of frequency-setting elements. To reduce the effect of the load on the operating frequency of the generator, an additional stage was introduced into the circuit - an emitter follower made on a bipolar transistor VT3. The generator is capable of operating up to frequencies above 150 MHz.

Among the various schemes of generators, it is especially necessary to single out generators with shock excitation. Their work is based on periodic excitation of an oscillatory circuit (or other resonant element) with a powerful short current pulse. As a result of the "electronic impact" in the oscillatory circuit excited in this way, periodic oscillations of a sinusoidal shape gradually damping in amplitude arise. The attenuation of oscillations in amplitude is due to irreversible energy losses in the oscillatory circuit. The damping rate of oscillations is determined by the quality factor (quality) of the oscillatory circuit. The output high frequency signal will be stable in amplitude if the excitation pulses follow at a high frequency. This type of generators is the oldest among those considered and has been known since the 19th century.

The practical scheme of the generator of high-frequency oscillations of shock excitation is shown in fig. 12.13 [R 9/76-52; 3/77-53]. Shock excitation pulses are fed to the L1C1 oscillatory circuit through the VD1 diode from a low-frequency generator, for example, a multivibrator, or other rectangular pulse generator (GPI), discussed earlier in chapters 7 and 8. The big advantage of shock excitation generators is that they work using oscillatory circuits of almost any kind and any resonant frequency.

Another type of generators is noise generators, the circuits of which are shown in Fig. 12.14 and 12.15.

Such generators are widely used to tune various electronic circuits. The signals generated by such devices occupy an extremely wide frequency band - from units of Hz to hundreds of MHz. To generate noise, reverse-biased junctions of semiconductor devices operating under the boundary conditions of avalanche breakdown are used. For this day, transistor junctions (Fig. 12.14) [Рl 2/98-37] or zener diodes (Fig. 12.15) [Р 1/69-37] can be used. To adjust the mode in which the voltage of the generated noise is maximum, regulate the operating current through the active element (Fig. 12.15).

Note that resistors combined with low-frequency multistage amplifiers, super-regenerative receivers, and other elements can also be used to generate noise. To obtain the maximum amplitude of the noise voltage, as a rule, an individual selection of the most noisy element is necessary.

In order to create narrow band noise generators, an LC or RC filter can be included at the output of the generator circuit.

low frequency generators.

Low frequency generators, or generators low frequencies(LFO), are sources of a sinusoidal signal in different frequency ranges: F<20 Гц (инфразвуковые), 20 Гц... 20 кГц (звуковые), 20...200 кГц (ультразвуковые). Диапазон частот может быть расширен до F>200 kHz. In some types of instruments, along with a sinusoidal signal, a signal is generated called meander.

Rice. 2.1. Structural scheme analog LFO

LFOs are used for a comprehensive study of the paths of radio receivers, for powering AC bridges, etc.

The master oscillator determines the shape and all frequency parameters of the signal: frequency range, frequency setting error, frequency instability, non-linear distortion factor.

If the waveform is not indicated on the front panel of the device, then it is always sinusoidal. Generators of the type rc, whose oscillatory system consists of phasing RC- chains. The entire frequency range of the generator is divided into 3-4 subranges. Each subrange corresponds to a certain value of the resistance of the resistor (Fig. 2.2), which allows you to change the frequency discretely.

Rice. 2.2. The principle of setting the frequency of the master oscillator

Smooth frequency setting is carried out by a variable capacitor, which serves all subbands. Master oscillators of the RC type are simple, cheap, have a low coefficient of non-linear distortion and small overall dimensions.

Oscillator type formula RC:

In some LFOs, discrete frequency control is carried out not by a resistor, but by a capacitor. Then a smooth frequency setting is provided variable resistor-potentiometer. The amplifier weakens the influence of subsequent blocks on the master oscillator, making its frequency parameters better, provides signal amplification in terms of voltage (power) and allows you to smoothly change the output voltage.

The matching transformer is designed for stepped matching of the generator output impedance with the connected load resistance.

The presence of a midpoint (s.t.) in the transformer allows you to get two identical in value, but opposite in phase output voltages (Fig. 2.3).

Rice. 2.3. Electrical circuit diagram center point matching transformer

The output matching transformer is used in generators with an increased level of output power. Most low frequency generators do not have an output transformer.

Load switch provides output impedance matching D out generator with load resistance R n. If coordination is not performed, then the output voltage does not correspond to that set by the generator indicator, the generator may even fail. The most common values D out are 5, 50, 600 and 6000 ohms. To match the resistances at output 1, a special load of 50 Ohm with a cable is supplied with the device.

Output voltage control is provided by an electronic voltmeter U-D type or an electromechanical voltmeter of the rectifier system. The output voltage indicator always shows the RMS value of a sinusoidal signal.

The attenuator ensures that the output voltages are different in value and vary discretely. In this case, the input and output resistances of the attenuator do not change and the matching is not violated. Sometimes attenuation is indicated not in volts, but in decibels.

The attenuation introduced by the attenuator is calculated by the formula:

, (2.2)

where U in(B) - voltage at the input of the attenuator; U out(B) - voltage at the output of the attenuator.

Let's consider two examples.

Example 1. Determine the voltage at the generator output in volts if it is 1 V at the input and U = 60 dB at the output. Based on the formula, we write:

Example 2. Determine the attenuation value introduced by the generator attenuator, if the voltage at its input is 1 V, and at the output 100 mV.

Based on the formula, we write

Digital LFO.

Digital LFOs, compared to analog ones, have better metrological characteristics: lower installation error and frequency instability, lower coefficient of non-linear distortion, stability of the output signal level.

Such generators are becoming more widespread in comparison with analog ones due to higher speed, simplification of frequency setting, elimination of subjective error in setting the output signal parameters. Thanks to the built-in microprocessor in digital LFOs, it is possible to automatically tune the signal frequency according to a given program.

The operation of digital LFOs is based on the principle of generating a numerical code and then converting it into an analog harmonic signal, which is approximated by a function modeled using a digital-to-analog converter (DAC). The block diagram of the digital LFO is shown in fig. 2.4.

Rice. 2.4. Block diagram of a digital LFO

The master pulse generator with quartz frequency stabilization generates short pulses in a periodic sequence, which are fed to the frequency divider. At the output of a frequency divider with an adjustable division ratio, a sequence of pulses is formed with a given repetition period, which determines the sampling step.

The counter counts the pulses arriving at it, the code combination of the pulses accumulated in the counter is fed to the digital-to-analog converter, which generates the corresponding voltage. After overflow, the counter is reset to zero and is ready to start the formation of the next period.

Topic 2.2. RF Signal Generators

High-frequency and microwave generators, or generators of high and microwave frequencies (HF and SHHF), are sources of sinusoidal and at least one signal modulated by any parameter (amplitude-modulated - AM signal, frequency-modulated - FM signal) with known parameters. The waveform at the output of the MHF is shown in fig. 2.5.

Rice. 6.5. Sinusoidal (a) and amplitude - modulated (b) signals at the output of the MHF

If the waveform is not indicated on the front panel of the device, then it is always a sinusoidal and AM signal.

The given signals are characterized the following parameters: f- carrier (modulated) high frequency, F- modulating low frequency, M-coefficient of amplitude modulation.

M=(A-B) 100%/(A+B) (2.3)

GHF and SHHF cover the following carrier frequency ranges: 200 kHz ... 30 MHz (high) and f> 30 MHz (ultra high). The frequency range can be extended up to f< 200 кГц. Такие генераторы применяются для всестороннего исследования высокочастотных трактов теле- и радиоприемных устройств, для питания схем напряжением высоких и сверхвысоких частот. Структурная схема ГВЧ приведена на рис. 2.6.

Rice. 2.6. Structural diagram of the GHF

The master oscillator I determines the value of the carrier frequency and the waveform. A generator of the type is used as a master generator. LC, whose oscillatory system is a parallel circuit consisting of an inductor L and capacitor FROM. The oscillation frequency is expressed by the formula:

(2.4)

The entire frequency range of the GHF is divided into sub-bands, the number of which can be up to eight. Each subrange corresponds to a specific inductor, and smooth frequency setting (within the boundaries of the subrange) is carried out using a variable capacitor. GHF has two outputs: microvolt and one-volt.

From the output of the master oscillator I, the voltage is supplied to two channels: the main and auxiliary. The main channel contains an amplifier-modulator and a high-frequency attenuator (“µV” output). An unmodulated sinusoidal or modulated regulated high-frequency oscillation, calibrated by voltage, is taken from this output. As with the LFO, the indicator shows the RMS value of the sinusoidal voltage.

The auxiliary channel contains an amplifier and a "1V" output. From this output, an uncontrolled, modulated (i.e. sinusoidal), unregulated high-frequency voltage of 1 ... 2 V is removed to the matching load

The AM input is intended for connecting an external modulating oscillator (master oscillator I) when the toggle switch is set to “Ext.” or internal modulating oscillator (master oscillator II) with the toggle switch in the "Int" position. Usually the value of the modulating frequency is fixed (400 or 1000 Hz). If it is not indicated on the front panel, then it is assumed to be 1000 Hz.

A feature of the SHHF is the use of special microwave amplifying devices: klystrons, reverse wave BWO lamps, avalanche-span diodes, Gunn diodes, magnetrons, as well as oscillatory systems on a cavity resonator or a quarter-wave segment of a waveguide, a coaxial line.

At the calibrated output of the SHVCH, the power does not exceed a few microwatts, and at the uncalibrated output - a few watts. In addition to the sinusoidal signal, SHVCH can produce a pulse-modulated signal (PM signal).

Topic 2.3. Pulse signal generators

Pulse generators, or pulse generators (GI), have found application in tuning and regulation pulse circuits used in television and communications, computers, radar, etc. Generators that provide rectangular voltages are widely used. The parameters of the pulse signal can be adjusted over a wide range.

GI is a source of two signals: main and additional (synchronized pulses - SI). The main parameters of these signals, adjustable over a wide range (Fig. 2.7), include U m- amplitude value of voltage, t and- pulse duration, t3- delay time (time shift) of the main pulses in relation to the clock pulses, T- pulse repetition period.

Rice. 2.7. GOP Output Parameters

Indirect (secondary) parameters of GI signals include - duty cycle, which must be ≥ 2 and is calculated by the formula:

, (2.5)

where F = 1/T- pulse repetition frequency.

The block diagram of the GI is shown in fig. 2.8.

>

Rice. 2.8. Structural diagram of GI

The master oscillator generates short pulses with a frequency F and can operate in self-oscillating (key position "1") or standby (key position "2") modes. In the external trigger mode, the pulse repetition rate is determined by an external generator connected to the “Input” jack. One-time start is provided by pressing the button of the external and one-time start device.

The block for the formation of synchronizing pulses (SI) provides the necessary form of SI.

The delay block creates a time shift by time t the main pulses relative to SI coming from the master oscillator.

The block of formation of the main impulses provides receiving on an output of impulses of the necessary form and duration.

The amplifier increases the amplitude of the pulses, allows you to change their polarity and performs resistance matching with the load supplied with the generator.

The attenuator reduces the amplitude of the pulses by a fixed number of times.

The measuring unit is a voltmeter that controls the amplitude value of the pulse signal.

The main metrological characteristics of generators that you need to know when choosing a device include the following:

Waveform;

Adjustment range of parameters;

Permissible error of setting each parameter;

Maximum allowable temporal instability of parameters;

Permissible waveform distortion.

The proposed high frequency generators are designed to produce electrical oscillations in the frequency range from tens of kHz to tens and even hundreds of MHz. Such generators, as a rule, are performed using LC-oscillatory circuits or quartz resonators, which are frequency-setting elements. Fundamentally, the circuits do not change significantly from this, therefore, high-frequency LC generators will be considered below. Note that, if necessary, the oscillatory circuits in some oscillator circuits (see, for example, Fig. 12.4, 12.5) can be easily replaced by quartz resonators.

High frequency generators (Fig. 12.1, 12.2) are made according to the traditional and well-proven in practice "inductive three-point" scheme. They differ in the presence of an emitter RC circuit that sets the operating mode of the transistor (Fig. 12.2) in direct current. To create feedback in the generator, a tap is made from the inductor (Fig. 12.1, 12.2) (usually from its 1/3 ... 1/5 part, counting from the grounded output). The instability of the operation of high-frequency generators on bipolar transistors is due to the noticeable shunting effect of the transistor itself on the oscillatory circuit. When the temperature and / or supply voltage changes, the properties of the transistor change noticeably, so the generation frequency “floats”. To weaken the influence of the transistor on the operating frequency of generation, it is necessary to weaken the connection of the oscillatory circuit with the transistor as much as possible, reducing the transition capacitances to a minimum. In addition, the change in load resistance significantly affects the generation frequency. Therefore, it is imperative to include an emitter (source) follower between the generator and the load resistance.

Generators should be powered by stable power supplies with low voltage ripple.

Generators made on field-effect transistors (Fig. 12.3) have better characteristics.

High-frequency generators assembled according to the “capacitive three-point” scheme on bipolar and field-effect transistors are shown in fig. 12.4 and 12.5. Fundamentally, in terms of their characteristics, the “inductive” and “capacitive” three-point circuits do not differ, however, in the “capacitive three-point” circuit, it is not necessary to draw an extra conclusion from the inductor.

In many generator circuits (Fig. 12.1 - 12.5 and other circuits), the output signal can be taken directly from the oscillatory circuit through a small capacitor or through a matching inductive coupling coil, as well as from the electrodes of the active element (transistor) that are not grounded in alternating current. In this case, it should be taken into account that the additional load of the oscillatory circuit changes its characteristics and operating frequency. Sometimes this property is used "for good" - for the purpose of measuring various physical and chemical quantities, controlling technological parameters.

The generator circuit shown in fig. 12.7, is operable in a wide range of values of the inductance of the coil of the oscillatory circuit (from 200 μH to 2 H) [R 7 / 90-68]. Such a generator can be used as a wide-range high-frequency signal generator or as a measuring converter of electrical and non-electrical quantities into frequency, as well as in a circuit for measuring inductances.

Generators based on active elements with an N-shaped CVC (tunnel diodes, lambda diodes and their analogues) usually contain a current source, an active element and a frequency setting element (LC circuit) with parallel or series connection. On fig. 12.8 shows a diagram of an RF generator on an element with a lambda-shaped current-voltage characteristic. Its frequency is controlled by changing the dynamic capacitance of the transistors when the current flowing through them changes.

The NI LED stabilizes the operating point and indicates the on state of the generator.

On fig. 12.10, in order to compare circuits according to their degree of complexity, a practical circuit of an RF generator based on a tunnel diode is given. A forward-biased junction of a high-frequency germanium diode was used as a semiconductor low-voltage voltage regulator. This generator is potentially capable of operating in the region of the highest frequencies - up to several GHz.

high frequency frequency generator, which is very similar to Fig. 12.7, but made using a field effect transistor, is shown in fig. 12.11 [RL 7/97-34].

The prototype of the RC oscillator shown in fig. 11.18 is the generator circuit in fig. 12.12.

This generator is distinguished by high frequency stability, the ability to operate in a wide range of parameters of frequency-setting elements. To reduce the effect of the load on the operating frequency of the generator, an additional cascade was introduced into the circuit - an emitter follower, made on bipolar transistor VT3. The generator is capable of operating up to frequencies above 150 MHz.

Another type of generators is noise generators, the circuits of which are shown in Fig. 12.14 and 12.15.

Such generators are widely used to tune various electronic circuits. The signals generated by such devices occupy an extremely wide frequency band - from units of Hz to hundreds of MHz. To generate noise, reverse-biased junctions of semiconductor devices operating under the boundary conditions of avalanche breakdown are used. For this, transistor junctions (Fig. 12.14) [Рl 2/98-37] or zener diodes (Fig. 12.15) [Р 1/69-37] can be used. To adjust the mode in which the voltage of the generated noise is maximum, regulate the operating current through the active element (Fig. 12.15).

In order to create narrow band noise generators, an LC or RC filter can be included at the output of the generator circuit.

Literature: Shustov M.A. Practical Circuitry (Book 1), 2003

We considered one of the varieties of generators using an oscillatory circuit. Such generators are mainly used only at high frequencies, but the use of an LC generator can be difficult to generate at lower frequencies. Why? Let's remember the formula: the frequency of the KC generator is calculated by the formula

That is: in order to reduce the generation frequency, it is necessary to increase the capacitance of the master capacitor and the inductance of the inductor, and this, of course, will entail an increase in size.
Therefore, to generate relatively low frequencies, RC generators
the principle of operation of which we will consider.

Diagram of the simplest RC generator(it is also called a three-phase phasing circuit), is shown in the figure:

The diagram shows that this is just an amplifier. Moreover, it is covered by positive feedback (POS): its input is connected to the output and therefore it is constantly in self-excitation. And the frequency of the RC generator is controlled by the so-called phase-shifting chain, which consists of elements C1R1, C2R2, C3R3.
With the help of one chain of a resistor and a capacitor, a phase shift of no more than 90º can be obtained. In reality, the shift is close to 60º. Therefore, to obtain a phase shift of 180º, three chains have to be set. From the output of the last RC circuit, the signal is fed to the base of the transistor.

Operation starts the moment the power supply is turned on. The collector current pulse arising in this case contains a wide and continuous frequency spectrum, in which the required generation frequency will necessarily be. In this case, the oscillations of the frequency to which the phase-shifting circuit is tuned will become undamped. The oscillation frequency is determined by the formula:

In this case, the following condition must be met:

R1=R2=R3=R
C1=C2=C3=C

Such generators can only operate at a fixed frequency.

In addition to using a phase-shifting circuit, there is another, more common option. The generator is also built on a transistor amplifier, but instead of a phase-shifting chain, the so-called Vin-Robinson bridge is used (Vin's surname is spelled with one "H" !!). This is how it looks like:

The left side of the circuit is a passive band-pass RC filter, at point A the output voltage is removed.
The right side is like a frequency-independent divider.
It is generally accepted that R1=R2=R, C1=C2=C. Then the resonant frequency will be determined by the following expression:

In this case, the gain modulus is maximum and equal to 1/3, and the phase shift is zero. If the divider gain is equal to the bandpass filter gain, then at the resonant frequency the voltage between points A and B will be zero, and the PFC at the resonant frequency jumps from -90º to +90º. In general, the following condition must be met:

R3=2R4

But there is only one problem: all this can be considered only for ideal conditions. In reality, everything is not so simple: the slightest deviation from the condition R3 = 2R4 will either lead to a breakdown in generation or to saturation of the amplifier. To make it clearer, let's connect a Wien bridge to the op amp:

In general, this scheme cannot be used in this way, since in any case there will be a spread in the parameters of the bridge. Therefore, instead of the resistor R4, some kind of non-linear or controlled resistance is introduced.
For example, a non-linear resistor: controlled resistance using transistors. Or you can also replace the resistor R4 with a micropower incandescent lamp, the dynamic resistance of which increases with increasing current amplitude. The filament has a sufficiently large thermal inertia, and at frequencies of several hundred hertz it practically does not affect the operation of the circuit within one period.

Wien bridge oscillators have one good property: if R1 and R2 are replaced by variables (but only doubled), then it will be possible to regulate the generation frequency within certain limits.
It is possible to divide the capacitances C1 and C2 into sections, then it will be possible to switch the ranges, and smoothly adjust the frequency in the ranges with a double variable resistor R1R2.

An almost practical circuit of an RC oscillator with a Wien bridge in the figure below:

Here: with switch SA1 you can switch the range, and with a double resistor R1 you can adjust the frequency. Amplifier DA2 is used to match the generator with the load.