input circuits. Input circuits of radio receivers of various ranges
The input circuit of the receiver on the DV, SV and HF bands, as a rule, consists of one or two resonant circuits tuned to the frequency of the received signal and antenna coupling elements with these circuits.
The input circuit of the receiver with inductive coupling with the antenna must be tuned to the frequency range from 3 to 15 MHz. Set the coupling coefficient as close as possible to the allowable value or equal to it, if possible.
The receiver input circuit has a circuit identical to the UHF circuit, so we can assume that the input circuit will have the same characteristic.
The input circuit of the receiver consists of one or two resonant circuits tuned to the frequency of the received signal and antenna coupling elements with these circuits. The connection of the input circuit with the antenna is carried out using a coil or a capacitor. In the first case, the connection is called inductive, in the second - capacitive.
The receiver input circuit usually contains two or three resonant circuits tuned to the frequency of the received signal. These circuits must provide protection from interference from mirror stations and powerful signals generated by transmitters that are part of the receiving and transmitting station. High frequency amplifiers are usually not used, and the mixer directly follows the input circuit.
The input circuit of the receiver is made according to the scheme of inductive coupling of the input circuit with the antenna Antenna coils Lt, Lk, Lt, C and Lt2 have their own resonant frequency below the lower frequency of the received range, which makes it possible to obtain a uniform gain over the range.
The input circuit of the receiver is made according to the scheme of inductive coupling of the input circuit with the antenna. The antenna coils L, Lt, L, La and Lit have a natural resonant frequency below the lower frequency of the received band, which allows you to get a uniform gain over the band. The wound coils and the coils of the input circuits of the non-working ranges are short-circuited.
The input circuit of the receiver is made according to the scheme of inductive coupling of the input circuit with the antenna. Antenna coils L, L4, Lt, Ls and L12 have a natural resonant frequency below the lower frequency of the received band, which allows you to get a uniform gain over the band. The antenna coils and coils of the input circuits of idle bands are short-circuited.
The receiver input circuit can serve as an example of a practical application of a series circuit.
The input circuit of the receiver on the DV, SV and HF bands, as a rule, consists of one or two resonant circuits tuned to the frequency of the received signal, and antenna coupling elements with these circuits.
Schematic diagram of a dual-band direct-gain broadcast receiver with a regenerative stage. The input circuit of the receiver is assembled according to the scheme of inductive-capacitive coupling with the antenna.
The input circuit of microwave receivers, being the first element, mainly determines its noise properties, therefore one of the main requirements for it is to ensure the maximum excess of the useful signal level over the noise level. The noise voltage developed in the input circuit is determined by its resonant impedance. When the connection is close to optimal, which ensures the mode of matching the antenna circuit with the input circuit, the useful signal voltage becomes maximum, and the noise voltage remains almost unchanged. To achieve optimal coupling, the active resistance introduced into the circuit from the antenna must be equal to the intrinsic resistance of the circuit loaded with the input impedance of the amplifying device.
Scheme input circuits. The input circuits of the receiver are the circuits connecting the receiving antenna with the first amplifying stage. The purpose of the input circuits is to transfer the signal voltage from the antenna to the grid of the first lamp and attenuate the interference voltage.
The input circuits of the receiver are the circuits connecting the receiving antenna with the first amplifying stage. The purpose of the input circuits is to transfer the signal voltage from the antenna to the grid of the first lamp and attenuate the interference voltage. Exist various schemes input circuits. The scheme shown in fig. 176, a, is called a capacitive coupling circuit with an antenna. The receiving antenna is usually not tuned to the frequency of the received signal.
The input circuits of the receiver are the circuits connecting the receiving antenna with the first amplifying stage. The purpose of the input circuits is to select a useful signal from a variety of signals of different frequencies acting on the receiving antenna and transmit it to a high-frequency amplifier.
A feature of the input circuit of the receivers is that only the coil of the input circuit of the CB range is placed on the ferrite rod of the magnetic antenna, which is made in the form of two separate coils LI and Z.
In the input circuit of the receiver, a useful signal is selected and the signals of other stations are preliminarily attenuated. The UHF high-frequency amplifier amplifies the signals coming from the input circuit and further attenuates the signals of interfering stations.
The task of the input circuits of meter and decimeter wave receivers as their first elements is not only to amplify the signal received from the antenna, but also to ensure the greatest excess of the useful signal level over the noise level. This is achieved by serial matching of the antenna with the feeder and the feeder with the receiver input.
By then designing the input circuit of the receiver in such a way that its input impedance is also purely active and close to 150 ohms, it is possible to provide a matching mode in the entire signal transmission path, induced in the antenna, to the input of the car receiver.
Amplifier circuit type grounded cathode - grounded grid. In the input circuits of highly sensitive receivers, it is desirable to use triodes, which are distinguished by a much lower noise level than pentodes. However, due to the large capacitance of the grid - the anode, triodes can operate in conventional circuits with a grounded cathode only when this capacitance is neutralized. If we take into account that neutralization is stable only at low gains of the first stage, then a new difficulty arises - the implementation of the second stage of the amplifier with a low noise level.
In the input circuits of the meter range receiver, oscillating circuits with transformer or autotransformer coupling are used. Coordination in these circuits is achieved by selecting the degree of coupling, which also determines the value of the voltage of the useful signal and the signal-to-noise ratio.
In the input circuits of the meter range receiver, oscillating circuits with transformer or autotransformer coupling are used. Coordination in these circuits is achieved by selecting the degree of coupling, which determines the value of the voltage of the useful signal and the signal-to-noise ratio.
Having established that the input circuit of the receiver is working, you should reduce output voltage GSS to 0 1 - 0 2 p and connect the LV between the grid socket of the second stage lamp and the chassis.
Tuning the local oscillator and the receiver input circuit without special measuring equipment is the most time-consuming operation and requires special attention.
The antenna is connected to the input circuit of the receiver and introduces active and reactive resistance into it.
Usually, fluctuations in the input circuits of the receiver consist of individual pulses, but these pulses occur one after the other so often that non-stationary phenomena in the receiver from individual pulses are superimposed on each other. At in large numbers overlapping transient processes, the central limit theorem of probability theory can be applied to their sum. This theorem says that the law of distribution of the sum of independent random variables having the same distribution functions tends to normal as the number of terms increases, regardless of what the law of distribution of terms is. Just such a case is the sum of a huge number of thermal fluctuations and the non-stationary processes caused by them.
In what cases - use the inductive-capacitive coupling of the antenna with the input circuit of the receiver.
The signal under investigation iic, mixed with the noise of the input circuits of the receiver, and the noise (signal) of the reference noise generator OG are periodically applied. In a synchronous LED detector, the reference oscillator noise is subtracted, which reduces the influence of the receiver's own noise on the measurement result.
Input circuits of radio receivers
Chapter 3 Input Circuits
radio receivers
Purpose of input circuits
The input circuits (IC) of a radio receiver are called circuits connecting
antenna-feeder system with the first amplifying or converting
receiver cascade.
The main purposes of the CC are:
transmission of the received signal from the antenna to the input of these stages;
preliminary filtering of external noise.
Usually, the VC is a passive quadripole containing oscillatory circuits. Single-loop CCs are the most widely used. Multi-loop input circuits are used only when there are high demands on the selectivity of the input circuit.
Typical schemes of input circuits 4.1 - 4.4 shows some common circuits of single-circuit input circuits. The schemes differ in the ways in which the input circuit is connected to the antenna. On fig. 4.1 shows diagrams with a transformer connection between the LKCK circuit and the antenna.
Rice. 4.1. Schemes with transformer connection of a single-circuit input circuit with an antenna: a - with a bipolar transistor; b - with a field effect transistor Input circuits of radio receivers In the circuits in fig. 4.2, the capacitive coupling of the input circuit with the antenna is used, and in the circuits in fig. 4.3 the input circuit is connected to the antenna feeder through an autotransformer.
The connection of the input circuit to the active element can be full or partial, depending on input impedance last having a low input impedance bipolar transistor usually connected partially, the field one can be fully turned on.
Rice. 4.2. Schemes with capacitive coupling of a single-loop input circuit with an antenna 4.3. Schemes with transformer connection of a single-circuit input circuit with a feeder 4.4 shows one of the most common schemes for a two-circuit CC.
Here, the connection of the primary circuit with the antenna is transformer.
The connection between the circuits is intracapacitive through the capacitor Csv1 and external capacitance through Fig. 4.4. Scheme of a two-circuit input circuit Csv2.
Input circuits of radio receivers A two-circuit VTS allows to obtain a frequency response closer to a rectangular one, therefore, to increase selectivity.
The main parameters of the input circuits The main electrical characteristics of the computer center are:
1. Voltage transfer coefficient, which is determined by the ratio of the signal voltage at the input of the first active element of the receiver (Uin) to the signal EMF in the EA antenna, and in the case of a magnetic (ferrite) antenna, to the signal field strength.
2. Bandwidth - the width of the frequency region with an acceptable transfer coefficient unevenness.
3. Selectivity, which characterizes the decrease in the voltage transfer coefficient for a given detuning K(f) compared to the resonant value K0.
The input circuit, together with the URCH, provides the specified selectivity of the receiver over the image channel and the intermediate frequency channel, as well as general preliminary noise filtering.
4. Overlapping the specified frequency range. The input circuit must provide the ability to tune to any frequency of a given range of the receiver, and at the same time its performance (transmission coefficient, bandwidth, selectivity, etc.) should not change noticeably.
The operating frequency range is characterized by the range overlap coefficient kd, equal to the ratio of the maximum tuning frequency to the minimum.
5. The constancy of the parameters of the input circuit when changing the parameters of the antenna and the active element. This is important for untuned antennas, which introduce active and reactive resistance into the CC. The introduced active resistance increases the losses of the CC, which leads to an expansion of the bandwidth and a deterioration in selectivity. The introduced reactance leads to a change in the setting of the CC.
Receiving Antenna Equivalents The receiving antenna equivalent circuit can be represented as a generator with EMF EA (or current IA) and internal resistance ZA.
The internal resistance of the EMF generator in general case contains active and reactive components, i.e. ZA = RA + jXA. If the antenna provides signal reception in the frequency range, then such an antenna contains a reactive component of resistance and is called untuned in the theory of radio reception. When the receiver is operating at a fixed frequency, tuned antennas are used. The internal resistance of a tuned antenna is purely active.
The internal impedance of untuned antennas depends in a complex way on frequency. However, if the dimensions of the antenna are small compared to the wavelength, then for the ranges of long, medium and short waves (LW, MW, HF), relatively simple antenna equivalents can be selected. So, for the LW and MW ranges, the equivalent antenna resistances can be represented as a first approximation in the form of capacitance CA, and the value of EA can be found from the expression where E is the magnitude of the electric component of the signal field at the reception point; lA is the effective antenna height.
Equivalent circuit of the input circuit The equivalent circuit of a single-loop input circuit is shown in fig. 4.5. Here, the antenna-feeder circuit is represented by a current generator IA with conductivities GA and BA, which generally include the parameters of the antenna-loop coupling elements. The input of the first active element of the receiver, together with the bias circuits, is represented by conductivities Gin and Bin.
The diagram shows the autotransformer connection of the circuit to the antenna circuit and to the input of the active element with transformation ratios m and n.
The equivalent circuit can be converted to the form shown in Fig. 4.6, if you recalculate the input and output parameters to the oscillatory circuit.
Rice. 4.6. Converted equivalent circuit of the input circuit Here, the recalculated parameters are GA = m2GA; BA = m2BA; Gin= n2 Gin;
Bin = n2 B in.
As can be seen from the equivalent circuits, the total capacitance of the circuit includes the capacitances of the antenna and the active element recalculated to the oscillating circuit, and the total active conductivity of the circuit includes the recalculated conductivities of the antenna and the active element. Consequently, the parameters of the antenna and the input conductivity of the active element affect the tuning frequency and the quality factor (selectivity) of the input circuit circuit.
Input circuits when working with non-tuned antennas Input circuit with capacitive coupling to the antenna 4.7.
Rice. 4.7. Capacitively coupled antenna and CC equivalent The input circuit consists of a coupling capacitance Cb and a circuit tuned to the frequency of the received signal by a variable capacitor C.
The value of the capacitance Cw is chosen small and amounts to units of picofarads. This is done because the receivers of the MW, LW ranges, as a rule, work with non-standard antennas, the value of the parameters of which can vary within wide limits. In order for the antenna to have little effect on the CC, its connection with the input circuit loop is made weak.
The module of the resonant transmission coefficient of the CC where EC is the equivalent attenuation of the circuit;
L is the inductance of the circuit.
The quadratic dependence of K 0 on the tuning frequency of the CC is explained by the fact that each of the quantities that determines the voltage at the output of the input circuit (current I A and the equivalent resistance of the circuit) is proportional to the value f0.
The change in the resonant transmission coefficient of the input circuit with capacitive coupling with the antenna over the range is shown in fig. 4.8.
Fig 4.8. Changing the modulus of the resonant transmission coefficient of the input circuit with capacitive coupling with the antenna during tuning; HDR VkhU - the operating frequency range of the input device The selective properties of the CC at large mismatches are determined by the contour of this circuit (Fig. 4.9).
Rice. 4.9. Changing the bandwidth in the frequency range When the circuit is tuned in the frequency range from f0min to f0max, if the attenuation of the CC circuit is unchanged in the operating frequency range, the amplitude-frequency characteristic of the circuit expands, which leads to a deterioration in the selective properties of the CC.
Antenna inductively coupled input circuit Elements CA and L1 form an antenna circuit.
Rice. 4.10. Equivalent of antenna and CC with inductive coupling The magnetic coupling M between the input circuit and the antenna circuit is chosen to be weak, so the resistances introduced from one circuit to another can be neglected in the first approximation.
CC transfer coefficient module There are two types of CC with inductive coupling. If fA f0 max, the input circuit is called an input circuit with a shortened antenna, if fA f0 min - an input circuit with an extended antenna. Here, f0 min and f0 max are, respectively, the maximum and minimum operating frequencies of the EC tuning range.
The case when fA is in the middle of the EC tuning range usually does not occur in practice. This is due to the significant non-uniformity of the CC transmission coefficient in the operating frequency range, when the resonant frequency of the antenna circuit is within its limits.
Consider two limiting cases: fA f0 max. and fA f0 min.
In the first case, with a strong shortening of the antenna, the value of K is proportional to 0. This is explained by the fact that the current I A with a strong shortening is proportional to the value of 0 and the EMF EWH introduced into the CC circuit is proportional to the square of the frequency.
In the second case, i.e., with a strong extension of the antenna, the current in the antenna circuit is inversely proportional to the frequency of the input signal. Therefore, the EMF introduced into the EC circuit turns out to be independent of the signal frequency. Consequently, the voltage at the output of the VC and the transmission coefficient are constant in the frequency range of the VC tuning, provided that the attenuation of the circuit is constant, which in most cases is an advantage.
Figure 4.11. Changing the modulus of the resonant transmission coefficient of the input circuit with inductive coupling with the antenna during restructuring; on the left - with a shortened antenna, on the right - with an elongated antenna. For this reason, such a relationship is rarely used in practice.
The selective properties of the input circuit with inductive coupling with the antenna are determined by the selective properties of the loaded circuit of this circuit.
For large detunings, the influence of the antenna circuit should also be taken into account.
Input circuit with inductive-capacitive coupling with antenna
capacitive and inductive. The circuit is linear, so the gains are summed. With inductive coupling, an "elongated" antenna with a small "extension" is used. Due to capacitive coupling, the transmission coefficient of the input circuit increases with increasing tuning frequency, and decreases due to inductive coupling.
Rice. 4.12. The input circuit and the equivalent of the antenna in inductive-capacitive coupling Joint electrical and magnetic coupling leads to the fact that the modulus of the resonant transmission coefficient weakly depends on the tuning frequency of the input circuit, and in magnitude it turns out to be much larger than in the case of inductive coupling with a strong "shortening" of the antenna . This type of CC finds wide practical application.
Adjusting the input circuit To obtain a given overlap ratio on all subranges in the circuits, additional capacitors C1 and C2 are used (Fig.
4.13), which reduce the effect of capacitance Sk on the tuning frequency of the circuit and, therefore, reduce the overlap ratio.
Rice. 4.13. Obtaining a predetermined range overlap coefficient In connection with the microminiaturization of equipment for tuning circuits, varicaps are usually used instead of bulky mechanical blocks of variable capacitors (KPIs). The main advantage of varicaps is their small size, mechanical reliability, ease of automatic and remote tuning control.
Schemes for including varicaps in an oscillatory circuit are shown in fig.
4.14. Regulating voltage to the diodes is supplied by a potentiometer from a stabilized source. The resistor is needed to reduce the shunt effect on the resonant circuit of the tuning control circuit.
The disadvantage of varicaps in comparison with KPI is non-linearity at high levels of signals and noise. Nonlinear effects can be weakened by a well-known technique - the use of balanced (push-pull) circuits. In this case, such a scheme is the counter-sequential inclusion of two varicaps (figure on the right).
To tune the circuits with the help of varicaps, it is advisable to use not mechanical voltage regulators with sliding contacts, but electronic sources of regulating voltage - voltage synthesizers. They contain a pulse generator and a digital-to-analogue converter (DAC), which is a source of a stepped tuning voltage of the oscillatory circuit. The control device for adjustment may contain a microprocessor (MP), memory and software devices, as well as manual controls.
Switching of subranges can be carried out both by switches with sliding contacts, and by electronic keys, which are switching diodes. Sliding contact switches have low reliability and make automatic and remote control of switching difficult. Electronic switches on switching diodes are simple and reliable, they allow automatic and remote control entirely on electronic components. Their disadvantage is the non-linearity of the diode switches with strong interference, which leads to a deterioration in multi-signal selectivity.
Switching subbands - more difficult task than electronic tuning. Therefore, radio equipment designers try not to use subband switching in the input circuits. Such a possibility appears with the infradyne construction of the receiver, when the first intermediate frequency is chosen above the maximum frequency of the receiver range and the image channel is located far beyond the range of received frequencies. As an input circuit in such receivers, non-tunable low-pass filters are usually used.
Input circuits with a tuned antenna Tuned antennas are used, as a rule, for reception at meter and shorter waves, as well as for professional reception at decameter waves, for example, on trunk communication lines. In these cases, there are usually high demands on the sensitivity of the receiver, which is limited by its own noise, so it is important to ensure the best signal transmission from the antenna to the input of the RF.
The transmission coefficient has a maximum value when matching the antenna with the feeder, and the feeder with the receiver input. In this case, a traveling wave mode takes place in the feeder, which is also necessary to eliminate signal distortions caused by reflections at a large feeder length.
Matching the feeder with the input of the receiver and obtaining a given resulting attenuation are achieved by choosing the transformation ratios. The resonant transfer coefficient at matching is determined by the expression Here, the parameter D determines the allowable increase in the attenuation of the input circuit circuit due to its load with the active conductivity of the antenna and the input conductivity of the active element: becomes big. In this case, the limiting gain of the input circuit is determined only by the antenna resistance and the input conductivity of the active element.
Tuned antennas usually have a wide enough bandwidth that changes in their impedance due to detunings can be ignored. Then the resonant curve of the input circuit will depend mainly on the characteristics of the equivalent circuit.
In addition to the matching mode, the optimal mismatch mode is used, which provides the minimum noise figure of the receiver and, consequently, its highest sensitivity. Usually, the calculation of the input circuit is carried out in the matching mode, and the optimal mismatch is achieved when tuning the receiver by slightly increasing the connection between the circuit and the antenna.
Input circuits of microwave receivers As well as at moderately high frequencies, the input circuits of the microwave range have the character of resonant systems or filters, however, the specificity of the microwave range makes their circuit and, most importantly, constructive implementation completely different, since at frequencies above 300 ... 500 MHz they are performed at elements with distributed parameters.
According to the method of implementation, microwave resonators are divided into planar and volumetric. Planar resonators are based on transmission lines various types: asymmetric and symmetrical microstrip (MPL), slot, coplanar, etc. In the decimeter range, segments of coaxial lines are used. Structurally, the resonator can be short-circuited or open-circuited at the end. With an electrical length less than a quarter of the wavelength, a short-circuited segment is equivalent to an inductance, and an open segment is equivalent to a capacitance. Lines whose length is a multiple of a quarter of the wavelength become resonant and equivalent to a parallel or series circuit, respectively. Short-circuited resonators on microstrip lines are small in size, have low radiation losses, and have a relatively high quality factor (Q = 200...300), but due to the presence of a short circuit, they are technologically more difficult. Easy-to-manufacture resonators open at the end have a lower quality factor (Q ≈ 100) due to radiation losses.
In the microwave path, the resonators are connected according to the two- or four-terminal circuit. On their basis, composite resonators can be formed. In addition to rectangular, flat resonators are also used in round, elliptical and annular shapes. The frequency tuning of flat resonators can be mechanical - by changing the dimensions, as well as electrical, usually with the help of a varicap included in the resonator.
The higher-quality cavity resonators used in the RPRU microwave are divided into solid-state and hollow. Solid-state resonators are small volumes of dielectric or ferrite in which the volume resonance of the electromagnetic field takes place. Dielectric resonators are discs, cylinders, bars, rings, etc., the shape, dimensions and permittivity of which are chosen so that they fulfill the conditions of electromagnetic resonance at a given frequency due to the phenomenon of total internal reflection of an electromagnetic wave.
Such resonators are very compact. The intrinsic quality factor of dielectric resonators at centimeter wavelengths reaches several thousand and can be further increased by cooling.
Dielectric resonators are included in the microwave path in two ways.
In the first method, the resonator is connected between two uncoupled transmission lines, for example, located orthogonally or separated by a section of the waveguide. At the resonant frequency, the dielectric resonator is excited, and the lines are coupled by its field. In the second method, the dielectric resonator is located outside the main path and is connected to it by electromagnetic fields. At the resonant frequency, the dielectric resonator is excited, its reradiated field compensates for the field of the incident wave, and a standing wave appears in the path.
Away from resonance, the dielectric resonator is not excited, and all the power in the path goes to the load. Tuning the frequency of the dielectric resonator is achieved by introducing into it electric field metal or dielectric bodies.
The most typical ferrite resonator is a carefully polished sphere with a diameter of 0.3 ... 1 mm from a single crystal of yttrium iron garnet (YIG), placed in the center of two orthogonally located communication loops, the plane of which coincides with the direction of the constant magnetizing field. Each of the loops is connected at one end to the inlet (outlet) line, and its other end is grounded via microwave using a quarter-wave segment. At a certain combination of external magnetic and microwave fields in such a sphere due to physical properties ferrite, a ferromagnetic resonance occurs, and if, due to the orthogonality of the location of the coupling loops, there is no connection between the input and output of the ferrite resonator, then at resonance, the microwave energy is transferred through the YIG sphere from the input to the output.
One of the main advantages of ferrite resonators is the possibility of obtaining high Q-factors (Q = 104) up to millimeter waves, and this is the only type of microwave resonators whose resonant frequency does not depend on size, but is determined only by the strength of the constant magnetizing field. Another important advantage of the PR is the possibility of a very wide-range tuning of the resonant frequency by changing the magnetic field strength.
The disadvantage of the PR is the strong dependence of the resonant frequency on temperature.
Hollow cavity resonators are currently used extremely rarely in radio receiving equipment due to poor weight and size indicators and low design and technological compatibility with other RPR units and units made using integrated technology. Their advantages include the possibility of implementing very high quality factors, as well as reliable shielding from the effects of external electromagnetic fields. Regular cavity resonators are segments of waveguide or coaxial transmission lines closed at the ends. There are also radial, spiral resonators, traveling wave ring resonators, etc. Connections of segments of transmission lines, including various types and in combination with elements with lumped parameters, form resonators of complex shape.
The most common components of CC microwave receivers are a variety of filters that differ in the type of electrical characteristics (bandpass, notch, high frequencies).
Bandpass and notch filters on microstrip lines are most widely used in microwave RPR. The simplest bandpass filters on microstrip lines are half-wave open resonators connected in series through end capacitances (Fig. 4.15).
The bandwidth of these PFs is determined by the width of the gaps between the resonators: the smaller the gap, the stronger the coupling and the wider the P band.
Rice. 4. 15. Designs of input circuits on microstrip lines 4.16 and 4.17 shows options for connecting resonators on coaxial lines and cavity resonators.
To eliminate the effect of changes in the complex output impedance of the antenna-feeder path on the characteristics of the first stage of the radio receiver, non-reciprocal ferrite devices - valves or circulators - are included between the path output and the stage input.
Fig.4.16. Input devices of decimeter range receivers on coaxial resonators with communication with the antenna using a loop (a), probe (b), direct contact (c) 4.17. Input devices of centimeter-range receivers on cavity resonators with communication with the antenna using: diaphragm (a), loop (b), probe (c) When the receiver and transmitter work with a common antenna, antenna switches, switches on p-i-n diodes or gas dischargers and limiters are used . In some cases, controlled attenuators are used.
Main Conclusions The input circuit should most fully transfer the signal energy from the antenna to the first stage of the receiver and carry out preliminary filtering of the signal from interference.
The input circuit contains a filter and a filter connection circuit with an antenna and an amplifying element of the subsequent stage.
The voltage gain of the input circuit can be defined as the product of the gains of the filter-to-antenna coupling circuit, the equivalent filter, and the filter-to-amplifier coupling circuit of the next stage.
The frequency response, phase response and selectivity of the input circuit are determined mainly by the frequency response, phase response and the selectivity of the resonant circuit and depend on its equivalent attenuation.
The equivalent attenuation of the input circuit loop is determined by its design attenuation and the insertion attenuation from the side of the antenna circuit and from the side of the subsequent stage.
The maximum transmission coefficient of the input circuit is provided with optimal coupling of the antenna circuit and the input of the subsequent stage with the circuit, at which the attenuation introduced into the circuit from the antenna circuit is equal to the attenuation from the subsequent stage. To obtain K0max, the circuit must have low losses.
The change in the resonant gain of the input circuit over the range is mainly determined by the change in the gain of the coupling circuit of its filter with the antenna.
With capacitive coupling with the antenna and with capacitive tuning of the input circuit, the transmission coefficient changes sharply over the range, therefore this type of communication is used in inexpensive receivers, with a small range overlap factor, etc.
A relatively small change in K0 in the operating frequency range can be obtained with a transformer connection with an "extended" antenna.
In the input circuit, it is possible to carry out matching in signal power corresponding to the maximum gain, and matching in noise, in which the noise figure of the receiver is minimal.
If the noise of the VRF is large compared to the noise of the input circuit, then the minimum noise figure of the input circuit, together with the VRF, is obtained almost at optimal coupling.
The improvement in noise figure over its optimal coupling value is proportional to the proportion of gain element noise in the overall circuit noise. If the RF noise is small, then the minimum noise figure is obtained with a connection stronger than necessary for signal power matching.
At frequencies below 100 MHz, the input circuit is contoured on lumped LC elements. In wavelength ranges shorter than 1 m, circuits with distributed parameters are used as the oscillatory circuit of the input circuit. In the range of decimeter wavelengths, segments of coaxial or strip lines are most widely used.
The use of strip lines makes it possible to make nodes and resonant circuits of the receiver in a single technological cycle using integrated technology. In the range of centimeter and shorter wavelengths, along with strip lines, cavity resonators are used as selective systems of input circuits.
Control questions 1. Indicate the purpose and list the main characteristics of the VC.
2. Why is tuning the VC circuit with a variable capacitance more preferable than tuning with a variable inductance?
3. Draw CC diagrams with different types connection of the circuit with the antenna and explain the purpose of the elements.
4. Make the equivalent circuits of the CC with different types of connection of the circuit with the antenna.
5. What parameters determine the transmission coefficient of the CC?
Conditions for obtaining the maximum transmission coefficient of the CC?
6. Conditions for matching the antenna with the input of the receiver?
7. From what considerations is the connection of the input circuit with a tuned antenna selected? Communication schemes?
8. From what considerations is the connection of the input circuit with an untuned antenna selected? Why? Communication schemes?
9. What determines the selectivity of VC?
10. What determines the bandwidth of the CC?
11. How is the connection of the input circuit with the active element selected?
12. List the main types of cavity resonators and microwave filters.
used as CC RPR. Describe their advantages and disadvantages.
13. What are the considerations when choosing the type of frequency response (maximum flat, equal-wave, elliptical) input filters for microwave RPR?
Tasks for self-control 1. The receiver preselector is tuned by a variable capacitor with a capacitance of 16 pF ... 318 pF.
Loop inductance 0.273 mH, mounting capacitance 20 pF.
Calculate the extreme frequencies of the receiver's tuning range.
2. In a single-circuit selective amplifier, the capacitance of the circuit is increased, and the inductance of the circuit is reduced by the same number of times.
How will the resonant gain and bandwidth change (decrease or increase) if the quality factor of the circuit and the circuit parameters have not changed?
3. Why is one of the capacitors of the circuit made trimmer in the range RPRU? Why is it necessary to provide for some change in the inductance of the loop coil?
4. Calculate the equivalent quality factor of the input circuit circuit for the broadcast RPR based on the required bandwidth, which in the MW band should be at least 9 kHz, and the frequency selectivity at the extreme frequencies of the passband (540 - kHz) does not exceed 3 dB.
References 1. Kolosovsky EA Devices for receiving and processing signals. Textbook for universities. - M: Hotline-Telecom, 2007. - 456 p.: ill.
2. Radio receivers: Textbook for universities / N. N. Fomin, N. N.
Buga, O. V. Golovin and others; Edited by N. N. Fomin. - 3rd edition, stereotype. - M. : Hot line - Telecom, 2007. - 520 p.: ill.
3. 1. Microelectronic microwave devices / G.I. Veseloye, E.N. Egorov, Yu.N.
Alekhin and others; Ed. G.I. Veselov. - M: Higher school, 1988. - 4. 2. Solid-state microwave devices in communication technology / L.S. Gassanov, A.A.
Lipatov, V.V. Markov, II.A. Mogilchenko. - M.: Radio and communication, 1988. - 5. 3. Microelectronic microwave devices / N.T. Bova, Yu.G. Efremov, V.V.
Konin et al. K.Tekhnika, 1984. 184p.
6. 4. Plaksneiko V.S. Devices for receiving and processing signals. Tutorial. Taganrog: Izd-vo TRTU 1999. 108 p.
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Page 1 of 3
Objective: experimental study of the main properties of input circuits operating from tuned and non-tuned antennas.
INTRODUCTION
In this paper, we study two types of input circuits.
1. Input circuits operating with an untuned antenna: with inductive coupling with the antenna, with external and intracapacitive coupling with the antenna. These schemes are collected in the upper left part of the layout.
2. Input circuit operating with a tuned antenna (the circuit is assembled in the upper right part of the plug-in).
1. BRIEF THEORETICAL INFORMATION
The input circuit is designed to transmit the received signal from the antenna to subsequent receiver circuits. The main functions of the input circuit:
a) pre-filtering the received useful signal from the set of signals coming from the antenna;
b) useful signal energy transfer to the input of the first stage with the least losses and distortions.
In general, the input circuit is a passive four-pole , including one oscillatory circuit or a system of oscillatory circuits, as well as elements of connection of this circuit with the antenna and the output of the next cascade .
1.1. Main characteristics of the input circuit
Resonance Gain - the ratio of the signal voltage at the output of the input circuit Uout to the value of the EMF E A , induced in the antenna by the electromagnetic field of the received signal:
In this case, the carrier frequency of the signal f c must be equal to the tuning frequency of the input circuit f 0 . When designing the input circuit, they strive to ensure a minimum change in K 0 in the operating frequency range.
Operating frequency range (fmin - fmax) - frequency range , within which the input circuit can be tuned to resonate with the frequency of the received signal. The restructuring of the input circuit is carried out by changing the capacitance or inductance of the circuit.
Frequency selectivity the input circuit is the ability to separate the useful signal and weaken the signals of interfering stations; it is determined by the amplitude-frequency characteristic of the input circuit K(f) .
In superheterodyne receivers, the main task of the input circuit is to provide selectivity for side reception channels, the most dangerous of which are the mirror (symmetric) channel and the forward channel (Fig. 1). The mirror channel f c differs from the main channel f c by two intermediate frequencies:
Usually the input circuit is a linear circuit, and its selectivity can be determined from the resonant characteristic (Fig. 2), while the selectivity value
When U in = const
or in decibels, also with U in = const.
However, with large detunings or high selectivity of the input circuit, the measured values of Uout become small and can be equal to or less than the noise level. In this case, measurements become impossible or unreliable.
In non-linear circuits, with a significant decrease in the signal level, their transfer coefficient changes, which leads to an error in measuring frequency selectivity. Therefore, the measurement of the selectivity of the entire receiver (incorporating a nonlinear element - a detector, the transmission coefficient of which depends on the signal level) is carried out according to a transposed (inverted) characteristic, which is called the selectivity characteristic (Fig. 3). Wherein
When U out \u003d const.
The selectivity of the range input circuit should be measured at those points in the range where this selectivity is the smallest. Image selectivity is measured in the upper part of the range
|
|
|
|
received frequencies, where the bandwidth is the widest, so the selectivity is the lowest. Selectivity through the forward channel is measured at frequencies closest to the intermediate frequency: at f pr \u003d 465 kHz, this will be the upper frequency of the LW range - f max and the lower frequency of the MW range - f min .
1.2. Input circuit classification
Input circuits can be fixed loop or ranged (tunable). According to the number of circuits used, there can be single-, double- and multi-circuit input circuits. According to the type of connection with the antenna, the input circuits are divided into two types:
Input circuits working with untuned antennas;
Input circuits working with tuned antennas.
1.3. Input circuits with untuned antennas
Untuned antennas are called antennas, the resistances of which have active and reactive components. It is these antennas that are usually used in the LW, MW bands. Antenna reactances for these frequencies are capacitive in nature (Fig. 4).
In this case, the connection of the input circuit circuit with the antenna is chosen to be weak in order to exclude the influence of the antenna parameters on the circuit (the reactive component of the antenna changes the tuning frequency of the circuit, the active component expands its bandwidth and reduces selectivity).
With a weak connection with the antenna, the transmission coefficient of the input circuit is small, but this is acceptable, since in receivers DV, MW sensitivity is mainly determined by external interference, and a decrease in the transmission coefficient of the input circuit can be compensated in the URF and IF.
1.3.1. Antenna inductively coupled input circuit
The scheme of the input circuit with inductive coupling with the antenna is shown in fig. 5. The main circuit that determines the parameters of the input circuit is the circuit L K C K , which is tuned to the frequency of the received signal. The coupling coil L St and the antenna capacitance C A form a series oscillatory circuit (antenna circuit), the tuning frequency of which is unchanged and different from the tuning frequency of the main circuit f 0 . If f A is less than the minimum frequency of the received frequency range f 0 min (wavelength λ A > λ 0 min), then the input circuit is called the input circuit with an "extended antenna" (Fig. 6, a). If f A is greater than the maximum frequency of the range f 0 max (wavelength λ A< λ 0 max), то входная цепь называется входной цепью с "укороченной антенной" (рис. 6, б).
Input circuit resonant gain
Where E A is the emf in the antenna.
The current in the antenna circuit I a induces an EMF in the coil of the main circuit, the value of which is determined by the known relation
where is the mutual inductance between L st and L K.
To St. - coupling coefficient between the coils L St and L K.
At the tuning frequency of the main circuit, the voltage in Q e times the induced emf:
where Q e - equivalent quality factor of the main circuit (we consider Q e frequency independent).
Thus, the dependence of the resonant transmission coefficient K 0 from frequency is determined by the product of I A (f) and Z St (f) (Fig. 6).
In the case of an elongated antenna, with increasing frequency, I A decreases, and Z w increases, as a result of which the resonant gain K 0 weakly depends on frequency (Fig. 6, a).
In the case of a shortened antenna, I A increases with increasing frequency. and Z sv, as a result of which the resonant gain K 0 depends sharply on the frequency (increases approximately according to a quadratic law) (Fig. 6, b).
The values of the active and reactive components introduced from the antenna into the main circuit depend on the degree of connection between the circuits K St.
1.3.2. Input circuit with external and intracapacitive coupling
External capacitance connection with the antenna (Fig. 7, a) is designed to reduce the influence of the antenna parameters on the input circuit circuit. This connection is made weak (with St. « C A) so that the antenna capacitance does not upset the input circuit circuit, and the active resistance of the antenna r A does not expand its bandwidth and does not worsen selectivity.
Input circuit resonant gain:
C a - antenna capacitance;
With sv - capacitance of the coupling capacitor ;
When tuning the input circuit with a capacitor of variable capacitance, the resonant transfer coefficient is proportional to the square of the frequency, if the quality factor of the circuit Q e is unchanged when it is rebuilt (Fig. 7, b):
since the resonant frequency is related to C e by the Thompson formula .
When tuning the input circuit with inductance, the resonant gain is constant if the quality factor is constant.
Intracapacitive coupling with an antenna (Fig. 8, a) is designed to reduce the influence of the antenna parameters on the input circuit circuit. This connection is made weak, for which the capacitance of the coupling capacitor is chosen much larger than the capacitance of the antenna: » C A. With a weak connection with the antenna, its parameters do not affect the input circuit circuit (they do not shift its resonant frequency and do not expand its bandwidth).
Resonance Gain
The resonant transfer coefficient does not depend on the values of L K and C K of the circuit (C K « C sv) and when the circuit is tuned in the frequency range K 0 changes only due to the variability of Q e (Fig. 8, b).
General information about input circuits
Receiver input circuit - a circuit by which an antenna or antenna-feeder system is connected to the input of the first stage of the receiver. The first stage can be a high frequency amplifier, a frequency converter or a detector. The location of the input circuit between the output of the antenna or antenna-feeder system and the input of the first stage determined its name (Fig. 3.3.8).
The main functions of the input circuit are:
a) in the preliminary selection of the received useful signal from the entire set of signals arising in the antenna circuit;
b) in the transfer of useful signal energy to the input of the first stage with the least losses and distortions.
In the general case, the input circuit is a kind of passive four-terminal network, which includes a resonant system and coupling elements. Depending on the frequency range, the resonant system is performed on lumped or distributed elements and consists of one or more oscillatory circuits or resonators. The coupling elements provide communication between the antenna circuit and the circuit or resonator, and with several resonant elements, also the connection between them and the first stage of the receiver. The main characteristics of the input circuit are: voltage (or power) transfer coefficient, constancy of the resonant gain over the range, operating frequency range, selectivity and bandwidth, the magnitude of the connection of the antenna with the input circuit.
Transfer ratio input circuit voltage TO. called the signal voltage ratio U c at the input of the first
cascade to the value e. d.s. E generator equivalent to an antenna or antenna-feeder system:
With the input circuit setting unchanged, the value To(f) changes with the frequency of incoming signals, reaching a maximum K 0 at the resonant frequency fo.
Addiction To(f) is called the amplitude-frequency (resonant) characteristic, and the dependence φ (f) is called the phase-frequency (phase) characteristic.
Frequency selectivity the input circuit is determined by the shape of the resonant curve. In superheterodyne receivers, the most important is the selectivity for two additional receiving channels - a symmetrical (or mirror) channel and a direct transmission channel at an intermediate frequency. To weaken the influence of a signal at a frequency fp, special filters are sometimes introduced into the input circuit (rejector, “plug” filter). By the shape of the resonance curve, one can both determine the selectivity of the input circuit and estimate the frequency distortion of the useful signal. A sufficient characteristic of the selective properties can often be the bandwidth P, usually determined by the level of 0.707. The uneven amplification of the components of the signal spectrum within the bandwidth does not exceed three decibels.
Operating frequency range(fomax - fmin) is provided if the input circuit can be tuned to any operating frequency of the receiver while satisfying the requirements for changing the gain, bandwidth and selectivity within the operating frequency range. The input circuit is more often rebuilt by a capacitor included in the block of variable capacitors of the receiver; in this case, a smaller change in the parameters of the circuit is provided in comparison with the change during the restructuring of its variable inductance.
Communication value with the input circuit is determined only by the parameters of the input circuit. At present, various antennas are used in radio reception technology, from the simplest wire vertical ones to parabolic reflectors and other complex antennas.
According to general theory antennas, input circuits can be considered in relation to two characteristic cases: the resistance of the antenna (antenna-feeder system) is active; antenna resistance is reactive. In the case of the reactive nature of the antenna resistance some reactance is introduced into the input circuit, due to which the resonant frequency of the latter changes; the introduced active resistance leads to a deterioration in the selectivity of the input circuit. The value of the inserted resistance can vary widely, since the receiver is often operated with different antennas, the parameters of which are not known in advance. Therefore, to reduce the influence of the parameters of untuned antennas on the input circuit, a rather weak connection between them is chosen. With active antenna resistance operating conditions of the input circuit are different. In this case, no detuning is introduced into the input circuit and the amount of coupling between the input circuit and the tuned antenna, the resistance of which is uniquely specified, is chosen from the condition for obtaining highest power signal at the input of the first stage. The connection under which this condition is ensured is called optimal.
Input circuit diagrams
The most common input circuit diagrams are capacitive, inductive (transformer), inductive-capacitive (combined), autotransformer connection with an antenna or antenna-feeder system (Fig. 3.3.9, a, b, c, Fig. 3.3.10).
Note that the simplest circuit is formed by directly connecting the antenna to the input circuit. Due to the absence of coupling elements, it is impossible to ensure a small influence of the antenna on the input circuit, and therefore such a scheme is rarely used in practice.
The input circuits differ from each other not only in the nature of the connection (Lb, Cb), but also in the number of circuits used in them. Currently, the single-circuit input circuit is most often used. The important advantages of such a circuit in comparison with a multi-loop circuit are the simplicity of the design and the provision of higher sensitivity. The latter is due to the fact that an increase in the number of circuits increases, as a rule, signal losses before the input of the first stage. The single-circuit input circuit also ensures the constancy of the resonant gain, combined with the convenience of tuning the receiver in the operating frequency range. The multiloop input circuit makes it possible to obtain the shape of the resonant characteristic, which provides the least distortion of the spectrum of the useful signal with high selectivity with respect to interfering signals, and this is its advantage. As a result, it is mainly used in high-quality receivers, usually operating at fixed frequencies. The most common is the two-loop input circuit shown as an example in fig. 3.3.10, a. In this circuit, the circuits Lk1, Sk1 and Lk2, Sk2 are tuned to the frequency of the received signal, and the intracapacitive connection between the circuits is carried out through the coupling capacitor C St. When using the specified bandpass filter in other input circuits, it can be coupled to the antenna in a different way, for example, through capacitive coupling.
Schematic diagrams of the input circuits shown in fig. 3.3.9 are typical for broadcast and other moderately high frequency receivers operating with untuned antennas.
Among them, the capacitive coupling circuit with the antenna (Fig. 3.3.9, a) is the simplest in design. In it, by choosing a sufficiently weak connection of the antenna with the input circuit, carried out through the coupling capacitor C s, it is possible to ensure, on the one hand, a small influence of the antenna on the circuit and, on the other hand, which is no less important, the constancy of the characteristics of the input circuit when the receiver is operated with different antennas . However, with a very small amount of coupling, the transmission coefficient decreases, and, consequently, the sensitivity of the receiver decreases. Usually C sv is chosen from the condition C sv< 10…40 пФ. К серьезному недостатку схемы относится значительное непостоянство To in the operating frequency range; the latter led to the use of the scheme for small values of the range overlap coefficient. An inductive coupling circuit with an antenna (Fig. 3.3.9, b) is the most common. With a sufficiently weak connection between the coupling coils and the input circuit, it is possible to obtain almost the same transmission coefficient over the operating frequency range, which is often used in practice. This is ensured, as we will see below, by an appropriate choice of the parameters of the antenna circuit (C A, L C B).
The scheme of combined communication with the antenna (Fig. 3.3.9, c) makes it possible to provide a sufficiently high and practically constant value of the resonant transmission coefficient Ko in the entire operating frequency range. The disadvantage of the scheme is the deterioration of the selectivity over the symmetrical reception channel compared to the selectivity provided by the transformer-coupled scheme. Incomplete connection electronic device the first stage (lamp, transistor) to the input circuit weakens the influence of its input resistance on the input circuit and allows you to provide a given bandwidth; this connection is carried out using an autotransformer connection (Fig. 3.3.9, b), using a capacitive divider (Fig. 3.3.9, in) or transformer connection.
Transformer and autotransformer coupling circuits with an antenna are widely used in professional receivers decameter and meter waves operating at a fixed frequency or in a narrow frequency range. When working with symmetrical tuned antennas, transformer connection allows you to use, and this is its advantage, symmetrical (Fig. 3.3.10, a) and asymmetrical receiving feeders. In the latter case, one end of the coupling coil connected to the output of an unbalanced feeder is grounded together with its outer shell. A circuit with an autotransformer connection (Fig. 3.3.10, b) is used when working with asymmetric (coaxial) feeders and is most often used in practice.
When working with tuned antennas, the coupling value is chosen, as already noted, from the condition of transferring the maximum power from the signal source to the input of the first stage, i.e., to the load.
Tuned antennas have a sharp radiation pattern, and in input circuits using transformer coupling, it sometimes becomes necessary to install an electrostatic shield between the L CB and L K coils (Fig. 3.3.10, a).
In the short-wave part of the meter wave range (λ = 1-3m), an input circuit circuit with a series connection of the inductance can be used (Fig. 3.3.10, c). In it, the input circuit is formed by an inductance L K and two series-connected capacitors C 1 and C 2 , and the capacitor C 2 is the input capacitance of the first stage. The circuit is tuned to the frequency of the received signal by changing the inductance. Due to this inclusion of contour elements L K , C 1 and C 2, the resulting capacitance of the circuit decreases compared to the capacitance of a conventional circuit when parallel connection From 1 and C 2 .
This allows you to increase the inductance of the circuit Lk or, at a certain value L kmin and the minimum possible capacitance Sk to increase the tuning frequency of the input circuit.
At frequencies f > 250 - 300 MHz, systems are used in the input circuits with distributed elements. At these frequencies, the quality factor of conventional circuits decreases sharply, which is associated with a reduction in the size of the inductors, an increase in losses due to the skin effect and radiation.
In the input circuits of decimeter wave range receivers, resonators are widely used in the form of half-wave segments open at both ends and mainly in the form of quarter-wave segments of coaxial lines closed at one end. They are made of hollow, usually copper, concentric tubes, the open ends of which are often mounted on the disk leads of the cathode and the grid of a special beacon triode used as an electronic device of the first stage.
On fig. 3.3.11 shows a circuit diagram in which the segment of the coaxial line l 1< λ 0 /4 along with the container FROM p represents a high-quality circuit (Q == 300 -400), and the antenna feeder, connected to this circuit at a distance l 2, forms an autotransformer connection with it. Tuning to the signal frequency can be carried out with a capacitance C n or a piston short-circuiting the end of the line segment. The connection of the feeder with the resonator can be transformer and capacitive and is provided with the help of a coil or a pin placed in the antinodes of the magnetic or electric field, respectively.
The input circuit must ensure the matching of the receiver with the antenna, i.e., the maximum transmission of the power of the signal received by the antenna to the input of the receiver, and protect the receiver from the effects of powerful radio signals operating out of range.
On fig. 2.4 shows a well-proven input circuit diagram that can be applied in shortwave amateur receiver any type. In this circuit, there are two resonant circuits tuned to the signal frequency, a wideband circuit C1L1-L3C2 and a narrowband circuit C4L4L5. These circuits are connected to each other only through the capacitor C5.
The broadband resonant circuit provides a choice of receiver-to-antenna coupling ratio and is usually adjusted to obtain the maximum received signal loudness. But if the selectivity of the receiver is insufficient, its coupling coefficient with the antenna can be reduced by increasing the capacitance of the capacitor C1 until the interference from the out-of-band signal disappears.
The decrease in the transmission coefficient of the input circuit during the transition to longer waves is corrected by the signal level meter (S-meter) of the receiver so that its readings on all ranges correspond to a qualitative assessment of the signal level (Table 2.1).
If the S-meter is calibrated in accordance with Table. 2.1 with a constant gain on all bands, then a signal with a level of 1.5 μV, which is easily received against the background of weak antenna noise in the 10 m band, will be evaluated as a weak signal, and with a small voltage, and a signal with a level of 25 μV, which against the background of interference in the range of 80 m, you can’t distinguish it, you will have to evaluate it as a “loud signal”. Using an S-meter without input circuit gain correction results in S1 ("barely audible, unreceivable") scores for good connections on high bands, and S9 scores for unreceivable low bands.
The input circuit diagram (Fig. 2.4) contains elements to protect the receiver from signals, the level of which can disable the source follower on a powerful field-effect transistor VT1. The protection unit consists of two diodes VD1 and VD2, shunting a high-Q circuit, to which a blocking bias of about 2 V is applied.
A source follower having a gain of approximately 0.7 is required if the input circuit is connected in front of a stage with low (hundreds of ohms) input impedance.
Structural data of the input circuit elements (Fig. 2.4): variable capacitors C1, C3, C4 - small-sized, dual with an air dielectric from transistor receivers (in the described design, tuning capacitors of the Sonata receiver were used).
Coils L1-L5 are wound on plastic frames with a diameter of 9 mm. Their winding is made in one layer turn to turn. Coil L1 contains 50 turns (wire PESHO 0.31); L2 (counting from the end connected to L1) - 10 + 5 + 5 turns (PESHO wire 0.44); L3 - 6 turns (PESHO wire 0.44). The L4 coil is wound on a ribbed polystyrene frame, the average coil diameter is 20 mm, the number of turns is 6, the winding length is 20 mm (silver-plated wire, copper or enameled wire can also be used, diameter 2 mm).
Coil L5 is wound on a toroidal magnetic core made of 30VCh2 ferrite, outer diameter 32 mm, inner diameter 16 mm, height 8 mm (hereinafter, the dimensions of such magnetic cores will be indicated as 32 X 16X8 mm). Wrap the magnetic circuit with a fluoroplastic tape and then winding evenly distributed over a 300 ° sector with a PEV-0.64 wire.
The number of turns (counting from the end connected to L4) 6 + 3 + 6 + 6.
Coils L4 and L5 made in the described way have a quality factor of 300 or more units. In the absence of the required toroidal ferrite magnetic circuit, L5 can be made without it, but to obtain the required quality factor, L5 must be composed of four coils connected in series, wound on a frame with a diameter of 70 ... 100 mm with a wire with a diameter of 1.5 ... 2 mm (number of turns pick up each coil until resonance is reached in the middle position of the rotor of the capacitor C4).
