Topic design of a radio transmitting device with amplitude modulation. Calculation, design and design of a radio transmitting device
Ministry of Education of the Russian Federation
Omsk State Technical University
Department of RTU and SD
Course project
Calculation, design and design of a radio transmitting device
Project Manager:
Eltsov A.K.
Developed by:
students of group RI-419
Kuprin V.I.,
Glazkov A.. V.
Omsk 2002
annotation
This course project examines the problem of designing a short-wave radio transmitting device with amplitude modulation. When designing, problems are solved, which include drawing up a block diagram, calculating a power amplifier, a quartz self-oscillator and a circuit for matching the active element with the load. The preliminary, intermediate and final power amplifiers are calculated according to constant and alternating current. At the next design stage, standard components were selected - capacitors and resistors, inductors were calculated, and an electrical circuit diagram of the designed radio transmitter was drawn up.
Introduction
Radio transmitters are devices designed to perform two main functions - generating high or ultra-high frequency electromagnetic oscillations and modulating them in accordance with the transmitted message. Radio transmitting devices are part of radio complexes, which also contain antennas, radio receivers and various auxiliary devices.
One of the main trends in technology development radio transmitting devices is the desire to make the radio transmitter, if possible, entirely on semiconductor devices and integrated circuits. If the required output power cannot be provided by existing generator semiconductor devices, then the output stages of the transmitter are made using vacuum devices: radio tubes, klystrons, traveling wave tubes, etc.
The development of a radio transmitting device is a solution to a complex of circuitry and design issues. The design of the amplifier, its manufacturability, and stability over time largely depend on how rationally the circuit is chosen and the operating mode of its elements is correctly calculated.
Radio transmitters are classified according to purpose, operating conditions, output power, frequency, type of modulation, etc. Based on output power, radio transmitters on semiconductor devices can be divided into low-power, medium-power and high-power; by frequency - high-frequency and ultra-high-frequency.
The development of the VHF band for radio communications and broadcasting purposes began somewhat later than the HF band. This is due to two reasons: the difficulties associated with amplification of VHF and UHF vibrations, and the limited propagation range of waves in these ranges. The difficulties associated with signal amplification were overcome by the creation of metal-ceramic generator lamps and devices, the operation of which is based on the use of the inertia of the electron flow. The relatively short range of VHF transmitters in many cases turns from a disadvantage into an advantage - it becomes possible to repeatedly use the same operating frequencies in different geographically distant points.
1. Selection and calculation of a block diagram
Let's consider the construction and calculation of the block diagram of the RPDU shown in Fig. 1. This option block diagram consists of:
ZG - master oscillator (autogenerator);
BU - buffer cascade;
Frequency multiplier;
PU - pre-power amplifier; final power amplifier;
M - modulating device;
Note that in more complex professional RPDUs, instead of a SG, an exciter is used, which is based on a frequency synthesizer, and the block diagram itself has a slightly different form.
The task of calculating the block diagram is to determine the optimal number k of high-frequency stages between the master oscillator and the final power amplifier.
Obviously, the value of the oscillatory power required from the active element of the driving stage can be calculated using the formula
;
where is the oscillatory power of the nth cascade
Power gain of the nth stage.
Having completed the solution to the issue of distribution of gain factors across all stages of the designed device, you can determine the power required from the master oscillator:
;
where i = n - 1 is the number of amplifier stages.
The specified stability of the operating frequency of the RPDU can only be achieved by using high-Q elements, for example, quartz resonators, in the master oscillator as an oscillatory system. It should be borne in mind that the power of the master oscillator should not exceed 20...50 mW, and the frequency of the quartz resonator should not exceed 10...15 MHz. In this case, you can get relative instability<1...2∙10-5.
The frequency multiplication factor in intermediate stages (frequency multipliers) is defined as the ratio of the frequencies of the output stage and the master oscillator.
Considering that the energy performance of frequency multipliers is worse than that of power amplifiers, multipliers of two and three are usually used.
Note that in RPDU with frequency modulation, frequency multiplication also makes it possible to increase the frequency deviation.
2. Calculation of high frequency power amplifier
.1 Calculation of PA using a circuit with a common emitter
The following initial data are required for the calculation:
Transmitter output power (90 W),
Transmitter operating frequency (103 MHz),
Load resistance (50 Ohm).
The electrical calculation of the operating mode of the active element is carried out separately for the collector and input circuits.
Let's consider the calculation of the cascade collector circuit:
1. To obtain maximum power gain and efficiency, the transistor must operate in a critical mode with a cutoff angle. For which we find the values from tables or graphs.
Let's find the output power of the amplifier
; 
,
where is the efficiency of the output oscillatory system.
Amplitude of the first harmonic voltage at the collector:
(supply voltage must correspond to the standard range of values given in GOST 21128-83. In our case, Ep = 27 V)
Maximum voltage on the collector should not exceed the permissible:
For our transistor.
If this condition is not met, it is necessary to reduce Ep or consider replacing the active element.
Amplitude of the first harmonic of the collector current
;
A.
Collector load resistance
Ohm.
DC component of collector current
;
where is the relation 
- output current shape coefficient for the 1st harmonic.
The maximum collector current (output current pulse height) is equal to:
;
Power consumption from power supply:
Efficiency of the collector circuit at a given payload:
Power dissipation at the transistor collector
;
.
Electrical calculation of the input circuit of a transistor when calculating the input circuit of a transistor connected according to a circuit with an OE assumes that a resistor Radd is connected between its base and emitter terminals, the resistance of which can be approximately determined by the formula:
,
where is the current gain in the circuit with OE;
Cutoff frequency;
Se is the capacitance of the emitter junction.
Amplitude value of base current:
where is the correction factor;
Sk is the barrier capacitance of the collector junction.
,
where: E'b - collector current cut-off voltage, equal (modulo) 0.6 ÷ 0.7 V for silicon transistors;
IN.
Constant components of the base and emitter currents:
4. Active component of the input resistance of the transistor at the operating frequency:
,
where: are found according to the formulas corresponding to the equivalent circuit of the input resistance of the transistor (Fig. 2):
where: Ska = (0.2) Ska =30 pf - barrier capacitance of the active part of the collector junction;
rb = 0.36 Ohm - resistance of the base material.
If rb is not given, then approximately it can be determined by the formula rb =
10.8 - time constant of the collector transition;
Emitter junction resistance (if not given, then can be taken = 0)
Note that the parameters and are used to determine the reactive component of the input resistance of the transistor.
Excitation power at operating frequency without taking into account losses in the input matching circuit:
6. Transistor power gain at operating frequency:
7. Total power dissipated by the transistor:
The Pras value is the initial parameter for calculating the thermal regime of the transistor and its cooling system.
.2 Calculation of the circuit matching the active element with the load
The matching circuit performs two main tasks. The first is converting load resistance into resistance, the second is filtering external harmonics.
In narrow-band transistor GVVs, especially in the output stages of radio transmitting devices, a U-shaped circuit, the diagram of which is shown in Fig. 3, is widely used.
Due to the geometric symmetry of the circuit, its implementation is possible at , including at . Obviously, if the resistances are equal, the main purpose of the circuit is to filter higher harmonics of the AE output current.
In a number of cases, for example, if the value of inductance L turns out to be too small, which makes its implementation difficult or impossible, then the equivalent inductive reactance is implemented in the form of a series connection of inductance LE and capacitance Se. The U-shaped circuit diagram in this case is represented in the form of a circuit shown in Fig. 4.
Below is the procedure for calculating the matching circuit shown in Fig. 4. Note that all calculations are carried out in basic units (Ohm, Hn, V, A, F, etc.).
We set the value of the wave impedance of the circuit:
where f is the signal frequency.
Let's determine the inductance of the circuit Le:
3. At signal frequency f, the calculated matching circuit is reduced to the form shown in Fig. 3, and the elements L, Le, Ce are in the ratio:
The value of L must be specified in accordance with the formula:
4. Determine the value of the capacitance of the capacitor Se:
5. Determine the value of the capacitances of capacitors C1 and C2:
C1=1010 pF, (1000 pF is the standard value);
pF.
C2=146 pF, (150 pF is the standard value).
The resistance introduced into the circuit will be equal to:
Rin = 2.323 Ohm.
Quality factor of the loaded circuit
where is the intrinsic loss resistance of the loop inductance, determined in the process of its structural calculation. For approximate calculations, you can take (Ohm).
8. Of particular interest is the calculation of the higher harmonic filtering coefficient for the output stage.
In a particular case, when you can use the expression
where: n=2 - single-cycle circuit.
Next, it is necessary to compare the obtained value of the filtration coefficient with the required value of this coefficient Ft, calculated from the literature. If F< Фт следует переходить к двух или трехконтурной схеме согласующей цепи.
In view of the fact that in a multi-stage transmitter all stages after the modulated one operate in the mode of amplifying modulated oscillations, it is necessary to check the load system to ensure the required bandwidth:
2.3 Selection and calculation of heat sink design
To remove heat from semiconductor devices, heat sinks are used, the action of which is based on various methods of dissipating thermal energy: thermal conductivity, natural forced convection of air and liquid, and changes in the state of aggregation of a substance.
There are two ways to calculate thermal conditions semiconductor device with heat sink:
at given values of power P dissipated by the semiconductor device, the temperature of the device body and the temperature of the pn junction and temperature environment Then the geometric dimensions of the heat sink are calculated;
for given geometric dimensions of the heat sink, ambient temperature Tc, p-n junction temperature or device body temperature, the power dissipated by a semiconductor device with a heat sink is calculated.
In particular, the following parameters are required for the calculation:
P - power dissipated by the device, W.
Ambient temperature, .
Maximum junction temperature, .
Thermal resistance transition - housing, .
Thermal contact resistance, .
1. To cool the transistor, a radiator is needed; its thermal resistance is calculated by the formula:
2. Average surface temperature of the heat sink:
Tsr= P∙Kt-s.out.d+ To.s=75.8° C.
Minimum rib length:
Fin thickness:
d=0.003 m=3 mm.
Heat sink plate thickness:
q=0.003 m=3 mm.
Distance between ribs:
b=0.012 m=12 mm.
Rib height:
h=0.025 m=25 mm.
Rib length:
L=0.13 m=130 mm.
Number of ribs pieces:
n=(l+b)/(b+d)=10.
Length of the heat sink plate on which the fins are developed:
l=b(n-l)+2d=0.11 m=110 mm.
Smooth surface area of the heat sink:
Sgl=L∙L=0.016 m2=16 mm2.
Heat sink finned surface area:
Sop=S1+ S2 +S3 =0.08 m2=80 mm2.
13. Radiation heat transfer coefficient:
αl=εφf(Тср+ Т.с)=8.1 W/(m∙С).
Convection heat transfer coefficient:
αк=А1*Тм[(Тср- To.c)/L]=3.96 W/(m∙С).
Heat transfer coefficient of a smooth surface:
αhl= αl + αk = 12.06 W/(m∙C).
Power dissipated by a smooth surface:
Rgl = αgl ∙ Sgl ∙ (Tsr-To.s) = 40 W.
Thermal resistance of smooth surface:
Rt.hl=1/(αhl ∙ Shl)= 4.98 C/W.
Ambient temperature between fins:
To.c1= Тср-Н∙ (Тср - Т.с)=61°С,
Тм1=0.5(Тср + To.с1)= 66° С.
Convection heat transfer coefficient:
20. Radiation heat transfer coefficient:
αl.or = εφf(Tsr+ To.s) = 1.6 W/(m∙C).
Power dissipated by the finned surface of the heat sink
RT.or=[ αk (Tsr- To.s) + αl (Tsr- To.s)] *S= 5 W.
Thermal resistance of the finned surface of the heat sink
Rt.or=(Ts-To.s)/ Rt.or= 21 C/W.
Overall thermal resistance of the heat sink
Rt.calc= (Rt.hl∙ RT.op)/ (Rt.hl+RT.op)= 18 C/W.
Power dissipated by smooth and finned heat sink surfaces
RT=Ht.gl+Rt.or= 58 W.
2.4 Selection and calculation of the inductor
After completing the electrical calculation, you need to select the type of capacitors. In this case, the capacitor must be selected from the corresponding TKE groups, have the required capacitance value (preferably from the E12 series), withstand the voltage acting on them and pass the corresponding current through them.
To meet reliability requirements, there must be a certain margin of voltage and current. If, instead of permissible current and voltage, permissible reactive power is indicated in the reference data, then the choice of design is made taking into account the value of this parameter.
amplifier power frequency oscillator
Inductors are not produced as standard, and the data found from the circuit calculations are used when developing the design of the coil. Inductors usually have cylindrical turns and are made as single-layer or multi-layer. Below we will consider the procedure for calculating a single-layer coil, a sketch of which is shown in Fig. 5.
We set the ratio of the length of the coils to its diameter within
.
2. Determine the longitudinal cross-sectional area of the coil S = lD using the formula
where is a coefficient characterizing the specific thermal load per 1 cm2 of the coil cross-section. Typical value of this coefficient:
3. Determine the dimensions of the coil in centimeters:
4. The number of turns of the coil W can be determined by the well-known formula
where LE is inductance, μH.
5. The diameter d of the coil wire (mm) is calculated using the formula:
where Ik is the amplitude of the loop current, A,
f - operating frequency, MHz.
We determine (clarify) the intrinsic loss resistance of the loop coil at the operating frequency.
where f is the operating frequency, MHz, d is the wire diameter, mm, D is the coil diameter, mm.
Circuit efficiency
3. Frequency multipliers
Frequency multipliers (MF) are called such GVV, the oscillation frequency, the output of which is 2, 3..., n times higher than the output. An amplifier differs from a power amplifier in that its output circuit is tuned to the second, third or nth harmonic of the input voltage. It should be noted that the energy indicators of the amplifier are lower than those of the power amplifier, which is due to a decrease in the amplitude of the harmonic components in the collector current pulse as the multiplication factor increases.
When constructing an HF, it is recommended to choose a transistor with a high cutoff frequency (), since with an increase in the operating frequency (), the collector current pulse expands and the content of higher harmonics in it sharply decreases. The calculation option given below assumes that the relation is satisfied, i.e. the active element is considered inertialess.
The following initial data are required for the calculation:
Output power,
Output frequency,
N is the multiplication factor.
The type of active element is selected based on the calculated output power and output oscillation frequency.
Let's consider the calculation of the cascade collector circuit.
1. The optimal cutoff angle at which the maximum values are obtained is determined by the formula
2. Find the amplitude of the N-harmonic voltage at the output of the active element operating in the boundary (critical) mode:
where is the voltage of the power supply of the radio transmitting device,
The steepness of the boundary mode line.
Determine the amplitude of the Nth harmonic of the collector current
4. The maximum value of the collector current is
5. DC component of collector current
6. Power consumption from power supply
7. Power dissipated at the collector
8. Efficiency
We calculate the input circuit
Determine the amplitude of the alternating voltage on the base
where = 4.1 is the slope of the flow characteristic.
3. Determine the required excitation power
4. Power gain
5. Stage input impedance
Calculation of values of multiplier circuit elements
DC component of base current
We find from the condition
Inductance Lr is found from the condition:
lb we find from the relation therefore
sbl we find from the condition therefore
4. Quartz oscillators
High stability of the operating frequency in multi-stage radio transmitting devices is ensured by a master oscillator. The current use of conventional LC generators as master oscillators, even when special measures have been taken to protect them from external influences, does not adequately meet the ever-increasing requirements for the stability of high-frequency oscillations.
The use of quartz resonators in self-oscillators as part of an oscillatory system makes it possible to build master oscillators with fairly high technical characteristics. With optimal selection and calculation of the parameters of circuit elements and their operating mode, the stability of the frequency of the CG without the use of thermal compensation and thermostatting is determined mainly by the stability of the resonator frequency. The stability of the CG frequency is usually assessed by changes in frequency due to changes in ambient temperature, the effects of mechanical and climatic destabilizing factors, as well as aging.
There are many types of CG schemes. Oscillator circuits are widely used, which are obtained by replacing one of the inductances of a three-point self-oscillator circuit with a quartz resonator. In particular, in the mid-frequency range, the capacitive three-point is most used, which allows for high frequency stability. A distinctive feature of oscillator circuits is that they operate only at the quartz frequency. If the quartz resonator malfunctions, oscillations occur in the self-oscillator.
Up to 15...20 MHz, quartz resonators operate according to the first (fundamental) harmonic; at higher frequencies, oscillations of odd mechanical harmonics are used. The quartz resonator and the active element (transistor) are selected based on electrical parameters, as well as operating conditions, dimensions and cost.
The approximate value of the relative instability of the CG frequency, for example, in the temperature range -10 °C to +50 °C can be 2...5∙10-5. Knowing this value is necessary when drawing up a block diagram and choosing the type of master oscillator.
5. Design of crystal oscillators with direct frequency modulation
.1 Features of the construction of voltage-controlled generators
When developing a frequency-controlled CG, it is necessary to correctly select the frequency of the generator, resonator and frequency control elements in order to ensure the necessary tuning limits with high frequency stability using the simplest circuit solutions. In addition to ensuring a certain frequency deviation to the CG when generating FM oscillations by the direct method, there is a requirement for minimal nonlinear distortions of the modulating channel, which are caused by the nonlinearity of the characteristics of the varicap and resonator. The most effective way to reduce them is to connect an inductor in parallel with the resonator.
For a number of objective reasons, frequency-controlled CGs are most widely used in the range of 5..20 MHz. In this range, quartz resonators operate, as a rule, at the fundamental frequency, the piezoelements themselves are flat plates, and the values of m and Co make it possible to obtain a frequency tuning of the order of ±1000∙10-6 with relatively high frequency stability. For more low frequencies ah piezoelements of T cut resonators have the shape of a biconvex lens, which reduces m and makes it difficult to obtain large frequency tuning limits.
5.2 Design of a voltage-controlled CG with frequency modulation
The compilation and calculation of the structural diagram in accordance with Chapter 2 of these instructions should have been carried out taking into account the real possibilities of constructing a master quartz oscillator. Using the results of this calculation, we clarify the necessary initial data.
Fig.6. Frequency modulated quartz oscillator.
Based on reference data, we select an AT resonator - a medium operating at the fundamental frequency. We write down the parameters of the resonator: Rkv, m, C0.
Select the active element. For example, transistor KT324, the slope of the static characteristic of which at a collector current of 1...2 mA is 35...50 mA/V. (Naturally, taking into account the specifics of a particular task, a transistor with the appropriate parameters should be selected).
We determine the control resistance of the self-oscillator:
,
Note that it should be calculated for the minimum value of the slope S and = 0.2 (excitation safety factor K3 = 5).
Let's find the values of the feedback capacitances (C3 and C4) of the generator.
What is necessary to prevent the varicap from opening by modulating voltage and high frequency voltage;
amplitude value of the modulating voltage.
We determine the reduced value of Xrn using the formula:
8. We calculate the capacitance of the varicap at a bias voltage of Evn - 4 V:
where: 1/2 - coefficient for sharp transitions.
From the commercially produced varicaps, we choose one such that series connections of two varicaps give a capacity approximately equal to St. Let's take the KV110B varicap.
To enable operation near the serial resonance frequency of the quartz resonator, inductor L2 is connected in series with it.
We determine the value of the tuning inductance for the two boundary values of the capacitance of the selected varicaps:
,
where SVN is the capacitance of the varicap when constructing a 4V bias. Moreover, at the bottom of the spread of parameters of semiconductor devices, substitute the lower and upper values of the capacitance in the formula for determining the limits of the change in inductance instead of the EHV.
After determining the upper and lower values of L2, we find the average value of inductance Lav.
We determine the coefficient of nonlinear distortion:
11. Since the coefficient of nonlinear distortion in the source data (Kf = 5%), to reduce it we connect the inductor L1 in parallel with the resonator. The value of this inductance is determined by the formula:
Where 
- reduced inductance resistance.
We calculate the coefficient of nonlinear distortion taking into account the inclusion of inductor L1 in parallel with the resonator:
Conclusion
In accordance with the technical specifications, the radio transmitting device was calculated. Thanks to the well-documented literature on similar devices and modern elementary base, a simple implementation of a radio transmitter has become possible. A variant of its design has been considered.
A calculation was made of a power amplifier, frequency multiplier and quartz self-oscillator of a radio transmitter broadcasting in the VHF band at a frequency of 103 MHz, providing an output power of 90 W. To power the device, a 27V DC source is required.
Note: the final configuration and selection of circuit elements is carried out during the manufacture of a mock-up of the radio transmitting device.
Literature
1. Design of radio transmitters: Textbook. manual for universities / Edited by V.V. Shahgildyan. - 4th ed., revised. and additional - M.: Radio and communications, 2000 - 656 p.
2. Design of radio transmitting devices: Textbook for universities / Ed. V.V. Shakhgildyan. - M.: Radio and communication, 1993. -512 p.
Design of radio transmitters: Textbook for universities / Ed. V.V.Shahgildyan. - 4th ed., revised. and additional - M.: Radio and Communications, 2000. - 656 p.
Design of microwave radio transmitting devices / Ed. G. M. Utkina.-M.: Sov. radio, 1979.-320p.
Design of radio transmitting devices using transistors. Guidelines for course design. - Rotoprit TIASUR. - Tomsk, 1987. - 79 p.
Providing thermal conditions for electronic products./ A. A. Chernyshev, V. I. Ivanov, A. I. Aksenov, D. N. Glushkova. - M.: Energy, 1980 - 216 p.
GOST 21128-83. Power supply systems, networks, sources, converters and receivers of electrical energy. Rated voltages up to 1000 V. - M.: Standards Publishing House, 1983.
GOST 22579-86. Radio stations with single-band modulation of the land mobile service. - M.: Standards Publishing House, 1986
GOST 12252-86. VHF radio stations of the land mobile service. - M.: Standards Publishing House, 1986
Coursework and diploma design. Guidelines for students of specialties 190200 and 200700. Omsk. - Omsk State Technical University Publishing House, 1997. - 44 p.
Radio transmitting devices. Guidelines for course design. - Ompi. - Omsk, 1985. - 27 p.
Altshuller G. B., Elfimov N. N., Shakulin V. G. Quartz oscillators: Reference. allowance. M.: Radio and communication, 1984. - 232 p.
Piezoquartz resonators: Handbook / Ed. P.E. Kandyba and G.P. Pozdnyakova. - M.: Radio and communication, 1992 - 392 p.
Semiconductor devices. Medium and high power transistors: Handbook / Ed. A.V. Golomedova. - M.: Radio and communications, 1989 - 640 p.
17. Electronic reference book on semiconductor devices. Shulgin O.A. v.1.02
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1 . Technical task
Design broadcast transmitter with AM (PRVAM) with the following parameters:
· Power in the antenna (load) P ~ =100 kW;
· Characteristic impedance of the feeder with Ф = 150 Ohm;
· Feeder efficiency z f = 0.80;
· Traveling wave coefficient KBB = 0.8;
· Maximum modulation index m = 1;
· Operating frequency range f min - f max, 0.1 - 0.3 MHz;
· Modulation frequency range DF = 50 10000 Hz;
· carrier frequency f 0 =200 kHz.
Analysis of technical specifications:
Radio broadcast transmitters (PRB) with AM used in the long, medium and short wave ranges must comply in their parameters with GOST 1392468. In tube versions of transmitters to receive an AM signal of a given power, the most common are anode, anode-screen or combined (over several electrodes) modulation in terminal stage, amplification of modulated oscillations (UMO) is less commonly used.
As part of this work, the following calculations were carried out:
· final stage at peak, minimum and telephone points, as well as at 100% modulation depth;
· modulating device and electrical parameters of its elements; transformer, chokes, blocking capacitors;
· output oscillatory system;
2. Choosing a construction method design of the designed device
For implementation of this device An anode modulation implementation was chosen due to its high energy efficiency, good linearity and wide application in radio broadcast transmitters. Structural scheme The designed device is shown in Figure 1.
Figure 2.1. Block diagram of the designed AM radio broadcast transmitter.
Approximate calculation of a radio transmitter with AM according to the block diagram
According to the technical specifications, the transmitter must have the following parameters: P ~ = 100 kW;
modulation index m = 1;
operating frequency range f min f max = 0.1 0.3 MHz.
Based on the parameters specified above, we will make an approximate calculation of the elements of the radio transmitter.
The peak power in the antenna will be:
The powers P 1 T and P 1 max supplied by OK devices are determined by the formulas:
where is the approximate efficiency of the output oscillatory system. selected from the table given in and , feeder efficiency.
Then P 1 T = 136 kW, P 1 max = 544 kW.
Due to the fact that anode modulation is implemented in the OK, the rated power of the electric generator is selected according to the rule P 1nom? 2P 1 T = 272 kW (rated power of generator lamps).
Because When developing the OK, a push-pull circuit was used, then P 1nom of the lamp = .
The choice of lamp type is carried out according to such parameters as P 1nom of the lamp and maximum operating frequency f max.
According to the reference tables presented in and, a GU 66 B lamp was selected, having the following parameters: E a nom = 10 kV; S = 0.16 A/V, P nom reference = 150 kW.
The description of the GU 66 B lamp is given in Appendix 1.
The schematic diagram of the designed radio broadcast transmitter is shown in Figure 2.2.
Figure 2.2 - Schematic diagram of the designed AM transmitter.
3 . Calculation of the final stage (OK)
At this point, the OK calculation is performed in the following modes:
· at the peak point;
· at the minimum point;
· at a telephone point;
· at 100% modulation depth.
Anode voltage modulation depth m = 1 in accordance with terms of reference.
The schematic diagram of the final stage is shown in Figure 3.1.
Figure 3.1 Schematic diagram of the final stage.
The anode supply voltage for telephone point mode is usually selected as:
The cutoff angle is selected within the range and = 80? - 90?. In this case, we will take the cutoff angle equal to 90?.
3 .1 Calculation of the final stage (OK) in maximum point
The calculation of the final stage at the maximum point is carried out according to the method outlined in and.
Anode supply and shielding grid supply voltage:
E a max = E a . t (1+m)=16 kV
Anode voltage utilization factor in boundary mode
Amplitude voltage at the anode:
U a max = E amax o max =15.7 kV
Amplitude of the first harmonic of the anode current:
I a 1 max =2=69.2 A
Anode current pulse amplitude
I amm == 138.4 A
Equivalent anode load resistance:
The upper cutoff angle is determined from the equation
Where do we get = 0.31 rad = 18 0
DC component of the anode current taking into account the truncated pulse apex
Power consumed by the anode circuit
Power dissipated at the anode
Efficiency of the anode circuit in maximum mode
Excitation voltage amplitude in the control grid circuit and bias voltage
Auto bias resistance
where, = 71.2 0, ? 0.66
Grid current components
where are the coefficients and, taking into account the non-sinusoidal nature of the current pulse, assumed to be equal? 0.66, ? 0.75
Power consumption from previous PC stage and bias source
Power dissipated on the control grid
3 .2 Calculation of final cascade(OK) at the minimum point
The calculation of the minimum point mode is carried out according to the methods outlined in -. The minimum point mode is characterized by low voltages at the anode. In the region e a >0, the regime intensity increases and the MX is slightly bent. To mitigate these phenomena, automatic bias resistance R c .. is included in the current circuit.
The minimum mode parameters are calculated only for the control grid circuit, . The initial data for this calculation are U c max, E c 0, S, R c. .
To find the parameters of the grid current, using the method described in we find from the equation
Power consumption from the bias source and from the PC.
3 .3 Calculation of final cascade(OK) at the telephone point
Calculation of the telephone point mode is carried out according to the methods outlined in and.
Components of anode current
Anode voltage and load voltage amplitude
Power consumption and output
3.4 Final calculation cascade(OK) in modulation mode
Calculation of OC in modulation mode is carried out according to the method described in and.
Average power consumed by the anode circuit
Power delivered by the modulation device
Average power output from OK lamps
Average power dissipated at the anode.
Average power dissipated on the control grid
4 . Calculation of the pre-terminal cascade
The EP for the pre-final stage is selected according to the following rule: according to the reference tables given in the power amplification factor N p = 30 .. 50 is found. Let us take N p = 50. Then the power of the previous stage required to excite the OK is
For this power, a GU-39 B lamp is suitable, with P nom = 13 kW. Characteristics of GU 39 B are given in Appendix 2.
The P chain can be used as a coordination chain for QAP and OK.
5 . R Modulation device calculation
The MMU is implemented using a class D amplifier. The operating principle of this MMU is described in detail in and. A push-pull class D amplifier is designed to amplify the modulating signal. To supply the constant component I a 0t to OK, a separate power source with voltage E at and inductor L d 4 are used. The modulating voltage U Ш is supplied to the pulse-width modulator and the subsequent pulse amplifier and then to the lamp V 2. The second lamp V 1 is controlled by the voltage falling across the resistance R 1 from the anode current of the lamp V 2 .
The schematic diagram of this device is shown in Figure 5.1.
Figure 5.1 Schematic diagram of an MMU with a push-pull class D amplifier.
The advantages of this scheme include:
· a significant increase in the efficiency of the amplifier, due to the fact that the cascade lamps operate in the key mode, and the direct current component I a 0 t OK passes through the inductor with low winding resistance;
· constant efficiency of the amplifier at different levels of the amplified signal (with a rational choice of lamps, the efficiency in such an amplifier can reach 95% - 97%);
· absence of a heavy, bulky, expensive modulation transformer.
The disadvantages of this scheme include:
· the need to carefully adjust the control of the lamps to prevent their simultaneous opening, which would lead to a short circuit of the 2E a power source.
Diodes VD 1 and VD 2 are designed to prevent interruption of the current in the L d 2 coil when the lamps are switched.
Since the calculation of the OK mode parameters has been completed, it is determined
Based on the calculated parameters, the GU-66 B lamp is selected.
Diodes VD1 and VD2 are selected according to the following parameters:
Reverse voltage E rev E p,
Maximum pulse current I D max = 38 A
The forward resistance of the open diode r D is preferably as low as possible. The inductance rating of the filter choke L d 1 is selected in several Henrys. L d 1 = 5 Gn.
Capacitor C 1 is selected from the condition then C 1 = 253 pF
The filter Ld 2, Ld 3, C 2, C 3 is made in the form of a half-link L d 2 C 2 according to Butterworth. Hence
The coupling capacitor C 4 is selected from the condition
Then C 4 = 688 nF.
is chosen from the condition Then we can put
Resistance R 1 is selected so that the inequality is satisfied
where is the cutoff voltage of the anode current of lamps VL1 and VL2.
Thus R 1 = 150 Ohm.
The clock frequency f t is selected from the condition f t = (5..8) F c. Choose f t = 70 kHz.
6 . Ra output loop system account
The calculation of the output oscillatory system is carried out according to the method outlined in and.
The purpose of output oscillatory systems in radio transmitters is to perform the following functions:
· approval active resistance R A antenna feeder with the necessary for normal operation output stage with equivalent load resistance R e in the anode circuit;
· compensation reactance X A of the antenna or feeder so that the VCS operates on an active load and delivers the maximum power to the antenna;
· filtering of harmonics generated electronic devices in the output stages.
To select a video conferencing design, let’s calculate the required filtration
Based on the graph of the dependence s VKS (F required), the design of the output oscillatory system is determined. For z VKS =0.92 and Ф required =2.1 10 3 in the VKS design will look like (Figure 6.1):
Figure 6.1 Schematic diagram of the output oscillatory system.
Maximum and minimum feeder input impedance
Calculation of VKS elements is carried out according to the methodology outlined in.
Then for the first P-chain we have
For the second P chain
Then the ratings of the VKS elements should vary within
7 . Conclusion
As a result of the work done, in accordance with the technical specifications, a broadcasting transmitter with amplitude modulation. The OK, modulation device and output loop system were calculated and the elements for constructing these devices were selected. The MMU is made according to a circuit with a push-pull class D amplifier, which helps to increase the efficiency of the amplifier and simplify its circuit. To match the active resistance of the antenna feeder with the equivalent load resistance in the anode circuit necessary for normal operation of the output stage, as well as to compensate for the reactance of the feeder and to filter harmonics generated by electronic devices in the output stages, an output circuit system with a double U-shaped circuit is used.
Annex 1
Characteristics of the generator triode GU 66 B
The GU-66B generator triode is designed to amplify power at frequencies up to 30 MHz in stationary transmitting radio devices, both in circuits with a common grid and in circuits with a common cathode.
General information
The cathode is thoriated carbided tungsten, directly heated. The design is metal-ceramic with ring leads of the cathode and grid. Cooling - forced: anode - water; legs - air. Height no more than 420 mm. Diameter no more than 211 mm. Weight no more than 23 kg.
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Electrical parameters |
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Filament voltage, V |
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Filament current, A |
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Characteristic slope, mA/V |
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Gain (at anode voltage 4 kV, anode current 8 A) |
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Interelectrode capacitances, pF, no more |
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day off |
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checkpoint, |
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Highest filament voltage |
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Largest starting current filament, A |
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Maximum dissipation power, kW |
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Highest temperature of the leg and ceramic-metal junctions, °C |
broadcast transmitter amplitude modulation transformer
Appendix 2
Characteristics of GU - 39 B
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Permissible influencing factors during operation |
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Ambient temperature, C 0 |
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Relative air humidity at temperatures up to 25 °C, % |
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Electrical parameters |
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Filament voltage, V |
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Filament current, A |
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Characteristic slope, mA/V |
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Output power kW, not less |
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Maximum permissible operating data |
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|
Highest anode voltage (constant), kV |
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|
Greatest operating frequency, MHz |
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Transmitter exciters are quite complex devices. They may include frequency synthesizers, a unit for generating types of work, a transfer unit, and a buffer amplifier. In Fig. Figure 2.1 shows a generalized block diagram of the exciter, which includes all of the listed blocks.
The exciter's task is to generate a high-frequency signal in certain range frequencies, ensuring the required nature of frequency tuning over the operating range, the required stability of the oscillation frequency, the formation of various types of work. In practice, there is a wide variety of ways to construct a pathogen. The choice of method for constructing an exciter can be significantly influenced by the requirements for the speed of switching the operating frequency, the level of by-products in the spectrum of the output signal, and the types of work that are formed in the exciter.
Rice. 2.1. Exciter block diagram
The types of work that are formed in the exciter mean different kinds modulation (manipulation) of a high-frequency signal. There are quite a lot of them. First of all, these are angle modulation, single-sideband modulation, amplitude modulation and others. Some of them are main, others are auxiliary for certain types of radio transmitters. Modulation is carried out at fixed subcarrier frequencies in a special block included in the exciter, which is called the block for generating types of work (BFVR). High frequency signals generated on fixed subcarriers using special block, called a transfer unit (BT), is moved to the working frequency range.
The output device of the exciter is a buffer amplifier (BU). Distinctive feature Control units from other types of amplifiers have a high input impedance. The high input impedance of the control unit ensures decoupling of the exciter from the subsequent amplification path of the RF signal.
The main part of the exciter in modern transmitters is the frequency synthesizer. The frequency synthesizer generates a grid of highly stable frequencies. The frequency grid replaces the continuous operating frequency range with discrete frequencies in increments of F, which is called the grid spacing. The grid step can be from fractions of Hz to tens of MHz. In some VHF communication systems, the grid step is taken to be 25 kHz. This step allows you to organize independent communication channels at adjacent grid frequencies without mutual interference with each other (the principle of frequency division of channels).
Any grid frequency can be represented as
where is a coefficient that can be changed. The required grid frequency is set by a control command (CU) coming from an external device, which sets the required coefficient value.
In addition, the synthesizer can additionally generate one or more fixed subcarrier frequencies for the BFVR, on which modulation is carried out.
The operating frequency is generated at the output of the exciter transfer unit. In transmitters, a transfer unit is a mixer equipped with a bandpass filter. A mixer is a nonlinear device. When signals arrive at the inputs of the mixer with different frequencies and at its output a signal appears, the spectrum of which contains harmonics of the form
where and are arbitrary integers. The main combination frequencies are the frequencies when and: - when transferring a signal up and - when transferring a signal down. In transmitters, the first option is more often used, in receivers - the second option. The operating frequency of the transmitter is formed by summing the signal with the grid frequency and a signal with one of the fixed frequencies coming from the BFVR:
The transfer block's bandpass filter clears the output signal of harmonics and other combinational spectral components. The filtered signal is supplied to the input of the control unit and then to the input of the RF signal power amplifier.
Transmitters in relatively low-power communication systems most often use one type of modulation, such as angle modulation. In this case, the BFVR turns out to be quite simple. For its operation, only one additional subcarrier frequency is formed in the synthesizer. Just such a case is considered below. However, in general, the proposed method for developing an exciter is acceptable for any transmitters.
The development of an exciter consists of the selection and calculation of its individual components.
2.1. Frequency synthesizers
If the transmitter is designed to operate in a frequency range, and the required instability value of the operating frequency is at the level of quartz self-oscillators (AG), then it is most advisable to use a frequency synthesizer in the transmitter exciter.
Basic parameters of synthesizers
1. Operating frequency range of the synthesizer……………….. 
.
2. The total number of frequencies generated by the synthesizer is…………..
3. Number of additional fixed frequencies
The oscillation power at the synthesizer output is usually a fraction of a mW. Currently, the formation of a frequency grid in synthesizers is carried out by two main methods:
1. By direct synthesis method.
2. By the method of reverse (indirect) synthesis.
Direct synthesis method
The direct synthesis method is based on the formation of a frequency grid through the use of simple arithmetic operations - multiplication, division, summation, subtraction. Based on the type of element base used, synthesizers of the direct synthesis method can be analog, digital, or combined.
A radio transmitting device (RTD) is a complex of equipment designed to generate and emit radio signals. The main components of the RPDU are the carrier frequency generator and the modulator. IN modern systems communication RPdU also contains other equipment that provides working together communications equipment: power supplies, synchronization systems, automatic control, control and alarm, protection, etc.
A generalized block diagram of a radio transmitting device with amplitude or phase modulation of signals is shown in Figure 7.9.
The primary signal to be transmitted enters the input circuit. The input circuit ensures the coordination of this signal with the radio control unit; ultimately, this is determined by the parameters of the modulated radio signal transmitted to the line.
The carrier frequency generator generates oscillations of the carrier frequency, which are the carriers of the message. In modern communication systems, the carrier frequency generator is designed as a frequency synthesizer. A frequency synthesizer is a device designed to generate highly stable oscillations in a given frequency range, determined by the stability of the parameters of the master oscillator.
Modulator is a node in which the transmitted message is superimposed on the parameters of the carrier oscillation. When generating radio signals with amplitude or phase modulation in the RPD, the frequency synthesizer produces oscillations with a constant frequency. With the additional influence of a modulating signal on the frequency of the output oscillation of the frequency synthesizer, it is possible to obtain radio signals with frequency modulation.
Rice. 7.9 Generalized block diagram of a radio transmitting device
The power amplifier is designed to increase the level of the radio signal to a value determined by the power of the emitted signal in the communication system. The necessary matching of the RPDU with the antenna is provided by the output circuit.
The advantages of digital methods of information processing (transmission, storage, conversion) have contributed to the widespread use of digital communication systems. The advantage of presenting signals in digital form is also its universality, that is, independence from the nature of the messages being transmitted. Modern communication systems are capable of transmitting not only discrete messages, but also continuous ones (both in time and level). To convert continuous signals into digital ones, use special devices- analog-to-digital converters (ADC).
In an analog-to-digital converter, from a continuous time signal, the signal values at certain points in time are first selected. Most often, such readings are taken at regular intervals. The selected signal values are called samples, and the operation of obtaining samples is called time sampling.
At the next stage of processing, the entire range of possible signal values is divided into a certain number of intervals and it is found out which of these intervals the value of the current sample belongs to. At this stage of processing, the signal value is not taken to be the actual sample value, but the closest rounded signal value to it. This value can correspond to the middle of the interval in which this sample falls, or to another value from this interval (the beginning or end of this interval). The operation of replacing the actual signal value with the nearest rounded value is called quantization, and the width of this interval is called the quantization step. If all the intervals into which possible signal values are divided are identical, then such quantization is called uniform. In some cases, for example, when transmitting speech, it turns out to be advantageous to make such intervals unequal. In this case, they talk about non-uniform quantization.
At the last stage, the analog-to-digital converter replaces the actual sample value with the number of the interval within which the value of this sample lies. The operation of replacing a sample value with a number (code) is called encoding. The most widespread in modern systems is the representation of samples in the form of binary codes. The received codes are then transmitted over the communication system.
A simplified block diagram of a digital communication system transceiver is shown in Figure 7.10. Let's consider the operation of this device.
Rice. 7.10 Digital communication system transceiver
A continuous message from a message source arrives at a device called an encoder. Coding in a broad sense is understood as the operation of converting samples of continuous signals into a sequence of code symbols. As a result, electrical signals corresponding to the code sequence determined by the transmitted message are generated at the output of the encoder.
The code signals in the form of a sequence of pulses are then fed to the modulator, the second input of which is supplied with a carrier frequency oscillation from the output of the frequency synthesizer. The modulator performs appropriate modulation (amplitude, phase, frequency, etc.) of the carrier frequency oscillations in accordance with the incoming code sequence. The modulated signals are then amplified to the required level using a power amplifier and radiated by the transmitting antenna.
Aimed at the receiving antenna electromagnetic radiation are supplied to the input of the amplifier and frequency converter, where oscillations of the carrier frequency of the useful signal are isolated and amplified. The demodulator demodulates the received message, and at the output of the demodulator a sequence of pulses is generated corresponding to the sequence of pulses of the transmitted message (at the output of the encoder), which is fed to the decoder. The decoder performs the reverse operation of encoding and the reconstructed message is sent to the message recipient.
In one transceiver device, the encoder and decoder are usually combined into a single structural unit (usually one chip) and the combined encoder-decoder block is called a codec based on the first letters of its components. Similarly, a combined modulator-demodulator unit is called a modem.
Radio transmitting devices differ in purpose, operating conditions, type of radio signal modulation and other characteristics.
The main energy indicators of the RPdU include the amount of signal power supplied to the antenna and the efficiency factor. A distinction is made between the peak power of the useful signal RpdU and the average power value over a certain time interval. Efficiency is the ratio of the useful power supplied to the antenna to the power consumed by the remote control unit from the power source.
The frequency range in which this RPDU operates is understood as the frequency band that is necessary for transmitting useful signals in the communication system and is allocated to this RPDU for generating radio signals. Unfortunately, in addition to useful signals, radio transmitting devices also emit unwanted vibrations.
Out-of-band emissions are those signals generated by the radio receiver, the spectra of which are located outside the band allocated for a given communication system. Out-of-band emissions are sources of additional interference for communication systems operating in other frequency bands.
An important characteristic of communication systems is the stability of the frequency of emitted oscillations. The frequency instability of the RPDU is understood as the deviation of the frequency of emitted oscillations relative to the nominal value. Insufficient frequency stability degrades communication quality and can cause interference for radio devices operating in adjacent frequency ranges.
Based on their purpose, radio transmitting devices are divided into communications and broadcasting. According to operating conditions, RPDUs are divided into stationary and mobile (installed on moving objects: aircraft, automobiles, portable, etc.). RPDUs also differ in the range of operating frequencies, the power of emitted oscillations, etc.
Coursework on the topic:
Communication radio transmitting devices with frequency modulation
Technical task
When designing a radio transmitting device, the following must be done:
draw up and justify a structural diagram of the PDP;
formulate requirements for individual entrepreneurs and provide diagrams.
Transmitter characteristics:
f = (160 ¸ 180) MHz
D f= 10 kHz
PVI = -50 dB
F mod = (0.3 ¸ 3) kHz
mains power - 220 V, 50 Hz
Introduction
Frequency modulation (FM) communication radio transmitting devices (RTDs) are designed to operate on one fixed frequency or over a range of frequencies. In the first case, the operating frequency is stabilized by a quartz resonator, and to generate FM oscillations, both direct and indirect frequency control methods can be used. The block diagram of a transmitter using the direct FM method is shown in Fig. 1.
Fig.1 Block diagram of a direct FM transmitter
The modulating voltage U W is supplied to the varicap, with the help of which the quartz self-oscillator (KG) is modulated in frequency. The quartz oscillator operates at frequencies of 10-15 MHz, then its frequency is multiplied n times to the operating value, the signal is fed to a power amplifier (PA) and through a communication circuit to the antenna.
The indirect FM method is based on converting phase modulation (PM) into frequency modulation by introducing an integrating element into the circuit, i.e. low pass filter (LPF). The block diagram of the transmitter using the indirect method of obtaining FM is shown in Fig. 2.
Fig.2 Block diagram of a transmitter using the indirect FM method.
A discrete frequency grid synthesizer is used as the exciter of the FM band transmitter, the slave oscillator of which is controlled by two varicaps (Fig. 3).
Fig.3 Block diagram of an FM transmitter with a frequency synthesizer
To build our connected transmitter, we will use a similar scheme, but we will clarify the composition and number of blocks included in it.
A discrete frequency grid synthesizer is used as the exciter of the FM band transmitter, the slave oscillator of which is controlled by two varicaps (Fig. 3). The modulating voltage U W is supplied to the varicap VD1, and the modulating voltage to the varicap VD2 control voltage phase-locked loop (PLL) systems. The separation of control functions is explained by the fact that the frequency deviation under the influence of the modulating signal is relatively small (3-5 kHz) in comparison with the tuning range of the slave oscillator (VCO) by the control signal from the output of the PLL system. Therefore, the varicap VD1 is connected to oscillatory circuit The VCO is significantly weaker than VD2. The frequency grid step at the transmitter output, depending on the operating range, can be 5; 10; 12.5; 25 kHz.
To increase stability, it is necessary that the final amplifier influence the operation of the VCO as little as possible, so they are isolated in frequency by introducing a frequency multiplier into the structure of the transmitter. In this case, the synthesizer grid step is reduced by n times, where n is the multiplier frequency multiplier.
In this course project, an analysis of a FM band transmitter was carried out. The explanatory note presents electrical calculations of the final stage, the communication circuit with the feeder, the self-oscillator and the frequency modulator, and provides structural calculations of the final stage and the communication circuit with the feeder. The explanatory note is accompanied by drawings with images of the complete electrical diagram and the design of the final stage of the transmitter.
1. Calculation of the final stage
1.1 Transistor selection
The power in the feeder of a communication transmitter operating in the range 160 - 180 MHz is 8 W. Let us accept the value of the efficiency of the communication circuit: h CS = 0.7. The power that the final stage should be designed for is:
Р 1max = Р Ф /h ЦС = 8/0.7 = 11.43 W.
The reference value of the power supplied by the transistor must be at least 10 W.
As a rule, a number of transistors can be selected to generate a given power in a load in a certain frequency range. From a group of transistors you need to choose the one that provides the best electrical characteristics power amplifier.
When choosing the type of power amplifier (PA) transistor, consider the following:
to reduce the level of nonlinear distortion, the transistor must satisfy condition 3. f t / β o > f;
transistor output power P out > P 1max.
The efficiency of the cascade is related to the value of the saturation resistance of the transistor - rus. The smaller its value, the lower the residual voltage in the boundary mode and the higher the efficiency of the generator.
Based on these conditions, we select the 2T909A transistor, which has the following parameters:
1. Parameters of idealized static characteristics:
transistor saturation resistance at high frequency r us » 0.39 Ohm;
current gain in a circuit with OE at low frequency ( f→0) β o = 32;
base resistance r b = 1.0 Ohm;
emitter resistance r e = 2.0 Ohm;
2. High frequency characteristics:
limiting frequency of current amplification in a circuit with OE f t =570 MHz;
collector junction capacitance C k = 30 pF;
emitter junction capacitance C e = 244 pF;
terminal inductance L B = 2.5 nH, L E = 0.2 nH, L K = 2 nH;
3. Acceptable parameters:
maximum voltage on the collector U ke add = 60 V;
reverse voltage at the emitter junction U be add = 3.5 V;
constant component of the collector current Iko. additional = 2 A;
maximum permissible value collector current I c. max. additional = 4 A;
operating frequency range 100 - 500 MHz;
4. Thermal parameters:
maximum permissible temperature transistor transitions t p. add = 160 ºС;
thermal resistance transition - housing R pc = 5 ºС/W;
5. Energy parameters
P out = 17 W;
Operating mode - class B.
Because The PA must amplify the signal from minimal distortion, i.e. to have a linear amplitude characteristic, and, in addition, the highest possible efficiency, let us take the collector current cut-off angle q = 90° (class B). Wherein
- Berg coefficients.1.2 Calculation of the collector circuit
1. Amplitude of the first harmonic voltage on the collector in the critical mode
IN2. Maximum collector voltage
INBecause condition not met
, it is necessary to reduce E
k, let's choose a standard constant supply voltage equal to 24 V. And also, if E
k choose equal to the greatest maximum permissible for of this type transistor, then we should expect a significant decrease in its reliability due to the danger of breakdown. In V. 3. Amplitude of the first harmonic of the collector current
A 4. DC component of the collector current
A; 5. Maximum power consumed from the collector voltage source
W6. Efficiency of the collector circuit at rated load