A simple 27 MHz transmitter circuit. Transmitter and receiver of the radio control system. Technical characteristics of the radio station

While the idea of ​​creating a wireless on/off switch may be trivial, designing, implementing, and understanding what's going on is a lot more complicated than it seems at first glance. For years I've wanted to build an RF transmitter and RF receiver from scratch, but it always turned out to be too complicated. This time everything will be different!

In this article we'll look at what it takes to build a simple 27 MHz RF transmitter. various processes what's going on in the transmitter, how everything interacts, and let's test it on some measuring equipment. The ultimate goal is to pair this transmitter with a receiver so that when transmitting, the LED on the receiver turns on. That's how simple it is.

Targetand an overview of this project

Purpose of this project is to create an RF transmitter that can send on/off pulses from its antenna to some receiver. The transmitter must be small and fit in the palm of my hand and must operate within government regulations of output power and frequency ranges. We will make this transmitter based on the fact that we want to make a receiver that turns on the LED during transmission. Simple idea, but not a simple implementation.
The transmitter must output a digital on/off signal at 350 Hz and use a carrier frequency of 27.145 MHz. It must be a continuous RF wave transmitter, so there is no modulation, the signal is simply turned on or off.

Circuit overview

The circuitry of this project is actually deceptively simple compared to the complexity of what's going on in the circuit.

Features of the scheme

Master oscillator

The first transistor T1 is configured to excite the 27.145 MHz crystal and cause it to oscillate at its natural frequency.

Creationon/off signal 350Hz

The 555 timer is configured to take a 350Hz signal from its pin 3 and feed it into our transmitter circuit.

Mixingsignals

The two signals we just generated are mixed at the base of T2 and once they exit the collector of the transistor, our RF signal is ready for transmission.

Board overview

The layout of the board was made so that all the parts were located very tightly. This is difficult to do with inferential elements, but not impossible.

Peculiaritiesboards

Earth
The ground spans the entire board (but is interrupted by traces) so that all elements that need to have access to the ground easily get it. Land is also very important because... acts as part of our antenna.

Trace Width
I just chose a good width for the beauty of the PCB, but it seems that less wide traces would be better for RF circuits... But I don't believe that on such low frequencies there will be a performance gain.

PCB assembly

Our board is ready, and now we will solder all the elements onto it. So put all the elements together like I have below:

First, we solder the on/off pulse generator on the 555 timer. Its operation can be easily checked by pressing the power button and measuring it with any voltmeter.

Now, solder the 27.145 MHz oscillator circuit.

Then solder the mixer circuit.

Finally, solder the last 10uH inductor and 12" antenna wire to the board.

Here's a view of the soldering from below:

Exactly the same view from above. Isn't it beautiful?

The transmitter is assembled! Now let's go over the theory of its operation.

Principle of operation

Rather than focusing on the math and raw theory behind this simple RF transmitter, we'll focus on the elements in each step. The math of how/why this circuit actually works is terribly ugly and way too complicated... so it's fun (for me) to just build and "feel" what works where and how.
So let's take some time to go through the circuit step by step to understand each part of the circuit, its purpose and the type of signal in important points. We'll go through 3 sections, first we'll take a look at how the signals we want to transmit are created, and then we'll move on to see what those signals look like when we want to transmit them, and then finally we'll look at measurements transmitter output power.

Carrier frequency generation

First of all, we need to generate the signal that we will transmit. Here is part of the circuit with a crystal oscillator:

Above you can see that the circuit produces a sine wave at the frequency we need. There is no filtering for many of the harmonics present, which slightly distorts our result, but this signal will work.

Generating On/Off Signals

The next signal we want to generate is a low frequency "digital" on/off signal. To do this we use a simple 555 timer:

At its exit we observe a meander, which is what we expected to see. Now, let's see what happens when these two signals are mixed.

Signal mixing

After the 27.145 MHz carrier frequency comes out of the 150 pF capacitor, it meets the 555 timer square wave after the 22 k ohm resistor and the two signals are mixed (multiplied if you like). Below you can see the end result of this mixing and where exactly in the diagram it happens:

The square wave from the 555 timer is still very noticeable and the signal is ready to go to the base of the transistor and will look like what we want to transmit.

The resulting continuous signal

Once the mixed signal goes into the transistor, the powerful on/off switching from the 555 timer helps make a nice continuous output signal at our carrier frequency, ready to hit our antenna (after passing one last DC blocking capacitor).


What comes out is either a giant sine wave with an amplitude of 2V between peaks or a main 0V. The on/off distance corresponds to our original 350Hz signal. So let's now take some power measurements to see how "powerful" our transmitter really is!

Spectrum Analysis

To make sure the transmitter outputs what we expect, a prototype transmitter I built was connected to a spectrum analyzer:



Our carrier frequency is certainly visible with the highest peak at 9dmb (about 10 mW), and then the harmonic frequencies are visible on both sides. Harmonics are always expected in systems that do not have filtering.

The last thing to do is to see what our capacities look like, whichto make sure the government doesn't hunt us down to create something too powerful. The power consumption at one peak frequency is analyzed. Note, high power was actually at 27.142 MHz and was not at 27.145 MHz. This is influenced by many factors.

The powerful output waves seen above look like the square wave we wanted to convey, which is pretty good considering we're looking at a mixed signal. This means that our receiver must have less demanding on/off detection circuitry that falls at 7dBm and -25dBm. The transmission power is within the tolerance of most countries.

Dataand observations

The transmitter itself is a boring thing to watch in action. You turn it on and it transmits... You must have a receiver. In the next article we will look at how to build a paired 27MHz receiver and when you do, you can watch the test video below:

As soon as you watch the video of the transmitter testing above, all doubts will disappeart you, because the system operates as designed and as required for the purposes of this project. You transmit, the LED lights up. You stop transmission, the LED goes out. Perfect!

List of radioelements

The transmitter can be made as an independent device or be part of a CB radio station, the number of channels is 4, the power source is a galvanic battery, the antenna is telescopic with a length of 750 mm.

Technical specifications

• Operating frequency in the range of 27 MHz
• Rated power 0.5W
• Current consumption no more than 100mA
• Supply voltage 6…12V
• Type of modulation - frequency with deviation - 2.5 kHz

The transmitter is made according to a two-stage circuit on VT2 VT3, with VT2 being the master oscillator and VT3 being the power amplifier. The frequency of the master oscillator is stabilized square. resonator in the base circuit. Channel selection - switch S1 - switching square. resonators. FM occurs by shifting the resonator frequency using external inductance L1 and varicap capacitance VD3, which changes under the influence of AF from a microphone amplifier.

VT3 operates without an initial bias; at the output there is a U-shaped circuit C16L4C17, which matches the output impedance of the power amplifier with the characteristic impedance of the antenna. L5 - extension coil.

The microphone amplifier is made using op amp A1. It receives an AF signal from a microphone that has a built-in amplifier. A field-effect transistor is used to limit the signal; it is connected to the OOS circuit A1 and, depending on the output signal, changes the depth of the OOS.
For a tone call, use the RC circuit R5 C10, which is turned on by button S1.

Transmitter coils are wound on plastic frames with a diameter of 7 mm with a 100HF trimmer core with a diameter of 2.8 mm and a length of 12 mm. Wound with PEV 0.31 wire.
L2 - 6 turns, L3 - 3...4 turns, L4-8 turns, L5 - 18 turns. Chokes L1 DL1 - type DPM 01 at 16 μH.

Tuning - by monitoring the signal strength and frequency using an oscilloscope or wave meter with a volumetric coil at the input.

Literature - Radioconstructor 1999-02

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500 mW transmitter

Schematic diagram

The transmitter (Fig. 3.30) can operate in the frequency range 27.12-28.2 MHz. The specific frequency value is determined by the quartz used. In the master oscillator, implemented on transistor vt1, quartz can be used both directly at the mentioned frequencies and at frequencies two (three) times lower. In the latter case buffer stage on transistor vt2 additionally performs the functions of a frequency doubler (tripler).

The values ​​of the elements of the master oscillator cascade are indicated in the diagram for the case of using a 14 MHz quartz resonator. The output frequency of the transmitter will be equal to 28 MHz. Amplitude manipulation is carried out by

switching the emitter circuit of the buffer stage transistor using an electronic key vt3, controlled by pulses from the output of the encoder.

Operating principle

The vt4 power amplifier operates without an initial bias at the base, which ensures collector current cutoff during negative half-cycles of the input voltage. The cutoff angle is chosen to be less than 90° due to the use of a low-resistance resistor r9 in the emitter circuit. The direct component of the emitter current creates a voltage drop across it, shifting the operating point of the transistor to the region of negative voltages at the base. Reducing the cutoff angle has a beneficial effect on the efficiency of the output stage, which, with careful tuning, can reach 70%.

The output P-circuit c10-l3-c11 provides suppression of higher harmonics of the collector current and matching of the power amplifier output with the active component of the antenna resistance. The reactive component of this resistance is compensated by extension coil l4. The buffer cascade uses partial inclusion of the circuit in the collector circuit, which provides better suppression of the fundamental harmonic when the frequency is multiplied.

Details and design

The printed circuit board is made of single-sided fiberglass. Its wiring is shown in Fig. 3.31. A quartz resonator at 14 MHz is used in a small-sized type RK-169. If it is intended to use resonators for the 9 MHz range with subsequent tripling of the frequency, then the values ​​of elements C2, SZ and r2 must be changed to 180 pF, 120 pF and 2 kOhm, respectively.

When using quartz directly at the radiation frequency, the circuit should be modified by including an oscillatory circuit in the collector circuit of transistor vt1. The left plate of capacitor C4 must be connected to the collector of the transistor. The coil of this circuit should contain 8 turns of wire with a diameter of 0.35 mm on a frame with a diameter of 5-6 mm with a tuning core made of carbonyl iron or high-frequency ferrite. The circuit capacitor should have a capacity of 27-33 pF.

Coils l1 and l4 are wound on the same frames and contain 2x5 and 15 turns, respectively, and in the first of them a wire with a diameter of 0.35 mm is used, and in the second - 0.18 mm. l2 is a standard inductor with an inductance of 20-30 μH. The l3 coil is frameless, contains 7 turns of wire with a diameter of 0.8 mm, wound turn to turn on a mandrel with a diameter of 6 mm.

It is useful to equip the vt4 transistor with a small radiator. If the transmitter housing is metal, and this is always desirable, then the output stage can be assembled using a KT644 transistor, fixing it directly to the housing. This transistor has pnp conductivity, and its collector is connected to the common wire. Since the collector is structurally connected to metal plate, available on the transistor body, then an insulating gasket between the transistor and the transmitter body is not needed. The output stage diagram for this case is shown in Fig. 3.31. Coil N contains 2-3 turns of wire with a diameter of 0.18 mm and is wound on top of coil L1. The printed circuit board must be adjusted in this case.

All capacitors in the circuit are ceramic, for example type KM-6. KT315 transistors can have any letter index or are replaced by KT3102. It is advisable to use a rod 1.2-1.5 m long as an antenna.

Settings

First, the parts related to the master oscillator are soldered into the board (everything up to and including resistor r4). A high-frequency oscilloscope is connected to this resistor, and the supply voltage is supplied to the cascade. The screen should display sinusoidal oscillations with an amplitude of 1.5-2 V and a frequency of 14 MHz. The reason for their absence can only be incorrect installation or a faulty quartz resonator.

Having verified the presence of oscillations, assemble a buffer stage up to and including resistor r6. The modulator input is temporarily connected to the positive of the power source. On the screen of an oscilloscope connected to resistor r6, frequency fluctuations of 28 MHz should be observed. Amplitudes in adjacent periods may differ from each other (every other period) due to poor filtering of the first harmonic of the master oscillator. By rotating the core of the coil l1 it is necessary to minimize these differences.

from the middle of the antenna. The sensitivity of the oscilloscope is set to maximum. After turning on the power, rotating the core of coil l4 sets the maximum amplitude of the observed oscillations. Then the position of the turns of coil l3 is specified. It is necessary to insert a carbonyl or ferrite core inside this coil with tweezers.

If, as the core approaches, the amplitude of the observed oscillations increases, then the inductance of the coil is insufficient. It is necessary to wind a new coil, increasing the number of turns by 1-2. If the amplitude decreases, it means that the inductance is greater than required and it is necessary to carefully stretch the turns of the coil until the maximum amplitude of the observed oscillations is fixed. After this, check the position of the extension coil core l4.

The current consumed by the transmitter at a supply voltage of 12 V should be in the range of 80-120 mA. output power in this case it will be about 500-700 mW (depending significantly on the length of the antenna).

Section: [Modeling]
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Technical characteristics of the radio station:

• Supply voltage, V 6
• Radio receiver sensitivity at s/n ratio 3:1, µV 0.2
• Transmitter output power, mW 250
• Current consumption in receive mode, mA 7
• Current consumption in transmission mode, mA 90
• Helical antenna length, cm 16
• Dimensions, mm 150x70x20 Weight, kg 0.25

The schematic diagram of the radio station is shown in the figure.

The signal from the WA1 antenna through the XS1 connector is sent to the SB2 button, which switches the antenna and the radio power supply when switching from reception to transmission. On schematic diagram SB2 is shown in radio receiving position.

In the receiving mode, the received signal from the SB2 button is sent to the communication coil L1, and the power supply voltage of the radio receiver is also supplied there. The input circuit C1, L2 is tuned to the operating frequency of the radio station. UHF uses full inclusion of the input circuit due to the high input impedance field effect transistor UHF VT1 type KP350B. The UHF gain is set by resistors R1 and R2.

Puc.1

The UHF load is circuit L3, C4, also tuned to the operating frequency. From the UHF output, the received, filtered and amplified signal through the L4 coupling coil is supplied to the DA1 chip of type K174ХА42 (its analogues are TDA7000, KC1066XF1). More details about this microcircuit can be found in.

Pin 6 of the DA1 chip receives RF voltage from the local oscillator on transistor VT5. The input signal arrives at pin 13. The output low-frequency signal from the volume control engine R5 goes to the ULF, made on a DA2 chip of type K174UN4A, and from its output to the dynamic head BA1.

In transmission mode, the SB2 button is moved to the lower position according to the diagram, while the supply voltage is supplied to microphone amplifier and the output stage of the transmitter. The master oscillator on transistor VT5 operates constantly. In transmission mode, diode VD3 is closed, and the frequency of the generator on transistor VT5 increases by intermediate frequency. Frequency adjustment over the range is carried out by the core of the coil L5, and its shift to an intermediate frequency is carried out by capacitor C36.

In circuit L7, C39, a signal is allocated with an operating frequency - in transmission mode, and with a shift down by the IF - in reception mode. From the communication coil L6, the quartz oscillator signal is supplied to the DA1 microcircuit, and from the collector of the VT5 transistor - to the base of the transistor of the output stage of the VT6 type KT646A transmitter. The output stage of the transmitter operates in C mode, its load is a double P-filter on elements L8, L9, C41...C44. Circuit L8, C42 is tuned to the second harmonic of the operating frequency. Next, the signal from the transmitter output through the SB2 button and the XS1 connector enters the WA1 antenna. A BA1 dynamic head with a resistance of 8...50 Ohms is used as a microphone.

The microphone amplifier of the radio station is built on transistors VT2 and VT3. It slightly limits the LF signal in amplitude, while expanding the signal spectrum. The low-frequency signal is supplied to the low-pass filter, made on transistor VT4 and elements C27...C29, R12, R13.

The filtered low-frequency signal from resistor R19 is supplied to a varicap VD4 type KV109G. Resistors R21 and R20 are installed on the varicap constant pressure+1.5 V. The master oscillator carries out frequency modulation of the operating frequency in transmission mode with a deviation of 2.3...3 kHz.

SA1 is used to turn on the radio station.

The printed circuit board of the radio station is made of double-sided foil fiberglass 1.5 mm thick, and the foil on the installation side of the elements is completely preserved and is removed by countersinking only under the terminals that are not connected to the common wire.

The radio station uses resistors such as MLT-0.125, S2-23, S2-33 or the like. Variable resistor R5 - type SP4-GM with a switch (SA1). Electrolytic capacitors- type K50-35, K50-41, K50-16 for an operating voltage of at least 6 V, other capacitors - types KM4, KM5, KM6, K10-17. Trimmer capacitor - KPKM type. Transistors VT3 and VT2 - type KT3102E (you can use others - KT315, KT342, KT358, etc.), VT5-KT368A, B, KT315, KT316, KT325, KT355, KT399, etc. Transistor VT6 - type KT646A, you can also use KT603, KT608, KT606, KT610, KT904, KT911, varicap VD4 - KV109, KV110, KV124, D901 with any letter index. When replacing components, it should be taken into account that the use of some of them will entail an increase in the size of the radio station and energy consumption.

Inductors L1, L2, L3, L4, L5, L6, L7 are wound on frames with a diameter of 5 mm with tuning cores from SB-9 or from intermediate frequency filters of CB and LW radio receivers. Coil L1 is wound on top of L2, L4 is wound on top of L3, and L6 is wound on top of L7. Reels L8 and L9 are frameless, on a mandrel with a diameter of 3 mm. Coils L10 and L11 are on ferrite rings of standard size K7x4x2 with permeability 300...1000. All coils, except frameless ones, are wound with PEV-2 wire and impregnated with BF-2 glue. The winding data of the inductors is given in the table. Frameless ones are wound with wire with a diameter of 0.5 mm.

Setting up a radio station you should start with the radio. By applying the supply voltage, check the operation of the low-frequency amplifier of the DA2 chip. When a signal is supplied from a signal generator with a voltage of 100 mV and a frequency of 1 kHz, there should be an output voltage of about 1 V at pin 8. The volume control should be at maximum. Next, voltage with an operating frequency and deviation of 3 kHz is applied to connector XS1, and by rotating the cores, circuits L2, C1 and L3, C4 are adjusted to maximum volume. In this case, you may need to more accurately tune to the operating frequency using tuning capacitor C36. Then the sensitivity of the receiver is measured - it must be at least 0.2 μV with a signal-to-noise ratio of 3:1.

Setting up the transmitting path begins with the master oscillator. It usually starts working right away. The L5 coil core must be used to set the operating frequency. Then, by rotating the core of coil L7, its output signal is maximized, monitoring the level at the collector of transistor VT5 using an RF voltmeter. Then, by connecting a resistor with a resistance of 51 Ohms and a power of 0.25 W as an antenna equivalent, by stretching and compressing the turns of coils L8 and L9, the maximum output voltage on it is achieved, which should be no less than 5...7 V.

Setting up the modulator is reduced to installing a voltage varicap of +1.5 V at the cathode. The low-frequency amplifier and filter begin to work immediately. You just need to check the operation of the tone generator by switching the SB1 button to the bottom position according to the diagram.

The spiral antenna is wound on a polyethylene rod of the central dielectric from a RK-50 or RK-75 cable with a diameter of 7...8 mm with PEV-2 wire with a diameter of 0.5 mm, turn to turn, for a length of 160 mm. One end of this winding is fixed to a rod, the other is connected to the input of the radio station. The core and braid have been removed. The antenna is wrapped with insulating tape on top, or a polyvinyl chloride tube of the appropriate diameter is placed on it.

The radio station's antenna is quite narrowband and requires tuning to operate effectively. To tune, you need a simple resonant wavemeter at the required frequency. By unwinding or rewinding the turns of the antenna winding, it is adjusted to the maximum deviation of the resonant wavemeter needle.

This completes the antenna setup.

Tests of this radio station have shown that the communication range in open areas reaches 7...8 km, and in densely built-up urban environments - 2.5...3 km.

Regarding the purchase of drawings printed circuit boards and assembly drawings, please contact the author with a return-addressed envelope and Russian stamps in the letter.

Literature

1. Polyakov V. Single-chip FM receivers. - Radio, 1997, N 2.

VHF FM mini transmitter

A. Kichigin
RL 7/2000

I propose a radio microphone circuit (Fig. 1). Power supply - 1.5 V from the battery. The range is about 100 m, in line of sight conditions. Current consumption - no more than 6 mA.

One flashlight battery lasts for 48 hours of continuous operation. The board (Fig. 2) is made of double-sided foil fiberglass. Coils L1 and L4 are wound on a plastic housing of transistor VT2 (KT368) (this transistor is not suitable for a metal housing).

Coil L1 contains 1 turn of PEL wire 00.3 mm; L4-4 turns PEL 00.3 mm. Coil L2 is frameless, wound turn to turn on a 06 mm mandrel and contains 22 turns of 0.6 mm PEL wire. L3 is wound in bulk on a resistor of 100...200 kOhm and contains 60 turns of PEL wire 00.11 mm. The battery is located directly on the board and is secured by a brass plate, which is curved to the shape of the battery. It is inserted into the holes of the board and soldered from below. Pieces of wire are inserted into points X1 and X2 and soldered to obtain the negative contact of the battery. When setting up, you need to shift or stretch the turns of coil L4, and, if necessary, select capacitor C5 to ensure that the carrier falls into the desired range (30, 66, 108 MHz). Resistor R5 sets the required level of frequency deviation. After tuning to the required portion of the range, capacitors C7 and C8 achieve maximum signal power.

To fix and stiffen the L4 coil, it is advisable to impregnate it together with VT2 with paraffin.