While the idea of ​​building a wireless on/off switch may be trivial, designing, implementing, and understanding what's going on is a lot more complex than it first appears. For many years I have wanted to build an RF transmitter and RF receiver from scratch, but it has always proved too complicated. This time it will be different!

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

Targetand an overview of this project

The goal 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 my palm and must operate within state regulation output power and frequency ranges. We will make this transmitter based on what we want to make a receiver that turns on the LED while transmitting. A 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 transmitter, so there is no modulation, the signal is simply switched on or off.

Circuit overview

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

Circuit Features

master oscillator

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

Creationon/off signal 350Hz

The 555 timer is configured to receive 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 as soon as they exit the collector of the transistor, our RF signal is ready to be transmitted.

Board Overview

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

Peculiaritiesfees

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

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

PCB Assembly

Our board is ready, and now we will solder all the elements on 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 is easy to check by pressing the power button and measuring it with any voltmeter.

Now, solder the 27.145MHz oscillator circuit.

Then solder the mixer circuit.

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

Here is a bottom view of the solder:

Exactly the same top view. Isn't it beautiful?

The transmitter is assembled! Now let's go through the theory of how it works.

Principle of operation

Instead of focusing on the mathematical and crude theoretical side of this simple RF transmitter, we will focus on the elements in each of the steps. The math of how/why this circuit actually works is horribly ugly and overly complicated... so it's interesting (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 signal type at important points. We will go through 3 sections, in the first one we will take a look at how the signals we want to transmit are created and then we will go further to see what those signals look like when we want to transmit them and then finally we will look at the measurements transmitter output power.

Carrier frequency generation

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

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

Generation of on/off signals

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

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

signal mixing

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

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

The resulting continuous signal

After 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).


The output is either a giant sine wave with an amplitude of 2V peak-to-peak or a fundamental 0V. The on/off distance corresponds to our original signal of 350Hz. 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 definitely visible with the highest peak at 9dmb (about 10mW) and then harmonic frequencies are visible from 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, whatto make sure the government doesn't hunt us down to build 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. Many factors influence this.

The powerful output waves seen above look like the square wave we wanted to transmit, which is pretty good considering we're looking at a mixed signal. This means that our receiver should have less demanding on/off detection circuitry that falls on 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 it is, you can watch the test video below:

Once you watch the transmitter test video above, all doubts will be gone.t you, because the system works as intended and as required for the purposes of this project. You transmit, the LED lights up. You stop transmission, the LED goes off. Excellent!

List of radio elements

Designation	Type	Denomination	Quantity	Note	Shop	My notepad
IC1	Programmable timer and oscillator	ICM7555	1			To notepad
T1, T2	bipolar transistor	2N2222	1			To notepad
D1	rectifier diode	1N4148	1			To notepad
C1	Capacitor	0.1uF	1			To notepad
C2	Capacitor	68 pF	1			To notepad
C3	Capacitor	150 pF	1			To notepad
C5	Capacitor	27 pF	1			To notepad
C6	Capacitor	100 pF	1			To notepad
C9	electrolytic capacitor	2.2uF	1			To notepad
R1	Resistor	100 kOhm	1			To notepad
R2	Resistor	100 ohm	1			To notepad
R5	Resistor	470 ohm	1

Transmitter block diagram with direct
frequency modulation shown in fig. 15.2. An integral part of such a scheme is the reactance scheme.
To obtain a frequency modulated signal, it is required to change the carrier frequency at a rate that depends on the frequency of the modulating signal. Thus, if the frequency of the modulating signal is
100 Hz, the carrier frequency after modulation will deviate from medium frequency both ways 100 times per second. Similarly, if the frequency of the modulating signal is 2 kHz, then the frequency of the modulated signal will change 2000 times per second. The magnitude of the frequency deviation from its average value is determined by the amplitude of the modulating signal. With an increase in the amplitude of the modulating signal, the deviation of the carrier frequency from the average value increases.
Since the carrier frequency changes continuously during frequency modulation, the carrier generator must be capable of frequency agility. In order for the carrier frequency to be stable, a quartz oscillator is used. In addition, an automatic frequency control circuit is used for the same purpose.
Rice. 15.2. Block diagram of a direct FM transmitter.
Variable frequency generator in the circuit in fig. 15.2 has a frequency equal to 1/18 of the carrier frequency.
Thus, if the carrier frequency is 90 MHz, then the oscillator frequency will be 5 MHz. The maximum deviation (deviation) of the frequency is maintained within 4.2 kHz in order to provide linear frequency modulation. If, for example, the frequency deviation of the generator is 4 kHz, then the frequency deviation at the output will be 72 kHz, since the frequency deviation also increases by 18 times due to multiplication.
In this circuit, a quartz autogenerator generates oscillations with a frequency of 2.8 MHz. This frequency is then doubled to 5.6 MHz and fed to the mixer, which also receives 5 MHz signals from a variable frequency oscillator. At the output of the mixer, a difference frequency signal of 600 kHz is generated, which is fed to the automatic frequency control (AFC) circuit.
When the circuit is running, it maintains a steady state. If the oscillator frequency deviates from 5 MHz, then the difference frequency signal at the mixer output will not match the resonant frequency.

frequency to which the AFC circuit is tuned. As a result, a voltage will appear at the output of the AFC circuit, which will act as a control signal, correcting the frequency drift of the generator (see also Section 4.6).
As shown in the figure, the control signal from the output of the AFC circuit passes through a low-pass filter and is fed to the reactance circuit. The latter corrects the frequency drift of the variable frequency oscillator (see Chap. 12). A low-pass filter is used to ensure that the modulating oscillations contained in the 0.6 MHz signal do not enter the reactance circuit. This filter usually passes signals with a frequency of no more than 10 Hz. By eliminating the audio frequency signals, they will not affect the control function. If the sound components are not filtered, then they will lead to the appearance of reactivity, opposite in sign to that which occurs under the influence of signals supplied from the modulating circuit. As a result, the frequency modulation of the carrier may be reduced to zero. Since the frequency drift of the frequency-controlled generator occurs at a very low rate, the voltage change at the output of the AFC circuit occurs at a frequency well below 10 Hz, i.e., within the band of the low-pass filter.
Another method for obtaining FM signals is shown in fig. 15.3. Amplitude modulation is performed first, which is then converted to frequency modulation by shifting the sidebands by 90° and reconnecting the sidebands and carrier. Low-power frequency modulation is used here, so only two side components of sufficient amplitude are formed. By shifting the phase of the side components, phase modulation is obtained, which can be converted to frequency modulation using a correction circuit. In the diagram in fig. 15.3, a quartz oscillator is used, the signals of which, after frequency multiplication, form a carrier. Sound signals from the amplifying output stage are fed to a balanced modulator, which also receives signals from a quartz oscillator. In a balanced modulator, amplitude modulation of the carrier is carried out by audio signals. Two side pieces
The AM signal is applied to a quadrature phase-shifting circuit. The two sidebands are then combined with a carrier that is fed from a crystal oscillator through a buffer amplifier. Thus, it is carried out indirect frequency modulation. In subsequent stages, the frequency is multiplied to the required value. In a balanced modulator, the carrier is suppressed so that only sideband signals are produced at its output (see Chapter 6).
Rice. 15.3. Block diagram of an indirect FM transmitter.
With phase modulation, the carrier deviation is a function of the frequency of the audio modulating signal, multiplied by the maximum allowable phase shift. Therefore, a higher frequency of the audio signal will correspond to a greater amount of carrier deviation, in contrast to frequency modulation, where the deviation depends only on the amplitude of the audio signal. To equalize the deviation so that it corresponds to the value that occurs at the FM, a corrective circuit is introduced, shown in Fig. 15.3.
This circuit is made up of series resistor and a parallel capacitor. The resistance pesncTqpa is chosen so that it is significantly greater than the reactance of the capacitor over the entire audio frequency range. Therefore, the characteristics obtained during the phase modulation of the signals are compensated, and at the output the signal acquires the properties of an FM signal.
The output signal from the correction circuit is taken from the capacitor, so the amplitude of the signals

varies with frequency. At low frequencies, the capacitor has a large reactance and has little shunting effect. In this case, the signal amplitude is essentially completely transferred to the next stage. However, at higher frequencies, the reactance of the capacitor decreases so that it has a stronger shunting effect. Therefore, as the frequency increases, the amplitude of the signals coming from the corrective circuit to the output amplifier decreases. This operation, the reverse of the process of phase modulation, leads to the compensation of the latter. As a result, a process is carried out that is equivalent to the standard frequency
MODULATIONS
,
at which the same amplitudes of the audio signals correspond to the same carrier frequency deviations, regardless of frequency.
15.3. Multi-channel FM transmitter
As shown earlier in sect. 6.4, in FM broadcasting systems, 100% modulation is defined as a frequency deviation of 75 kHz either side of the carrier. In FM stereo or other multi-channel systems, the transmission must be carried out in such a way that the frequency spectrum remains within the specified limits defined by the specified 100% modulation. Thus, during stereo transmission, the various modulating signals should not lead to exceeding the limits defined by 100% modulation.
In systems High Quality modulating sound signals are usually in the frequency range of 30
Hz - 15 kHz. Higher modulating frequencies can also be used, provided that their amplitude is not too high and the frequency band does not exceed the specified limits. At higher baseband frequencies, the carrier deviation rate increases. Thus, the use of higher-frequency modulating signals makes it possible to implement a convenient method for generating signals in multichannel (stereo) systems.
Rice. 15.4. Stereo transmitter with FM.
When transmitting stereo signals, compatibility must be ensured, i.e., the possibility of receiving both stereo and conventional single-channel receivers. To ensure compatibility, stereo stations transmit a mono signal obtained by combining two signals from different sources. In this case, the sound signals from the left and right microphones are fed to the modulating circuit of the main FM transmitter, which

is the main channel. Such a method is illustrated in Fig. 15.4, where the signals of the left (L) and right (R) channels are fed to the monomixer. These signals are then fed to the modulator carrier generator and other circuits that make up the main FM transmitter.
For the transmission of stereo signals, additional circuits are required, which form separate left and right channels. For this purpose, a difference signal is formed by subtracting the right signal from the left
(right and left signals are applied to the mixer with a phase shift of 180°). The difference signal is used to modulate an additional carrier (called a subcarrier) in amplitude (AM), resulting in sidebands. These sidebands individually modulate the carrier in frequency.
The subcarrier frequency is suppressed, and therefore, when receiving stereo signals, it must be restored in the receiver (see Section 15.7).
The subcarrier frequency is 38 kHz (the generator generates a frequency of 19 kHz, which is then doubled to obtain the desired frequency of 38 kHz). A 19 kHz signal is also transmitted (by carrier modulation) to synchronize the stereo detector in the receiver. In this case, the 19 kHz signal, called the pilot signal, carries out a shallow carrier modulation (approximately 10%). This is sufficient to double this frequency to recover the 38 kHz subcarrier at the receiver. At the receiver, the subcarrier is demodulated along with the stereo sidebands (see Figure 9.6).
The sidebands that result from modulating the 38 kHz subcarrier with a difference signal are not the same as mono modulating signals; side components are located in the frequency range 23 - 53 kHz. As in the case of a mono signal, the frequency range of stereo audio signals is in the range of 30 Hz - 15 kHz. Thus, a multi-channel modulating signal in FM stereo transmission consists of a mono signal (L + R), the frequency of which lies in the audio range of 30 Hz - 15 kHz, a pilot signal (subcarrier) with a frequency of 19 kHz and (L - R) signal (23 - 53 kHz) with a carrier frequency of 38 kHz suppressed during transmission. When transmitting musical recordings, the main carrier is also modulated by signals over two channels using an auxiliary generator, as shown in the figure with dashed lines.
The subsidiary communications authorization (SCA) method allows the transmitting station to use additional channels in addition to the conventional broadcast channel. The FM channel is used for broadcasting, and the combined (SCA) channel is used only for transmitting signals from the pickup, for example, for sound accompaniment and other auxiliary purposes. As shown in fig. 15.4, the auxiliary oscillator is essentially a miniature FM transmitter (compared to the main transmitter) with a subcarrier frequency of 67 kHz.
15.4. TV transmitter
In television, the image is transmitted using the carrier amplitude modulation method, as in conventional
AM radio broadcast. Frequency modulation is used to transmit audio signals.
The difference between the frequencies of the image carrier and the sound carrier is 4.5 MHz (see Fig. 5.14, a).
When transmitting a black-and-white image, it is also necessary to transmit signals for synchronizing vertical and line scans. However, in color television, carrier modulation also uses chrominance signals and additional clock signals.
In a black-and-white television receiver, the master oscillator generates fundamental frequency oscillations, from which signals are obtained for scanning circuits. The oscillation frequency of the master oscillator is 31.5 kHz.
To obtain a line frequency (15750 Hz scan, it is divided by two, and to obtain a vertical scan frequency of 60 Hz, it is divided by 7, 5, 5, and 3. In the case of color image transmission, these frequencies are somewhat different due to the peculiarities of the spectrum width and synchronization. In color transmission, a sub-carrier needs to be generated and modulated to produce chrominance sidebands, and then the carrier needs to be suppressed due to the limited bandwidth available for transmission.Therefore, at the receiver, the carrier must be recovered and mixed with the sidebands for subsequent color difference demodulation. signals.
Thus, the horizontal scanning frequency in a color television receiver is 15734.264 Hz, and the subcarrier frequency is 3.579545 MHz (3.58 MHz). The frame rate in a color television receiver is 59.94 Hz. Since the frequencies of lowercase and personnel scan in a color receiver are close to the corresponding frequencies in a black and white receiver, then under normal operating conditions there are no problems when switching from receiving a black and white image to color.
The main blocks of a color television transmitter are shown in fig. 15.5. A color television transmitting camera with a special transmitting tube and lens system perceives the three primary colors of the image. Based on the principle of color additivity, these colors are red. (R) blue (IN)
and green (G).
As follows from the diagram shown in Fig. 15.5, the amplifying and scanning circuits output three components (red, green, and blue signals) of the transmitted image. Signals R, G And IN then they are fed to three matrix circuits, two of which contain phase inverters. The output signals of the matrices are designated Y, 7 and Q. The Y signal, as noted above, is called the luminance signal. It turns out

by adding three signals of primary colors - red, green and blue - in the ratio 0.3:0.59:0.11.
Compliance with this ratio is necessary to compensate for the unequal sensitivity of the human eye to different colors.
Rice. 15.5. Block diagram of a color television transmitter.
The two main color difference signals consist of an I-signal (in phase) and a Q-signal (quadrature). Signal I contains 0.6 red signals, 0.28 green signals and 032 blue signals. The ratio of these components for the Q signal is as follows: R:G:B = 0,21: 0,52: 0,13.
The I and Q signals are fed to balanced modulators where they modulate two subcarriers at a frequency of 3.58
MHz shifted in phase by 90°, with the I signal leading the Q signal. In balanced modulators, the subcarrier and the I and Q signals are suppressed, and only subcarrier lateral oscillations pass to the output. The Y signal passes through the filter to the adder, where the output signals from the balanced modulators are also fed.
Color burst signal generator, which receives signals from a generator with a frequency
3.58 MHz, produces a 9-cycle 3.58 MHz signal, which is transmitted on the back step of the horizontal quenching pulse and serves to synchronize the subcarrier generator in the receiver (see Section 4.6).
All signals, including clock signals and blanking pulses of lines and fields, are added in the adder. Thus formed complete television signal is fed to a modulating amplifier, where it is amplified if necessary, and then fed to the final modulation stage operating in class C amplification mode. As with other AM transmitters, a crystal-stabilized oscillator is used here. The signals from this generator are multiplied in frequency, amplified and fed to a class C amplifier. A separate FM transmitter is used to transmit audio signals. Thus, two transmitters are used in a television transmitter, one with amplitude modulation and the other with frequency modulation.
15.5. AM receiver

The block diagram of the AM signal receiver is shown in fig. 15.6. Presented here superheterodyne the reception scheme that underlies most receivers used in communication systems.
The signal from the antenna output through the RF amplifier (see Fig. 3.4) is fed to the frequency converter, which includes a local oscillator and a mixer. In receivers with low sensitivity high frequency amplifier may not be; then the signal from the antenna output is fed directly to the transducer, as shown in the figure by the dashed line (see also Fig. 4.2).
The local oscillator of the converter generates oscillations of the required frequency, which, mixing in the mixer with the received oscillations of the modulated carrier, form oscillations of the intermediate (difference) frequency at the output of the mixer. An intermediate frequency value of 455 kHz is standard for broadcast receivers [The intermediate frequency of receivers used in various areas of radio electronics varies over a very wide range. - Note. Ed].
Rice. 15.6. Block diagram of a superheterodyne receiver.
From the mixer, the signal is fed to an intermediate frequency amplifier for additional amplification and filtering of interfering signals that appear during the heterodyning process. After amplification, the intermediate frequency signal is demodulated in the detector, and an audio signal is extracted. Since the audio signals at the output of the detector are rather weak, they are amplified in a conventional audio amplifier to the level necessary for their further reproduction in a loudspeaker.
Regardless of the frequency of the received signals, the intermediate frequency of the receiver retains a certain value. To do this, the tuning capacitors of the high-frequency amplifier, mixer and local oscillator are interconnected, so that during the tuning process their rotors rotate simultaneously. In parallel with each of the main tuning capacitors, a small tuning capacitor is included to ensure fine tuning over the entire range of the receiver (see Fig. 4.2). Thus, regardless of the frequency of the received signal, the local oscillator provides an intermediate (strictly fixed) frequency signal; typically the local oscillator frequency is higher than the carrier frequency of the signal. Therefore, if the station is transmitting at a carrier frequency of 1000 kHz, then to obtain a difference frequency of 455 kHz, the local oscillator frequency must be equal to 1455 kHz.

The proposed FM transmitter has an output power of 15 mW at a current consumption of 15 mA, a frequency deviation of -+ 3 kHz. It is simple in design, has small dimensions and consists of accessible elements.
The figure shows circuit diagram FM transmitter. The signal from the microphone through the isolation capacitor C2 is fed to the AF amplifier on the transistor VT1, and then through the resistor R4 - to the varicap matrix VD1, VD2. Resistor R2 determines the operating point of the amplifier and at the same time the initial offset of the varicap matrix.

The quartz oscillator is made on a transistor VT2.

The quartz resonator is included in the base circuit and is excited at the parallel resonance frequency at the first harmonic. In the collector circuit of the transistor, there is an L1C6 circuit tuned to the resonator harmonic frequency in the frequency range of 72.0 ... 73.0 MHz.

A frequency doubler VT3 is inductively connected to the coil of this circuit, where a voltage with a frequency of 144.0 ... 146 MHz is released. Amplified voltage through the L3C11C12 low-pass filter, which performs the functions of suppressing higher harmonics and matching with the load, is fed into the antenna. Capacitor C13 is separating.
The microphone amplifier and the crystal oscillator are powered by a parmetric voltage regulator, made on the VD3 zener diode.

Details

Resistors - MLT-0.125 (0.25). Capacitors: trimmers - KT4-23, KT4-21 with a capacity of 5 ... 20.6 ... 26 pF, the rest - KM, K10-17, KD, C5 - K53-1A. Microphone BF1 - MKE-84-1, MKE-3, DEMSh-1A. Zener diode VD3 - KS 156, KS 162, KS 168.

VD1, VD2 - varicap matrix KVS111A, B or varicaps KB 109, KB 110, in the latter case, R5 is removed, the varicap is switched on in place of VD2, and the left (according to the diagram) output of the capacitor C4 is connected to the C3R4VD1 node.

Transistors: VT1 - KT3102, VT2, VT3 - KT368, KT316, KT325, KT306, BF115, BF224, BF167, BF173. Quartz resonators - in a small package for frequencies of 14.4 ... 14.6, 18.0 ... 18.25, 24.0 ... 24.333-MHz. Fundamental frequency and harmonic (overtone) - at 43.2 ... 43.8, 54.0 ... 54.75, 72.0 ... 73.0 MHz (3rd harmonic for the first two and third and the fifth harmonic of the third).

The transmitter coil L1 has 11 turns of PEV wire - 2 0.64, wound on a frame with a diameter of 5 mm, turn to turn. L2 is wound over L1 and has 6 turns of PELSHO 0.18 wire. A 20Wh ferrite core is screwed inside the frame. L3 - 5 turns of silver-plated copper wire with a diameter of 0.8 mm, wound on a mandrel with a diameter of 5 mm. L4 - 3 turns of silver-plated copper wire, winding diameter 5 mm, winding length 10 mm.

Setting

All details are assumed to be correct. Before setting up with a magnifying glass, you need to check the board for short circuits. Then determine the average nominal voltage at which the radio microphone will operate. It is equal to the arithmetic mean between the upper and lower allowable supply voltages.
For example, the upper voltage is -9V (fresh battery), the lower voltage is 7V (discharged battery): Un rms. \u003d (9 + 7) 2 \u003d 8 V. At this voltage, you need to adjust the transmitter.
An equivalent is connected to the output of the transmitter (two MLT-0.5 100 Ohm resistors connected in parallel).

The output of the VD3 zener diode is unsoldered from the common wire, a milliammeter with a limit of 30-60 mA is turned off in series with it. The transmitter power is turned on.
By varying the supply voltage from the minimum allowable to the maximum, by selecting the resistance of the resistor R10, they ensure that the zener diode does not leave the stabilization mode at extreme supply voltages (the minimum stabilization current for KS 162A is 3mA, the maximum is 22mA. The connection is restored.

With proper installation and serviceable parts, the microphone amplifier does not need to be adjusted at the first stage of setup.
We control with a wavemeter (or, in extreme cases, on a broadcast VHF radio receiver, placing its antenna near the transmitter), the appearance of a signal with a frequency of 72.0 ... 73.0 MHz in the L1C6 circuit. By rotating the core and coil L1, we achieve the maximum value of this voltage, then go to the L3C9C10 circuit, controlling the voltage, now with a frequency of 144.0 ... 146.0 MHz. With the help of a wavemeter or a two-meter receiver, we achieve its maximum level.

Having adjusted all the stages several times to the maximum output voltage, we select the resistance of the resistor R7 in the quartz oscillator, then go to the doubler and balance it according to the maximum suppression of the signal with a frequency of 72.0 ... 73.0 MHz at the output. The presence of harmonics and their absolute level is convenient to observe on a spectrum analyzer, which, unfortunately, has not yet become a device for mass use. With precise balancing of the doubler, all odd harmonics are suppressed, and even ones, except for the second one (for which the transmitter was built), are filtered out, like the harmonics of the quartz resonator itself.

For more "meticulous" tuners, we can recommend choosing the value and the ratio of the capacitances of capacitors C4 and C5 according to the maximum power of the transmitter. Frequency adjustment can be done by slightly shifting the core of the coil L1, as well as changing the capacitance C3, remembering that when the capacitance of this capacitor changes, the overlap of the varicap matrix in frequency also changes. Consequently, the maximum frequency deviation will also change, which, if necessary, can be corrected by selecting the resistance of the resistor R2.
An interesting option is to turn on the transmitter frequency multiplier to quadruple the frequency. In this case, the tuning frequency of the L1C9 circuit should be 36.0 ... 36.5 MHz, and quartz resonators can be used starting from 7.2 .... 7.3, 9.0. ..9.125, 12.0...12.166, 18.0...18.25 MHz and overtones 21.6...21.9, 27.0...27.375, 36.0...36, 5 MHz (3rd harmonic) and 36.0...36.5, 45.0...45.625, 60.0...60.83 MHz (5th harmonic). Naturally, the greater the frequency multiplication, the lower the power received at the output of the transmitter and the more carefully tuning is required.

The transmitter antenna can be a quarter-wave vibrator shortened by a coil at the base, or a helical antenna. In a stationary position, the entire arsenal from GP to multi-element and multi-tiered antennas is acceptable.

When powering the transmitter from a 12-wave source, you should install a VD1 zener diode with a high stabilization voltage, for example, D8 4A, D81 4B, D818, choosing R177 again.

V.N. Shostak, Kharkov

In amateur radio practice, the generator high frequency is one of the most responsible nodes. The final parameters of the designed devices depend on the thoroughness of its manufacture. Requirements for the RF generator: high frequency stability, no modulation of the output signal by background and interference, as well as high purity of the spectrum. In addition, in some cases, a low level of intrinsic noise.

Fig.1 AL2602 chip structure

In practice, either quartz oscillators are used (with subsequent frequency multiplication to the required value), or LC oscillators. The advantage of quartz oscillators is high frequency stability. There are several disadvantages: elevated level noise, performance complexity caused by the need to multiply the frequency, and the impossibility of quickly changing the output frequency over a wide range.

LC oscillators are simpler in design, they can use frequency multiplication stages and adjust the output frequency over a wide range. Their main drawback is the increased instability of the output frequency compared to quartz oscillators. True, with the application of certain measures, this disadvantage can be minimized. Structurally, LC generators are made on bipolar or field-effect transistors, but RF generators made on integrated circuits (ICs) are of greater interest.

As a rule, RF generator ICs are broadband, have electronic tuning output frequency and provide high output parameters. A class of such devices is collectively called "Voltage Controlled Oscillator" or VCO. Of the most well-known and affordable, we can name VCO microcircuits from Motorola MC12100, MC12148, as well as MAX2432 manufactured by MAXIM. They operate over a wide frequency range and usually have a buffered RF output. But, in my opinion, the AL2602 integrated microassembly, which has recently appeared on sale, deserves the most attention.

Functionally, the integrated microassembly AL2602 is a voltage-controlled RF FM buffer generator. It contains a master oscillator operating in the frequency range of 80-220 MHz, an FM modulator, a 3 V voltage regulator, a buffer and a power amplifier. Unlike the above VCOs, this IC does not require external frequency setting circuits. Only the frequency setting resistor is needed. In the absence of this resistor output frequency is equal to the minimum, i.e. 80 MHz. Thus, the IC contains nodes that allow it to be successfully used in many amateur and professional radio transceiver designs. The structure of the AL2602 microcircuits is shown in Fig. 1, and the pin assignment is given in the table.

The supply voltage of AL2602 is 3 ~ 9 V. However, it remains operational when the voltage drops to 1.8 V. The current consumption when pin 4 is not connected is not more than 5 mA.

Output number	Designation	Purpose
1; 7; 8	GND	Minus, power ("ground")
2	Vref	3 V reference voltage stabilizer output
3	Vss	Plus supply (3 - 9 V)
4	RF OUT	Powerful RF output (open collector)
5	OSC Monitor	Low current RF output (frequency control)
6	v mod	Control voltage (modulator, frequency setting)

The use of ICs as a VHF generator, a voltage-controlled generator together with a synthesizer, as well as as part of portable VHF transmitters, which we will consider in more detail, was tested.

A miniature FM modulated transmitter (Fig. 2) contains a minimum number of parts, but, despite its simplicity, has high parameters. The transmission range in open areas exceeds 200 m. The operating frequency in the range of 80-220 MHz is set with a tuning resistor R2. The microphone is electret, but it is also possible to use a dynamic microphone with an additional single-transistor amplifier. The setting comes down to setting the operating frequency. The design of the board is arbitrary, taking into account the requirements for the installation of RF devices. The transmitter works stably over the entire range of supply voltages.

Fig. 2 Miniature transmitter with FM modulation

A portable VHF FM transmitter (Fig. 3) delivers a power of 5 W to the load, while due to the use of unpackaged parts it has small dimensions. The left side of the circuit is discussed above, and the right side is a power amplifier Philips BFG591 (Umax = 120 mA) and BLT81 (Imax = 500 mA) transistors can be replaced with domestic ones such as KT606 and KT911, but the dimensions of the board will increase. When replacing transistors with domestic ones, another transistor may be needed to achieve the same output power. Setting up the device comes down to setting the operating frequency and adjusting the current of the transistor VT1 within 50-80 mA by resistor R3.

Together with the transmitter, you can use a frequency synthesizer. In this case, the RF frequency comes from pin 5 to the divider of the synthesizer, and the tuning voltage from the synthesizer goes to pin 6 of the IC. In all other respects the design is the same.

Fig.3

In many cases, for example, when designing radio telephones, portable radio stations with a range of up to 1 km, transmitters that are part of security systems, etc., circuits with a single transistor - a power amplifier - work very effectively. The scheme of this option is identical to the scheme portable device, but the transistor VT2 is not used, and the antenna is connected to the connection point of capacitors C4 and C5. The collector current of the transistor in this case is set to 100 mA. The dimensions of the board of this version of the device do not exceed 30-40 mm.

The FM transmitter circuit is shown in Fig. 2 and Fig. 3. A simple FM signal transmitter can be assembled according to the circuit shown in the figure.