Experimental FM transmitter at 145 MHz. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Transmitters

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The proposed transmitter is simple in design, small in size, assembled on quite accessible parts. It can be recommended as an integral part of a portable radio station or as an experimental one for working in local VHF networks, when tuning antennas, etc.

The transmitter has an output power of 1 W at a supply voltage of 9,5 V, a frequency deviation of +/- 3 kHz.

The block diagram of the transmitter is shown in Fig.1. The signal from the microphone is fed to the amplifier A1 and from it to the modulated oscillator G1 with quartz frequency stabilization. The third, fourth or fifth harmonic of the FM signal (depending on the frequency of the applied quartz resonator) is fed to the frequency doubler U1. The converted signal within the two-meter amateur band is amplified by a two-stage amplifier and fed into the antenna.

Fig. 1

Figure 2 shows a schematic diagram of the transmitter. The signal from the BM1 microphone through the decoupling capacitor C1 and the resistor R1, which cover the lower frequencies of the AF range, is fed to the operational amplifier (op-amp) DA1 and amplified by it. Capacitor C2 protects the amplifier input from RF interference. Resistor R4 in the negative feedback circuit of the op-amp determines its gain. Resistors R2, R3 balance the op-amp for direct current and, at the same time, set the operating point on the capacitance change characteristic of the varicap matrix connected to the op-amp for direct current through low-pass filter resistors (LPF) R5C4R6.

Rice. 2 (click to enlarge)

The voltage on the varicaps pulsates in time with the frequency of the audio signal. Their capacitance is connected in series to the capacitive divider in the feedback circuit of the quartz oscillator and, therefore, when the latter is excited, its frequency will also change in time with the sound signal. The master oscillator is made on the transistor VT1. The quartz resonator ZQ1 is included in the base circuit and is excited at the parallel resonance frequency. The L1C9 circuit in the collector circuit of the transistor emits a voltage with a frequency in the range of 72:73 MHz. The input of a paraphase balanced frequency multiplier (in this case, a frequency doubler) operating on even harmonics is inductively connected to the coil of this circuit.

The bandpass filter (PF) L3C13C15L4C16 allocates a voltage with a frequency of 144:146 MHz (depending on the frequency of the ZQ1 quartz resonator), which, from part of the turns of the L4 coil, through an isolation capacitor, enters the input of the first stage of the amplifier, made on the transistor VT4. It operates in class AB mode with a small initial bias obtained on a parametric voltage regulator - a silicon diode VD3, connected in the forward direction of current flow. The amplified and filtered (PF L5C20L6C21) voltage is supplied to the final power amplifier assembled on the VT5 transistor. The cascade does not have any features, it works in class C. The amplified RF voltage (here it is better to talk about current or power) through a low-pass filter that suppresses higher harmonics and a matching stage with a load is fed into the WA1 antenna. Capacitor C26 is separating.

The microphone amplifier and the crystal oscillator are powered by a parametric voltage regulator made on the VD1 zener diode. LED HL1, connected in series with the zener diode, indicates the inclusion of the transmitter.

RC filters R10C10, R12C14, R16C22, as well as R14C18 and capacitors C3, C5 and C23 increase the stability of the transmitter by decoupling its power stages.

The transmitter antenna can be a quarter-wave vibrator, a whip antenna with a shortening coil, spiral. In stationary conditions, the entire arsenal of antennas is acceptable: from GP to multi-element and multi-tier. The author tested the transmitter with antennas: GP and 16-element F9FT.

The transmitter is made on a board made of double-sided foil fiberglass with dimensions of 137,5 x 22 x 1,5 mm (Fig. 3). From the upper side of the board (the parts are installed on it) around the holes into which the leads of the elements are inserted, isolated from the common wire, the foil was removed by countersinking. All soldering to the case is made on the top side of the board, except when it is structurally impossible (for example, when mounting a quartz resonator vertically), the "grounded" points on the top side of the board are connected by wire jumpers to the foil on the bottom side of the board (these places on the drawing of the board marked with crossed out circles).

Fig. 3

The transmitter uses small-sized parts, the installation is tight. If installation is difficult, some of the resistors and capacitors can be placed on the side of the printed conductors. The VT5 power amplifier transistor is installed upside down on top of the board (screw up). The lid of its crystal is recessed into a hole with a diameter of 7 mm in the board. The planar base and collector leads are soldered overlapped to the etched or cut conductors on the top side of the board, the emitter leads are soldered on both sides of the body to the "ground" foil. Capacitor C26 is installed outside the board (between the board and the antenna jack).

The microphone is located at the bottom of the transmitter (portable radio) to keep the operator's brain away from antenna radiation. It is even better to use a remote microphone with a "reception-transmission" switch located on its body, the latter will allow you to raise the radio station with an outstretched arm above your head and thereby "move the radio horizon", providing radio communication over a greater distance.

The design uses resistors MLT-0,125 (MLT-0,25), R11-SP3-38, trimmer capacitors KT4-23, KT4-21 with a capacity of 5:20, 6:25 pF, C1, C7, C8, C17 - KM, C15 - KD, C5 - K53-1A, the rest of the capacitors - KM, K10-7, KD. Microphone VM1 - electret capsule MKE-84-1, MKE-3 or, in extreme cases, DEMSh-1a. Zener diode VD1 - KS-156A, KS-162A, KS168A. In the absence of the HL1 LED, you can refuse the indication by increasing the resistance of the resistor R17. Diode VD3 - any silicon low-power small-sized, VD2 - varicap matrix KV111A, KV111B. When using a separate varicap (KV109, KV110), it is switched on in place of VD2.1, the resistor R7 is removed, the output of the capacitor C7, left according to the scheme, is soldered to the connection point of the elements C6, R6, VD2.2. Operational amplifier DA1 - any of the series K140UD6 - K140UD8, K140UD12. OA K140UD8 is recommended to be used at an increased transmitter supply voltage (12 V and higher with a zener diode VD1 - KS168A). At pin 8 of the K140UD12 OU, a control current should be applied through a 2 MΩ resistor from the positive bus of the power source.

As VT1, you can use any low-power transistor with a cutoff frequency of at least 300 MHz, for example, KT315B, KT315G, as well as from the KT312 and KT368 series. Transistors VT2: VT4 are also low-power, but with a cutoff frequency of at least 500 MHz, for example, from the KT368, KT316, KT325, KT306, BF115, BF224, BF167, BF173 series. Transistor VT5 - KT610A, KT610B, KT913A, KT913B, 2N3866, KT920A, KT925A. Not all of the transistors recommended for use are the same size as those used in the author's version of the KT610A transmitter. This must be taken into account when iterating the design. It is undesirable, in order to reduce the size of the transmitter design, to use one transistor assembly in several high-frequency stages, since due to the strong interstage coupling, the transmitter parameters will deteriorate: spectral purity, sub-excitation will appear and the inability to achieve maximum output power.

The transmitter can use quartz resonators for fundamental frequencies: 14,4:.14,6; 18,0:18,25; 24,0:24,333 MHz or harmonic (overtone) at frequencies 43,2:43,8; 54,0:54,75; 72,0:73,0 MHz.

The transmitter coils, except for L1 and L2, are frameless. L1 and L2 are located on a frame with a diameter of 5 mm with a ferrite tuning core from VHF radio stations, preferably no worse than 20 HF. If this is not the case, then you can use brass, aluminum, or abandon the core altogether by counting the number of turns of the coils L1 and L2 proportionally and soldering a small trimmer capacitor from the side of the printed tracks of the board. L1 is wound turn to turn on the frame, L2 is wound over L1. Between the coils L1 and L2, it is advisable to place an electrostatic screen in the form of one open loop of foil, "grounded" at one point (on one side). Coils L3:L8 are placed at a distance of 0,5:1,0 mm from the board. The winding data of the coils are shown in the table. If coils with microwave ferrite tuning cores are used in the transmitter circuits, and capacitors with a capacity of no more than 10 pF (instead of tuning ones) are hidden under the screens of the corresponding coils, then the output power of the transmitter will increase, the installation volume will decrease, the circuits will be tuned by coil cores.

Before setting up the transmitter, it is necessary to check the board for the absence of short circuits between the printed conductors. Then, the voltage at which the radio station will operate is determined as the arithmetic mean between the voltage of a fresh and discharged battery, for example: the voltage of a fresh battery is 9 V, a discharged battery is 7 V,

(9 + 7) / 2 = 8 V

At a voltage of 8 V, the transmitter should be tuned, this will ensure the minimum dependence of the transmitter parameters on the supply voltage and a compromise in terms of economy. The fact is that with an increase in the supply voltage, the current consumed by the transmitter increases, not only due to the increasing buildup power of the final stage, but also due to an increase in the stabilization current VD1, to increase the efficiency of the transmitter, it is useful to reduce this current, but then there is a risk of jumping out for the lower limit of the stabilization current of the zener diode when the supply voltage decreases, when the battery is discharged. An equivalent is connected to the transmitter output: two MLT-0,5 resistors with a resistance of 100 ohms connected in parallel. From the common wire (when the power is off!) Solder the output of the zener diode VD1 and turn on a milliammeter in series with it with a full deflection current of the arrow 30:60 mA. Then turn on the power of the transmitter. By varying the supply voltage from the maximum to the minimum allowable, by selecting the resistance of the resistor R17, they ensure that at the extreme allowable values ​​of the supply voltage the zener diode does not exit the stabilization mode (the minimum stabilization current for the KS162A is 3 mA, the maximum is 22 mA). After that, turning off the power, the connection is restored.

With proper installation and serviceable parts, the establishment of the transmitter is continued by tuning the circuits, using a resonant wavemeter for control. First, by rotating the tuning ferrite core of the L1 coil, the maximum voltage value is achieved with a frequency of 72:73 MHz (depending on the frequency of the quartz resonator) in the L1C9 circuit. Then, the circuits L3C13, L4C16, the bandpass filter and the low-pass filter are sequentially tuned to the maximum voltage with a frequency of 144:146 MHz. If, at the same time, any trimmer capacitor is in the position of maximum or minimum capacitance, then it is necessary to compress or expand the turns in the corresponding loop coil, respectively, using, for example, a fiberglass plate (dielectric).

Sharp changes in the readings of the wavemeter, deviation of the arrow of the measuring head in it, even when the quartz resonator is short-circuited and (and) the wavemeter is detuned in frequency from the working transmitter, extraneous overtones that occur when listening to the transmitter signal on the receiver indicate parasitic self-excitation of the transmitter. If this occurs, you should lower the mounted components as low as possible to the "ground" foil of the board, shorten the leads of all capacitors to the required minimum, setting the decouplers as shields (at right angles to the plane of the circuit board, not laying them horizontally). It can affect the stable operation of the transmitter and the reduced quality of capacitors: cracks on them, dielectric leakage, the use of low-frequency types of capacitors, their large dimensions.

After tuning the circuits, the resistance of the resistor R9 in the quartz oscillator is selected, also focusing on the maximum output voltage of the transmitter, then the frequency doubler is balanced with a tuning resistor R11 according to the best suppression at its frequency output in the region of 72:73 MHz (depending on the used quartz resonator). It is convenient to observe the presence of harmonics and their absolute and relative levels on the screen of a spectrum analyzer, which, unfortunately, has not yet become a device for mass use. For the most "meticulous" tuners, we can also recommend choosing the resistance of the resistor R8 and the ratio of the capacitances of the capacitors C7 / C8 according to the maximum output power.

In a balanced multiplier (doubler) of the frequency, the tuning resistor R11 can be replaced with two constants and their values ​​can be selected individually. In this case, it is necessary not only to proceed from the maximum frequency suppression in the range of 72:73 MHz, but also to obtain the maximum output voltage in the range of 144:146 MHz, controlling it with a resonant wavemeter on the L3C13 circuit or at the transmitter output. Field-effect transistors can also be used in the multiplier, but, in this case, it will be necessary to increase the number of turns of the L2 coupling coil.

If necessary, the transmitter frequency can be (within a small range) adjusted by detuning the L1C9 circuit, however, operation in this mode is undesirable due to the risk of generation failure in the crystal oscillator during modulation. In the transmitter, instead of a doubler, you can use a frequency quadrupler. In this case, the L1C9 circuit must be tuned to 36,0:36,5 MHz. In the master oscillator, you can use quartz resonators for fundamental frequencies: 7,2: 7,3; 9,0:9,125; 12,0:12,166; 18,0:18,25 MHz or overtones: 21,6:21,9; 27,0:27,375; 36,0:36,5; 45,0:45,625; 60,0:60,83 MHz. However, it should be taken into account that the output power of the transmitter with a frequency quadrupler will be less than with a doubler, in addition, it may be necessary to include additional links in the PF and LPF of the transmitter.

When the transmitter is powered from a 12 V source, in order to obtain savings, it is possible to use Zener diodes D1A, D814B, D814 as VD818, while it is necessary to select the resistance of the resistor R17, as mentioned above. When connecting an additional power amplifier, the transmitter should be completely shielded from it. The transmitter can have several channels, for this, as many L1 coils should be placed on the L2L1 RF transformer as there will be generators (channels) switched by power supply with parallel connection by AF.

To adjust the frequency of the transmitter, in addition, in series with the ZQ1 quartz resonator, you can turn on a tuning capacitor or an inductor with a tuning ferrite core, in the first case, the frequency increases, in the second, it decreases. The board of the mounted transmitter can be located in its case both horizontally and vertically. Capacitor C15 is installed on the side of the printed tracks. The upper (according to the diagram) terminal of the capacitor C17 is soldered directly to the turns of the coil L4. Coil L2 is wound with a double wire to ensure symmetry, then the beginning of one wire is connected to the end of the other. The article contains the names of foreign transistors that remain from imported equipment, are commercially available, a paradox: sometimes a foreign transistor is easier to find than a domestic one, and the former costs less than the latter.

If you want to operate the transmitter in a wide range of supply voltages, you should abandon the HL1 LED, reselect the resistance of the resistor R17, introduce a decoupling capacitor with a capacity of 0,47: 0,68 uF between the connection point of the resistor R4 to terminal 6 of the op-amp and resistor R5, connect it in parallel to the zener diode VD1 is a tuning resistor with a resistance of 200:220 kOhm, with which to "hang out" the middle of the modulation characteristic of the varicap matrix. The additional trimmer slider must be connected to connection point R5C4R6. The bias on the base of the transistor VT1 can also be applied from a resistive voltage divider, which allows you to work in a larger range of supply voltages, with a more stable operating point. For precision operation of the FM modulator, it may be useful to include a current stabilizer in the VD1 zener diode circuit, for example, from [2]. The latter can be explained by the desire to get a very small change in the supply voltage, within the stabilization characteristics: for a parametric stabilizer on a zener diode, this is 30:40 mV, for a current stabilizer - 1 ... 2 mV. In practice, the diagram in Fig. 1 of [2] is switched on instead of R17, the KP303E transistor, a resistor with a resistance of 100:150 Ohm (selected according to the rated stabilization current of the Zener diode VD1).

If the transmitter does not require full power, then you can do without the final stage by connecting the antenna through the C24L8C25 low-pass filter to the collector of the VT4 transistor or connect the antenna to the tap of the L5 coil (no more than 1: 1,5 turns from its "cold" end), keeping capacitor C20, the right (according to the diagram) output of which is connected to a common wire: we get an economical pocket-type transmitter that can do a good job when, for example, tuning antennas. When the transmitter is self-excited, as already mentioned above, lower the mounting closer to the foil, shorten the leads of the parts to the minimum reasonable length, for parts installed vertically, the lower lead closest to the board should be "hot" by RF, decoupling capacitors should be of RF types and have a capacitance of 1000:68000 pF. As can be seen from the circuit diagram, the transmitter consists, as it were, of two parts, relative to the coils L1 and L2: a quartz oscillator with an FM modulator and a microphone amplifier and a frequency multiplier with a two-stage power amplifier. This construction allows the designer to use parts of the transmitter on a block principle, replacing them with the same type, at his own discretion.

Relative to the specified "intersection point" (L1 and L2), you can "multiply" - use several crystal oscillators with a common microphone amplifier, frequency doubler and power amplifier - a measure when several (up to five) channels are required for transmission with their switching over direct current , this will require as many L1 coils as crystal oscillators are used. You can also connect two power amplifiers to, for example, a single-channel transmitter and feed each antenna through its own antenna, for example, in a stack, or directed in different directions, to increase efficiency (instead of GP). You can also use the master oscillator as part of the radio station to work through repeaters. The local oscillator voltage (its role, in this case, is played by the transmitter's quartz local oscillator on VT1) is fed through the coupling coil (several turns over L1) to the receiver mixer, which operates on the principle of a superheterodyne with a low intermediate frequency of 600 kHz. The mixer must provide operation on the second harmonic of the local oscillator (direct conversion technique).

It is possible to use the SYNTEX-72 principle with voltage applied simultaneously to two mixers [3]. By the way, the SYNTEX-72 system does not give a gain in the suppression of the image channel in IF2 in terms of frequency - this is my mistake - XCUSE! But since the IF is "hidden" further into the radio receiver circuitry behind the underlying circuits and bandpass filters, nevertheless, the image channel over IF2 is suppressed much better than with a single conversion with a low IF, when the usual conversion method is used.

Winding data of the coils of the experimental FM transmitter at 145 MHz:

In conclusion, I would like to thank V.K. Kalinichenko (UA9MIM).

Literature

1. Tranzistorski predajnik za 432 MHz, RADIOAMATER, 1977, No. 1, p.3:6
2. A. Mezhlumyan. Microcurrent stabilizers on field-effect transistors, RADIO. 1978, No. 9, pp. 40:41
3. V. Besedin. On the suppression of the mirror channel. RADIO HAMMER, 1994, No. 3, p. 62:63
4. V. Besedin. FM transmitter, RADIO AMATEUR, 1995, No. 2, p. 42:44
5. V. Besedin. FM transmitter, RADIO HAM. HF and VHF. 1997, No. 1, p. 32:33

Author: A.Besedin

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