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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Frequency synthesizer for KB transceiver. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Civil radio communications The frequency synthesizer in communication equipment, being the heart of the tuning system, determines not only consumer, but also selective characteristics of a particular device. In recent years, amateur radio synthesizer designs have appeared using direct digital synthesis chips from Analog Device (analog.com). Microcircuits differ from each other in the limiting output frequency, the quality of the synthesized signal, the "tricked out" service and, no less important, the price. Let's try to figure out how and which DDS chips it is advisable to use when building a frequency synthesizer for a shortwave transceiver. Direct digital frequency synthesis - DDS (Direct Digital Sinthesys), a rather "young" synthesis method, the first publications about which began to appear in the late 70s. The frequency resolution of DDS reaches hundredths and even thousandths of a hertz at an output frequency of several tens of megahertz. Another characteristic feature of DDS is the very high hopping speed, which is limited only by the speed of the digital control interface. PLL-based synthesizers use feedback and error filtering, which slows down the frequency hopping process. Because the DDS output is digitally synthesized, various kinds of modulation can be performed. Both technically and economically, the DDS satisfies most of the criteria for an ideal frequency synthesizer: it is simple, highly integrated, and small in size. Many DDS parameters are program-controlled, which allows you to add new features to the device. All this makes DDS synthesizers very promising instruments. There are some limitations associated with the sampling and digital-to-analog conversion processes that take place in DDS:
Without going into details of the structure and principle of operation of DDS microcircuits (all this is described in detail in the specialized literature), we will dwell only on general issues of their application and characteristics. The main problem that still hinders the use of DDS microcircuits as a local oscillator of a KB transceiver is the presence of components in the spectrum, the level of which is about -80 dB. They are heard almost in a continuous sequence (like a "fence" from the affected points) when rebuilding the transceiver with the antenna turned off. You can get rid of these components only with a DDS filter that monitors the output frequency, but the manufacture of such a filter greatly complicates the design. The author tried to use in self-made transceivers the synthesized signal directly from the output of DDS microcircuits, instead of the local oscillator signal based on the "classic" single-loop synthesizer. The output signal of the DDS synthesizer was filtered by a low-pass filter with a cutoff frequency of 32 MHz. The transceivers in which the synthesizers were tested were built according to a single conversion scheme and an IF in the range of 8,321 ... 8,9 MHz. The first mixer is passive, made on KP305B transistors or on a KR590KN8A microcircuit, controlled by a meander. The level of the RF signal on the mixer - no more than 3 V (eff). Sensitivity - 0,3 μV. The dynamic range for intermodulation is not lower than 90 dB when two signals are supplied with a spacing of ± 8 kHz, which, according to the author, will suit most radio amateurs working on the air. It was these parameters that all tested transceivers with a "classic" one-loop synthesizer had. Its detailed description can be found at cqham.ru/ut2fw. There you can also find a DDS synthesizer circuit based on it. Tests of synthesizers showed that, for example, with the AD9850 microcircuit, the level of components was fixed at the level of 2 ... 4 points on the S-meter scale. With the antenna connected, in total with the level of on-air noise, the S-meter showed from 4 to 7 points at frequencies below 10 MHz. On the bands of 160 and 80 m, the "fence" was practically not noticeable. With the AD9851 microcircuit, whose nominal noise characteristics are 10 dB better, the average level of combination components did not exceed 1...3 points on the S-meter scale. When operating on the air at frequencies below 10 MHz, they are almost impossible to detect by ear, but this, in turn, depends on the value of the selected intermediate frequency (for example, 8,363 MHz). The quality of the signal synthesized by the DDS chip itself is excellent, the tone is "ideal", the width of the "noise" is minimal. The resolution of the SK4-59 spectrum analyzer did not allow us to find the difference between the signal of this synthesizer and the signal of the classical GPA on a field-effect transistor (KP307G, inductive three-point, tuning using KPE). If not for these, albeit rather weak, "peak, peak, peak" during tuning, one could throw out the one-loop synthesizer from the transceiver and install a DDS synthesizer in its place. The work carried out allows us to speak about the impossibility of using AD9850, AD9851 direct digital synthesis chips in a transceiver with a sensitivity of about 0,3 μV without degrading its characteristics. It is possible that with less stringent requirements for the sensitivity of the transceiver and another version of the mixer, these microcircuits can be used in the local oscillator. Probably, this will be a good version of a microtransceiver synthesizer for field conditions with all kinds of services (control from the processor), practically without input filters (upconversion), with a continuous operating range from 0 to 15 MHz. The dimensions of the synthesizer together with the control controller are no more than a matchbox. The maximum synthesized frequency can be over 75MHz, and the transceiver's intermediate frequency can reach 60MHz! A perestroika step - at least a fraction of a hertz! In the descriptions of DDS microcircuits, the manufacturer offers two options for their use in PLL synthesizers with increased requirements for the quality of the output signal: use it as an "adjustable reference oscillator" or as a variable division ratio divider (VDC) in a single-loop synthesizer. Information about the difference in the qualitative characteristics of the synthesizers of both versions could not be found. Analyzing the circuitry of imported transceivers, the author found there the implementation of only the second option (for example, in FT-100, FT-817 transceivers), on the basis of which the proposed synthesizer was built. It should also be noted the versatility of this version of the synthesizer. Depending on the control program and VCO tuning frequency, it can be used for either a low IF transceiver or an "upconverted" transceiver. In the synthesizer for low IF, the VCO operates at frequencies four times higher than required, and when a signal is applied to the mixer, its frequency is divided by 4 by an additional divider. Eliminating the divisor by 4, the synthesizer can be used for reworking and expanding the capabilities of decommissioned military communications equipment, for example, "R-143", "Kernel", "Crystal", "R-399" and the like, with a high first IF. In table. 1 shows the "standard" frequency layout for low IF (8,863 MHz).
In table. 2 - frequency layout for IF 90 MHz, which can also be used for any other frequency (there are no restrictions in the program), and its use in a transceiver with a low IF will greatly facilitate the problem of suppressing mirror and side reception channels.
The block diagram of the synthesizer is shown in fig. 1. The 20MHz crystal oscillator signal is used simultaneously for the operation of the DDS chip and the PIC controller.
Depending on the selected range and the control program of the controller, the DDS chip generates frequencies from 80 to 500 kHz, which are fed through a low-pass filter (LPF) to one of the inputs of the frequency-phase detector (PD). The output frequency of the VCO is divided by 256 and fed to the second input of the frequency-phase detector. The voltage from the output of the FD, after passing through the low-pass filter, is supplied to the varicap of frequency tuning of the VCO. The voltage change occurs until the frequencies at both PD inputs match. When the frequencies match, the PLL closes and holds the frequency. The output frequency of the DDS is controlled by the microcontroller, in accordance with the program embedded in it and the state of the external control circuits. To make the VCO frequency suitable for building a low IF TRX, it is additionally divided by 2 or 4, depending on which mixer is used in the transceiver. In the author's transceiver, the formation of antiphase control signals for the mixer is performed on a 74AC74 microcircuit, which divides the frequency by 2. The synthesizer tuning step is selected by software and can be set with a resolution of 1, 10, 20, 30, 50, 100,1000 or 5000 Hz. The frequency stability of the synthesizer, which depends mainly on the stability of the clock crystal oscillator, is comparable to the stability of synthesizers of imported industrial transceivers. At a constant ambient temperature, frequency drift is possible within a few hertz. When the clock generator is heated with a soldering iron to +70 ° C, the frequency drift in the 28 MHz range is no more than 140 Hz. For example, in an expensive transceiver "IC-756" (according to the company) in the first hour after switching on, the frequency change is ± 200 Hz, and after warming up - ± 30 Hz per hour at a temperature of +25 °C. When the temperature changes from 0 to +50 °C, the frequency can vary within ±350 Hz. The synthesizer uses a hybrid TTL generator from the computer motherboard. With very stringent requirements for frequency stability, a thermally compensated highly stable generator can be used, although the author has very serious doubts about the appropriateness of its use, and the cost of such a generator is comparable to the cost of the entire synthesizer. Schematic diagram of the synthesizer controller is shown in fig. 2. The synthesizer uses a DD1 PIC16F628 microcontroller, although there is a control program for PIC16F84A. Programs for these microcontrollers were written by Vladimir RX6LDQ (develop-pic@yandex.ru).*
It makes no sense to describe in detail the operation of the DD1 microcontroller, let it remain a "black box" that works according to the program hardwired inside it and issues control signals to the HG1 display, the DDS chip and external devices. To obtain the best noise characteristics of the synthesizer as a whole, the DDS AD9832 chip was chosen, which forms the widest frequency spectrum. In addition, the cost of this DDS chip is significantly lower than others. The operation of the synthesizer is controlled by the keyboard SB1 - SB 18 and the encoder, made on optocouplers U1, U2 (Fig. 3). The number of control buttons in the synthesizer was not reduced - 12 buttons control the operation of the synthesizer, and six buttons (A1 - A6) are used to control the transceiver operating modes.
Why are there so many buttons? It was possible to stop at a step-by-step menu, when each of them performs several functions. So, for example, imported portable transceivers work. It seemed to me extremely inconvenient when, for example, for operational tuning to the other end of the range, you need to enter the menu, change the tuning step to a coarser one, turn the tuning knob, then enter the menu again, return the original tuning step, and only after all these manipulations work quietly . In the description of the keyboard of the synthesizer for each control button, the following are sequentially indicated: its serial number and main function (command executed when the button is pressed), the range to be switched on when entering the "BAND" function and the reference designation on the circuit diagram (see Fig. 2 in the first part articles). "1 RIT"; 1,8 MHz; SB11 - detuning enable button. The frequency displayed on the display at the time the button is pressed is memorized and will be used in transmit mode. The amount of detuning is entered with a rotary encoder. Whether you stay on the band where the detune was turned on, or switch to a different band, when you switch to transmit, the synthesizer will return to the frequency that was on the display at the time the detune was turned on. This provides SPLIT and CROSSBAND modes. When the detuning is turned on, a dot after tens of MHz lights up on the display. The detuning is turned off by pressing this button again. "2 FREQ"; 3,5 MHz; SB12 - operational on / off software increase (quadruple) frequency tuning step. When this button is pressed, the display briefly shows "2p". There is no multiplication of the number of pulses from the shank and, for example, with 60 teeth of the shank disk and a tuning step of 10 Hz, we have 600 Hz per revolution. When you press this button again, the display will show the inscription "4p" and the number of pulses will be multiplied by 4, i.e. we will already get 2400 Hz per revolution. "3 BAND"; 7 MHz; SB13 - button for enabling range switching. When it is pressed, the display shows the inscription "Band", and then, after pressing one of the buttons "1-9", the display sets the frequency corresponding to the middle of the selected range. "4 IN"; 10 MHz, SB 14 - saving the current tuning frequency and the state of six transceiver control buttons to one of 16 memory cells. When you press SB14, the display shows the inscription "Push" and is expected to press the button with the number of the required cell. To enter numbers from the 10th to the 15th, within a second after pressing the number 1, enter the second digit, from 0 to 5. The display will show the number of the cell. Cell 0 stores information used to set the initial state of the synthesizer when the power is turned on, i.e. you can write the desired values into it, for example, the tuning step and the inclusion of any mode in TRX, the frequency to which the synthesizer will switch when the transceiver is powered on. For example, you have an agreement with a correspondent to meet at a frequency of 21,225 MHz. You switch the transceiver to this frequency, turn on the UHF (by pressing the SB3 button), select the tuning step you want to work with, and then press the "IN" and "0" buttons. All settings are recorded in cell "0". Now you can turn off the transceiver, and the next time you turn it on, the processor will set all the modes that you saved in the zero cell - turn on UHF, frequency 21,225 MHz, tuning step. "5A-B"; 14 MHz; SB15 - exchange with an additional receive frequency. This is the so-called "second local oscillator" mode. To memorize the value of the frequencies in the "virtual" cells "A" and "B" you need to tune in to the desired frequency and press this button. The frequency will be stored in cell "A". The same frequency value on the display will “jump” to cell “B”, i.e., virtually we, as it were, “switched” to the second local oscillator. Here you can make any changes to the frequency - memorization in the cell "B" will occur only when the button A-B is pressed again, i.e. in the cells "A and B" the values of the two frequencies that were on the digital scale at the time the button A was pressed -IN. Perhaps for radio operators who did not use synthesizers in their transceivers, such a description of the operation of this button will not give a clear understanding of its purpose. I'll try to describe this mode in a different way. Imagine that two VFOs are installed inside the transceiver and this button switches one tuning knob to VFO "A" or to VFO "B". To make it clear which "local oscillator" you are working on, the display shows in mode "A" a dot near the UNITS of the MHz scale, in mode "B" - the dot near the UNITS MHz goes out and three dots light up near the UNITS, TENS and HUNDREDS hertz scale. "6 SCAN"; 18 MHz; SB16 - scan button. After pressing it, the inscription "Scan" is displayed on the indicator. There are three scanning subfunctions: A. When you press the "8" button, 15 memory cells are scanned, with stops for 3 seconds on each cell. b. When button "2" is pressed, scanning is performed from the lower frequency recorded in cell 1 to the higher frequency recorded in cell 2. If the frequency in the 1st cell is greater than in the 2nd, when pressing SCAN, the message "Error" appears. Scanning is possible only within one range. V. When button "3" is pressed, the included range is rebuilt from the lower limit to the upper one and vice versa. Scanning can be interrupted by pressing any button on the keyboard, turning the encoder, or pressing the PTT. Scanning can be resumed at any time from where it left off by double-clicking the SCAN button. "7RT"; 21 MHz; SB17 - exchange of receive and transmit frequencies, with detuning enabled. When the button is pressed, the transmit frequency becomes the receive frequency, and the receive frequency becomes the transmit frequency. Pressing SB 17 again returns everything to its original state. If the detuning is not enabled, then pressing the "7" button will show the message "Select" on the display. This is a menu of two basic settings that can be accessed by pressing button "1" or "2". "1" - intermediate frequency input mode. The value of the set intermediate frequency of the transceiver appears on the display (by default, the initial frequency in the program can have values from 8,3 to 8,9 MHz). The frequency is set by the encoder. Fixing the inverter and exiting the mode by pressing the button "1" again. After the final setting of the frequency of the reference oscillator of the transceiver, measure the frequency with a frequency meter to units of Hz and set it by turning the knob of the encoder, entering this mode. You should first select a synthesizer tuning step of 1 Hz. "2" - 20 MHz reference oscillator constant correction mode. The synthesizer displays the "fixed frequency" value of 10 Hz and automatically turns on the VCO of the 300 m range. The frequency at the output of the VCO board must be measured with a frequency meter, and if it differs from 000 MHz, correct by rotating the encoder. Exit and storage - by pressing button "160" again. These synthesizer settings are "basic" and should be adjusted more carefully. To do this, we connect a frequency meter warmed up for at least an hour (preferably industrial) to the output of the F / 2 synthesizer and, by rotating the encoder in the correction mode, set the frequency to 10,30 MHz with an accuracy of one hertz. This function was required due to the fact that the synthesizer reference oscillator does not have additional tuning and frequency spreads for different instances can reach several kilohertz. "8 OUT"; 24 MHz; SB 18 - restoration of the frequency and state of six transceiver control buttons from one of the 16 memory cells. When pressed, the display shows "Pop" and the button with the corresponding cell number is expected to be pressed. To enter numbers from 10 to 15, it is necessary to press the second one, from 1 to 0, within a second after pressing number 5. After entering the number, the number of the memory cell will appear on the indicator for a short time. "9 T=R"; 28 MHz; SB1 - the mode of setting the transmission frequency equal to the reception frequency. Works with detuning enabled. If the detuning is turned off, then when you press the "9" button, the inscription "Step" is displayed on the indicator and you can select the desired synthesizer tuning step with the LEFT and RIGHT buttons: 1, 10, 20, 30, 50, 100, 1000 and 5000 Hz. The selected step is memorized when this button is pressed again. "0 STEK", SB10 - extracting the frequency from the stack. There are five stack cells, which can be viewed by successively pressing the button. Before the output of frequencies from the stack cells, the indicator briefly displays the inscription "Stec" with the cell number. The input to the stack is carried out automatically when changing the range, when extracting from a memory cell and when scanning. "LEFT"; SB9 - quick frequency reduction button. "right"; SB8 - quick frequency increase button. When you press the buttons "A1" - "A6" (SB2-SB7), the logic levels at the outputs ATT, AMP, U/L, VOX, AF BW, PROC change accordingly, which, in turn, control the functional units and modes of the transceiver. When the synthesizer is initially turned on, these outputs are logic zero. All user settings and information in the memory cells are stored in the RAM of the microcontroller without additional external power supply. When you turn on the power of the synthesizer, the program retrieves from the "0" memory cell those parameters of the transceiver that you would like to have immediately every time you turn it on, namely: frequency and tuning step, transceiver modes (state of six transceiver control buttons); "multiplying" by 4p the number of valcoder pulses and "zeroed" stack cells. In the program, when the synthesizer is first turned on, the first ten memory cells contain the frequencies at which you can most often hear the call sign UT2FW. In the remaining cells - the frequencies of the ranges. This is done so that the first time you turn on the synthesizer, it starts to work correctly and it is easier for the user to get used to its control. The DDS chip is controlled by serial code on the RAO, RA1, RA3 buses. The DDS output signal is filtered by low-pass filter elements R7, R8, L2, L3, C7, C8, C9 with a cutoff frequency of about 700 kHz. As the display of the HG1 controller, it is acceptable to use different types of LCD indicators, since their control, as a rule, is the same. The synthesizer uses an inexpensive "telephone" LCD - MT-10S1 of the Moscow company MELT. Such an indicator is controlled via four buses - these are the outputs QE, QF, QG, QH of the DD2 microcircuit. A more expensive option is the use of matrix indicators from foreign companies Powertip, Sunlike, Wintek, Bolymin, and from MELT. But the cost of such LCDs today is quite high. It should also be noted that not all models of matrix indicators are suitable in terms of speed. For example, the WH1602J indicator does not "keep up" with the restructuring of the encoder, and when the knob of the encoder is rotated quickly, incomprehensible signs and symbols begin to "jump out". Exactly the same type of indicator VS1602N, from another company, works without problems. The D0-D3 buses supply control signals to the band switching decoder on the transceiver band pass filter board and the band switching decoder of the VCO board. Chip DD6 - pulse shaper of the valcoder. At the moment of restructuring the synthesizer, in front of the optocouplers U1 and U2 (see Fig. 3), a disk with holes or teeth cut along its edge, rigidly connected to the transceiver tuning knob, rotates. In the case when the reflective surface of the disc is opposite the optocoupler, the resistance of the photodetector of the optocoupler is minimal, when the disc hole is located, the resistance of the photodetector is maximum. The elements of the DD6 microcircuit, due to resistance drops, form a sequence of rectangular pulses on the RB6, RB7 buses, which are read by the PIC controller. The control program contains two reading algorithms - along the leading edge of the pulses and along both drops. By pressing the button "2" of the keyboard, we switch these algorithms. The key on the transistor VT1 when the transceiver is transferred to the transmission blocks the keyboard. LED HL2 - indicator of this mode. For additional isolation and reduction of mutual interference, LC filters are included in all power circuits of the controller unit - L1, L4-L6, C2, C3, C17-C23. The voltage-controlled oscillator, VCO (Fig. 4), operates at frequencies four times higher than those required for transceivers with an intermediate frequency of 5 ... 10 MHz.
This is done for two reasons: firstly, at higher frequencies, the master oscillator coils are smaller; secondly, such a generator is more versatile, and, depending on the required tasks, frequencies of more than 100 MHz can be obtained. The generator itself is made according to the scheme of a capacitive three-ton circuit on a field-effect transistor VT1. Almost all "field workers" offered by Kyiv firms were tested - BF966 showed the best results. Buffer stages are made on transistors VT2 and VT3. Sufficiently powerful BFR96 transistors were used, in class A. The VCO frequency when switching ranges is changed by switching coils L1-L5 with relay contacts K1-K4, which, in turn, are controlled by decoder DD1. Since the heterodyne frequencies for some ranges practically coincide, we managed to get by with five coils. Filtering RC and LC circuits are installed at the input and output of the DD1 chip. As mentioned earlier, in the author's transceiver, the local oscillator frequency must be 2 times higher than the required one. The signals of these frequencies are removed from the outputs Q0 and Q1 of the counter DD2. At the output of Q0 DD2 we get the frequency divided by 2, at the output of Q1 - by 4. The output of Q1 is used to operate in the range of 20 m, where the frequency of the VCO is additionally divided by 2. The DD3 microcircuit, controlled through the VD7 diode, when a logical zero appears on its pins 12 and 13 allows the passage of the VCO signal from the output of Q1 DD2. If you use the synthesizer in the transceivers "RA3AO", "Ural", "KRS", "UA1FA", then the required grid of heterodyne frequencies can be obtained using the Q2 output of the DD2 microcircuit (divider by 8). To do this, pin 1 of the DD3.1 chip should be connected to pin 13 of DD2, and pin 5 of DD3.2 to pin 12 of DD2. Now, at the output of the F/2(4) synthesizer, we will receive a signal of the form F/4(8), i.e. directly those frequencies that are indicated in Table. 1 in the column "Restructuring the GPA". The phase detector is made on a DD4 chip. The VCO frequency before being fed to the phase detector is pre-divided into 256 counters DD2 and DD5. At the output of the DD5 chip, the low-pass filter L13-L14, C51-C53 is turned on. A signal from the DDS is fed to the second input of the phase detector, through an additional amplifier on the VT4 transistor. This cascade was introduced due to possible losses in the cable that will connect the DDS output to the PD input. Transistor VT5 controls the LED HL1 "LOCK" on the controller board. The LED indicates the lock of the PLL loop, if the LED is off - the ring is closed, if it is lit - this indicates a malfunction. The control voltage is generated by the operational amplifier DA4 and through the filter elements R7, R8, C15, C16 is supplied to the generator varicap VD5. Additional filtering RC circuits R4-R36, C38-C48 are also installed at the DA50 inlet. Digital and analog components of the device, in order to avoid interference, are powered by separate stabilizers DA1, DA2, DA3. There are no special features in the manufacture and tuning of the synthesizer. The digital part, when using serviceable radio elements, works immediately. It should be noted that the capacitors C7-C9 in the low-pass filter at the output of the DD5 microcircuit (see Fig. 2) should be taken with a minimum TKE so that the filter characteristic does not change when the transceiver warms up. The same requirement must be met by capacitors C17, C19-C21, C51-C53 of the VCO board (Fig. 4). The PIC controller can be soldered to the board, but given the possible firmware update, it is advisable to install it on the panel. Two types of interference were detected from the synthesizer. When turning the encoder at some frequencies, there are very short clicks that cannot be tuned in. They disappear when the rotation of the encoder stops. These are sequential codes that enter the registers of the indication board. The method of struggle is to power the HG1 indicator from a separate stabilizer on the KREN5A chip with an RC filter at the input (a 10 ... 15 Ohm resistor with a power of 1-2 W and a high-capacity oxide capacitor). The capacitance of the capacitor (2200-10000 uF) is selected by ear for maximum suppression of clicks. If clicks appear only when UHF (AMP) or some other TRX mode is turned on, additional LC or RC filters should be installed in the corresponding control circuits (QC-QH outputs of the DD3 chip). It should also be noted that the outputs of the DD3 chip are designed for a load current of not more than 5 mA. To connect a more powerful load, it is necessary to additionally turn on the K555LN5 or 47NS06 chip in series with the controlled circuits (load current up to 40 mA at voltage up to 15 ... 30 V). The second type of interference is affected points, which are most common on the 20 m band. They occur as conversion products in the mixer and pickup from the 20 MHz reference oscillator. The cardinal method of dealing with these interferences is the complete shielding of the controller board (a box made of tinned sheet or foil fiberglass). The screening of a separate generator does nothing, the pickup "spreads" along the printed conductors of the DD1 and DD5 microcircuit board. When wiring board-to-board connections, wires should not be bundled into tight bundles, and even more so, wires connecting digital and analog circuits should not be combined. Power is supplied to each board by a separate twisted pair, stranded wire. One wire is common, the second is the supply voltage. To get the "ideal" tone of the output signal, you need to eliminate all possible (and impossible) pickups on the circuits associated with the VCO varicap. And use only high-quality elements in these chains. This is especially true for capacitors C14, C15, C16, C47, C48, C49, C50 of the VCO board. The synthesizer signal from the VCO board is fed to the transceiver mixer via a coaxial cable with a diameter of 3 mm. To precisely match this line, a resistor R27 is selected. In case of poor matching, affected frequencies most often appear, so we tune the transceiver to such a frequency and select R27 for its maximum suppression. For the recently "popular" IF, determined by the choice of quartz for PAL decoders of 8,867 MHz TVs, the winding data of the VCO coils are as follows: L1 - 5 turns, L2-L3, L5 - 4 turns each, L4 - 3 turns. The coils are frameless, wound on a mandrel with a diameter of 4 mm with wire PEV-2 0,8. The exact frequency of each generator is selected by moving apart the turns of the coils, after the final tuning of the generators. Pieces of foam rubber are inserted inside the coils and filled with paraffin. If this is not done, a microphone effect will be observed. Inductors L6-L9, L11-L14 of the VCO unit are wound on ring ferrite magnetic cores M2000NM, size K7x4x2. The number of turns - 10 ... 15 for L6-L9 and L11; 30 turns for L12-L14, PEV-2 wire 0,15. Throttle L10 - DM 0,1. You can also use small-sized imported chokes with inductances shown in the diagram. Relay K1-K4 - RES49 with a winding resistance of 1 kOhm (selected from the relay for an operating voltage of 24 V). It is desirable to use microcircuits in the synthesizer of the types indicated in the diagram. This will eliminate problems in further configuration. Instead of the 74NST9046 chip, it is still quite rare on sale, you can use HEF4046 (Philips Semiconductors) or CD4046. In case of replacement, you should slightly change the layout of the board, since not all of the pins of these microcircuits match the 9046. The SIGIN input (pin 14), which receives a signal from DDS, has a maximum sensitivity of 150 mV. Therefore, the amplitude of more than 4 V should not be set at the output of the amplifier on the transistor VT0,3. The selection of this mode is carried out by resistors R28, R29. With some instances of 74NST9046, it was not possible to ensure the closure of the PLL ring on all ranges - this malfunction was avoided by including an additional 1500 pF capacitor between pin 14 of the microcircuit and the common wire. Optocouplers U1 and U2 are reflective. The resistance of resistors R13, R15 connected in series with the emitters must not be less than 470 ... 510 Ohm, otherwise the emitting diodes may fail. The spread of the characteristics of AOT137A optocouplers requires their individual adjustment, according to a clear response to the passage of a "clove" of the disk near the optocoupler. The valcoder mechanism itself can be performed in various ways. In the author's version, the optocouplers are soldered directly to the controller board, in front of which a 65 mm diameter disk made of 0,7 mm thick duralumin with 60 teeth evenly cut along the edge of the disk rotates. The middle of the teeth is aligned with the centers of the optocouplers, the distance between the optocouplers is 15 mm. You can drill holes in the disk or stick paper with white and black sectors drawn, but the width of the drawn sectors should not be narrower than 3 mm, otherwise the encoder will not clearly work out each sector. The disk is located at a distance of 1,5...2,5 mm from the surface of the optocouplers. When the disc rotates, the advance shift must be set to 90 degrees, i.e. half a tooth lead. We temporarily solder the tuning resistors instead of R13, R15 and select the current through the emitters of the optocouplers according to the precise operation of the encoder. The sensitivity of triggers and their characteristics can be selected with resistors R9-R12, R14. If they fail to achieve accurate work, one of the optocouplers should be moved, since the required shift of 90 degrees is not provided. The quality of the output signal of the synthesizer can be estimated from the spectrogram shown in Fig. 5 obtained using the spectrum analyzer SK4-59.
Control programs for microcontrollers Author: Alexander Tarasov (UT2FW), Reni, Ukraine
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