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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING The local oscillator of an amateur transceiver. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Knots of amateur radio equipment. Generators, heterodynes A smooth range generator (GPA) is one of the most critical components of an amateur transmitter, receiver or transceiver. The problem of high-quality GPA is especially acute in modern equipment, where high-frequency quartz filters are increasingly used. In this case, you need a GPA operating at relatively high frequencies (tens of megahertz). It is difficult to obtain good parameters from GPAs made according to traditional schemes at such frequencies. The frequency shaper can be made according to the block diagram shown in Fig.1. Here G1 is a reference oscillator, D1 is a frequency divider, (U1 is a phase discriminator, Z1 is a low pass filter, G2 is a voltage controlled oscillator, D2 is a frequency divider with a variable division ratio. This device is an active digital frequency synthesizer with a divider with a variable division ratio.Such a synthesizer allows you to receive at the output of the device, depending on the selected division coefficients D1 and D2, a frequency grid in steps of units of kilohertz.So, if the reference oscillator GI operates at a frequency of 5 MHz, the divider D1 reduces the frequency by 500 times , then by changing the division factor D2 from 2000 to 2100, you can get a frequency grid at the G2 output from 20 to 21 MHz in 10 kHz steps.
If we take a highly stable GPA as a reference oscillator, then by changing the operating range G2 and the division factor D2, we can obtain the heterodyne frequencies necessary for the transceiver. In this case, the divisors are quite simple, since the required division factor is usually small. It was this principle that was used in the local oscillator of the transceiver exhibited at the 30th All-Union Radio Exhibition. Schematic diagram of its frequency shaper is shown in fig. 2.
With the first IF equal to 8750 kHz, and signal formation on the upper sideband, heterodyne frequencies of 19,25 ... 20,25 MHz are required for the 28 MHz range; 12.25...12.75 - for 21 and 3,5 MHz bands; 5,25 ... ... 5,6 MHz - for the 14 MHz band; 15.75 ... 16.25 and 10,5 ... 11 MHz - respectively for the 7 and 1,8 MHz bands. GPA, covering the frequency band 5,25 ... 5,6 MHz, is assembled on a V5 transistor. The stability of the GPA is ensured by a rigid structure, the use of a loop coil L1, tightly wound on a ceramic frame, the use of thermal compensation (capacitor C5 has a negative TKE); small coupling of the generator with subsequent stages and stabilization of the supply voltage. By frequency, the GPA is tuned by section C6.1 of a built-in block of capacitors of variable capacitance. For detuning during reception (or transmission), a voltage is applied to the varicap V1, either set by resistor R3 when setting the unit, or changed by resistor R41 when tuning. A buffer stage is assembled on transistor V6, loaded on a broadband circuit L2C29R31. and on the transistor V7 - an emitter follower. From the repeater, the signal goes to the pulse shaper, assembled on the D7.2 element, and then to the frequency divider (D3 chip). The controlled generator is made on the transistor V11. The desired range is selected by connecting one of the coils L13-L17 to the generator circuit through diodes V24-V28 (they are supplied with a voltage of 12 V through resistors R4-R8, which opens them). From the broadband amplifier on the V10 transistor, the signal generated by the controlled generator is fed to the mixers and to the pulse shaper (element D8.1) and then to the digital scale and frequency divider on the D4 chip. Experiments have shown that the shaper based on the "2I-NOT" element of the K155 series in combination with the K155IE5 divider works stably at frequencies up to 35...40 MHz. The divider with a variable division ratio is assembled on the D-flip-flops of the D1 and D2 microcircuits. To obtain the required division factor, elements D5.1, D5.2, D6.1, D6.2, D7.1 are used, included in the divider feedback circuit. So, to obtain a division factor of 11 (for a range of 10 m), element D5.1 is used. One of its entrances is the control one. With the arrival of the divider of each eleventh pulse, a logical 5.1 appears at the three inputs of D1. If the fourth input of D5.1 also has a logical 1 (10 m range is on), then the difference from the output of D5.1 will set the divider to zero. The reset pulses are also the output pulses of the divider with a variable division ratio, which are fed through the element D7.3 to the divider D4.2 (the first trigger of the eight divider of the K155IE5 chip is used). With D4.2, rectangular pulses are fed to the phase discriminator, the functions of which are performed by the element "2I-NOT" D8.3. The signal from the divider D3 of the GPA frequency comes to the second input of the element. The choice of the division factor is due to the frequency band of the GPA and the required gating frequency on the phase discriminator. The latter, in turn, tend to choose this one. to combine on the scale the beginning of the ranges, as well as to simplify the divider with a variable division factor as much as possible. These claims are contradictory. With an intermediate frequency of the transceiver of 8750 kHz and an initial frequency of the GPA of 5250 kHz, the ratio of the initial frequencies of the controlled generator and the GPA on the bands of 10, 15, 20, 40, 80 and 160 m, respectively, is: 19,25 / 5,25 -11/3; 12,25 / 5,25 \u7d \u3d 5,25/5,25; 3/3=15,75/5,25; 9/3=12,25/5,25; 7/3=10,5/5,25; 6/3=11/7. This shows that the division factor of the divisor with a variable division factor (the number in the numerator) should be equal to 3, 9, 7, 6, 3 and 3, and the division factor of the divisor D4 (the number in the denominator) should be XNUMX. Considering that before the divisor with variable division ratio and after it there are dividers by two, which improve the conditions for its operation and the phase discriminator, then in the frequency divider it is necessary to increase the conversion factor by XNUMX times. It should be noted that in the above case, you can choose other division factors of the divisors. If the frequency of the signals coming from D3 and D4.2 coincides, the output of element D8.3 will be rectangular pulses of the same frequency, but the duty cycle of which depends on the phase ratio of the input signals, and as a result, on the phase ratio (taking into account the dividers) of the GPA and the controlled generator. The DC component of the output signal voltage also depends on this. After passing the inverter (element D8.4) and the amplifier on the transistor V9, the signal enters the low-pass filter R18C16, whose task is to suppress the pulses coming from the discriminator and skip the constant component and the limited low-frequency band. The signal from the filter is fed to the V12 varicap, which is included in the frequency setting circuit of the controlled oscillator. To facilitate the capture of the frequency in the ring of the phase AFC, without introducing an auto-search device, a free section of the block of variable capacitors is connected to the circuit of the controlled generator. It uses the fact that the tuning factor is the same on all ranges. If elements with the ratings indicated in the diagram are used in the low-pass filter, side signals that arise due to phase modulation of the frequency of the controlled oscillator by pulses that have passed through the filter will be suppressed in the output signal of the local oscillator by at least 75 dB. At the same time, the capture and hold bands are sufficient for reliable capture and retention by the GPA signal of oscillations at any point in the ranges. The tuning band of the controlled generator on individual ranges is obtained with the selected circuit more than necessary. However, with the electronic frequency indication of the transceiver, this does not really matter. The winding data of the coils are given in the table. Coil L2 has a trimmer from SB-12a, and L4-L5 - SCR-1. Choke L3 - DM-0.1.
In accordance with the selected range, the range switch should supply voltage to one of the resistors R24-R28, as well as logic 1 to the control input of the corresponding logic element (B5.1. D5.2, B6.1, D6.2, D7.1 ). At the same time, logical 0 must be applied to the control inputs of the remaining logic elements. Blocking capacitors with a capacity of at least 1000 pF are connected in parallel with the power supply outputs of the microcircuits. Other pins of the microcircuits that are not indicated on the diagram can be left free. Correctly assembled digital part starts working immediately. Setting the GPA consists in setting the boundaries of its restructuring and ensuring the thermal stability of the generator by selecting capacitors C4 and C3. The broadband circuit L2C29 is tuned to the middle frequency of the GPA range. When setting up a controlled generator, resistor R9 is disconnected from transistor V18, a constant voltage of 5 V is applied to it and the circuits are tuned to the desired frequencies. On any of the ranges, by adjusting the corresponding coil and capacitor C20, the frequency overlap of the controlled generator is set equal to the GPA overlap multiplied by the division factor in this range. On the remaining ranges, pairing is achieved only by adjusting the coils. Having restored the connection of the resistor R18, a voltmeter is connected to the collector of the transistor V9 and the coils L4-L8 are adjusted again. When the cores are screwed in and the entire assembly is working correctly, the frequency capture and out of synchronization should be clearly indicated on the voltmeter. In the working area (from 2 to 10. V), an increase in inductance should lead to an increase in voltage at the V9 collector, and hence at the V12 varicap. The coils should be adjusted so that the voltage at the collector of transistor V9 is about 5 V. In the future, the correct operation of the PLL ring can be controlled by rotating the variable resistor R41. A change in the frequency at the output of the controlled generator will indicate the normal operation of the system. When setting up a controlled generator, it may be necessary to select the resistor R15. With a decrease in its value, the output voltage increases, but the signal shape deteriorates. In conclusion, it must be said that this device is also applicable for the synthesis of a frequency grid (for example, with a step of 500 kHz). To do this, in accordance with Fig. 1, instead of the GPA, install a quartz oscillator and select the parameters of the frequency dividers and the controlled oscillator accordingly. Author: V. Tereshchuk (UB5DBJ), Uzhhorod; Publication: N. Bolshakov, rf.atnn.ru
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