Encyclopedia of radio electronics and electrical engineering / Civil radio communications

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This generator is designed to generate signals of two switchable stable frequencies. In particular, it can be used as part of synthesizers for individual radio broadcasting when forming both a medium-wave broadcasting grid with a step of 9 kHz, and a short-wave one with a step of 5 kHz. But its scope is not limited to this. The ability to use integrated oscillators and quartz resonators for various frequencies, together with a wide range of division coefficients, allows this design to be used in other devices.

Relative generator frequency stability 0,5 10^{-6 о}С^-1 in the temperature range from -10 to +60 ^оC is provided by a thermocompensated quartz oscillator GK321-TK-K-9M-5V [1]. It is possible to replace it with a conventional crystal oscillator on logic elements. However, the frequency stability in this case will be worse.

The generator has a frequency divider with a variable division ratio, set by two switchable sets of jumpers corresponding to two values. frequency) is selected with a two-position switch.

The generator circuit is shown in Fig.1. The integrated thermally compensated quartz oscillator G1 (GK321-TK-K-9M-5V) is connected according to the scheme recommended by the manufacturer. Additionally, a decoupling filter from the inductor L1 and capacitors C1 and C5 is installed in its power circuit. If the jumper S1 is set to position 2-3, the generator signal is fed to the buffer amplifier on the logic element 3I-NOT DD1.3, turned on by the inverter.

Rice. 1 (click to enlarge)

An alternative quartz oscillator is made on the logic elements DD1.1 and DD1.2 according to the scheme of an asymmetric multivibrator with a quartz resonator in the feedback circuit. In the second arm of the multivibrator, the simplest low-pass filter R4C7 is installed with a cutoff frequency equal to twice the frequency of the quartz resonator, which prevents the excitation of this resonator at the harmonics of the fundamental frequency. When using quartz resonators for other frequencies, the capacitance of the filter capacitor C7 must be changed in inverse proportion. For example, a 4,5 MHz quartz resonator requires a 30 pF capacitor.

The programmable frequency divider is made on two microcircuits of parallel synchronous binary counters 533IE10 (DD4, DD5) and two flip-flops of the microcircuit 533TM2 (DD3). On overflow of the counter DD5, a high logic level is set at its CO transfer output, which comes to the input (pin 13) of the DD2.1 element. The signal from the output of the high-order digit of the counter DD4 (pin 11), supplied to the inputs (pin 1 and 2) of the element DD2.1, prevents desynchronization (delay accumulation) of the falling transfer pulse drop, which improves the stability of the time position of the rising pulse drops at the output of this element and, as a result, reduces the phase noise of the generator output signal.

The pulse from the output of the element DD2.1 is fed to the inputs of the parallel load L of the counters DD4 and DD5 and allows them to write codes pre-set by jumper sets S2 and S3. On the next clock pulse, the codes are loaded into the counter, further counting starts from the loaded number.

For example, if a log is applied to all inputs D of the counter. 1 (high level), then the number 255 will be written to it and only one will remain to count until overflow. In this case, the division factor will be equal to 256 - 255 = 1. The logic levels on the contacts 1-4 of the groups of jumpers S2 and S3 at different positions of the SA1 switch are given in Table. 1. By installing jumpers between these contacts and contacts 5-8, you can get combinations of levels at inputs 1, 2, 4, 8 of DD4 and DD5 microcircuits, corresponding to any X numbers from 0 to 255. The division ratio will be equal to N = 256 - X.

Table 1

At the output of the frequency divider on the counters DD4 and DD5 there is an additional two-digit binary counter on the D-flip-flops DD3.1 and DD3.2, which increases the overall division factor by two or four times. If switch SA1 is in position F₁ the logic level at the inputs (pin 10, 11) of the element DD2.3 is low and the signal from the output of the trigger DD3.2 to the output F₂ does not pass. At the same time, the level at the inputs (pin 3, 4) of the DD2.2 element is high, so the output F₁ pass pulses with a duty cycle of 2 from the output of the trigger DD3.1. They follow with frequency F₁ = F_square/((256 - X₁) - 2), where F_KB - frequency of the quartz oscillator; X₁ - the number set at the inputs D of the counters with the switch SA1 in position F₁.

When switching switch SA1 to position F₂ pulses at the output of the element DD2.2 will stop, and at the output of the element DD2.3 they will appear and will follow with a frequency F₂ = F_square/((256-X₂) 4), where Х₂ - the number at the inputs D of the counters at position F₂ switch. Outlet F₃ regardless of the position of the switch, there are short (with a duration of one cycle of oscillations of the clock generator) pulses. The frequency of their repetition is less than the frequency of the quartz oscillator by a number of times equal to the currently set frequency division ratio by the counter on the DD4 and DD5 microcircuits.

Suppose, it is supposed to use the described generator as a source of the reference frequency of 45 kHz for the synthesizer described in [2]. In this case, the frequency of the 9000 kHz crystal oscillator must be divided by 9000/45 = 200 times. Taking into account the division by four triggers of the DD3 microcircuit, we obtain that the frequency division ratio of the counter on the DD4 and DD5 microcircuits should be equal to 200/4 = 50. This means that at each overflow it is necessary to write the number 256 - 50 = 206 into its microcircuits₁₀ = 1101110₂. To do this, you must install jumpers in accordance with the table. 2. Since it is not required to switch the division factor in this case, contacts 2 and 3 are not used for setting jumpers, the logical levels on which depend on the position of the SA1 switch. Only the generator outputs will switch, and the pulse frequency at the output F₁ will be equal to 90 kHz, and at the output F₂ - 45 kHz.

Table 2

If it is necessary to program the generator to receive two frequency values, for example, 10 and 36 kHz (this may be required to create a frequency synthesizer with a grid step of 5 and 9 kHz), then it is advisable to form a lower frequency at the output F₂, which has an additional divisor by four, and a higher one - at the output F₁ dividing by two.

For F₁ = 36 kHz total division factor 9000/36 = 250, and without additional division by two - 250/2 = 125. The number that should be written to the counter when overflowing is - 256 - 125 = 131₁₀ = 10000011₂. For F₂ = 10 kHz the total division factor is 9000/10=900, and without additional division by four - 900/4 = 225. The number that should be written to the counter when overflowing is - 256 - 225 = 31₁₀ = 00011111₂. The positions in which, in the case under consideration, it is necessary to install the jumpers of sets S2 and S3, are shown in Table 3. It is in these positions that they are depicted and highlighted in color in the diagram of Fig. 1.

Table 3

If a crystal oscillator is used for a different frequency (it can reach 20 MHz) or it is necessary to obtain other frequency values ​​\u16b\uXNUMXbat the outputs, then calculations similar to those given above will have to be done independently and jumpers should be installed in accordance with their results. If necessary, you can get more than two values ​​of the output frequency and quickly switch them by using two code switches with XNUMX positions each instead of sets of jumpers.

All parts of the generator are mounted on a double-sided printed circuit board (Fig. 2) with dimensions of 90x35 mm made of foil fiberglass 1,5 mm thick, manufactured using the technology with metallized holes. If it is not possible to metallize them, then you will have to solder the leads of the parts on both sides, and solder pieces of tinned wire into the vias.

Fig. 2

The location of the parts on the board is shown in fig. 3. When using a temperature-compensated crystal oscillator G1, elements ZQ1, C7, C8, C11, R2 and R4 are not mounted on it. In addition, it is necessary to install two additional jumpers: one between the contact pads intended for the capacitor C7, and the other between the left ones in fig. 3 contact pads for resistors R2 and R4, Jumper S1 is set to position 2-3.

Fig. 3

If a generator based on a quartz resonator ZQ1 and logic elements DD1.1 and DD1.2 is used, then the generator G1, inductor L1, capacitor C5 and resistors R1 and R3 are not mounted on the board, and the jumper S1 is set to position 1-2. For the conclusions of the quartz resonator, two pairs of contact pads are provided, which are used depending on its size. The resonator itself is mounted on the board with a loop of tinned wire with a diameter of 0,6 ... 0,7 mm, on which a thin tube of cambric, polyvinyl chloride or fluoroplast is put on. The loop is stretched and its ends are soldered into the holes on the board. Under the quartz resonator in a metal case, it is necessary to place an insulating gasket made of fiberglass or thick cardboard. A quartz resonator in a glass bottle should be wrapped with three or four layers of varnished cloth before installation.

The board is designed to install resistors MLT or C2-23. Capacitors (with the exception of C10) - K10-17-1b. Oxide capacitor C10 - K53-18 with axial leads, which can be replaced by K50-35 with leads in one direction or a similar imported one. There is an additional hole on the board for the negative terminal of a capacitor with such an arrangement of pins. The 2D212B diode can be replaced by any silicon diode with a permissible forward current of at least 500 mA. Instead of an integral stabilizer KR142EN5A, an imported 7805 is suitable. Choke L1 - DM-0,1. The conclusions of digital microcircuits before installing them on the board must be molded according to Fig. 4 using tweezers, thin long-nosed pliers or a special tool.

Fig. 4

In the case of using an integrated quartz oscillator, it is necessary to accurately select the value of the corrective resistance formed by the series connection of resistors R1 and R3. It must correspond to the value specified in the passport of a particular instance of the generator. The frequency is precisely set using a frequency meter by selecting these resistors at a temperature of 20 ^оC.

If a quartz resonator and a logic gate oscillator are used, the exact generation frequency is set by a selection of capacitors C8 and C11. Trimmer resistors and capacitors are not specifically used, which eliminates the influence of the instability of their moving contacts on the frequency and increases the reliability of the generator.

The proposed universal design makes it possible to assemble and debug a synthesizer (for which the described generator is intended) with any available quartz resonator, and then order a highly stable integrated oscillator for the exact frequency and install it on the same board.

Literature

1. Thermally compensated quartz oscillators GK321-TK-K - URL: bmg-quartz.ru/gk321_tk_k.html.
2. Komarov S. Medium-wave broadcasting frequency synthesizer. - Radio, 2012, No. 9, p. 19-23; No. 10, p. 21-23.

Author: S. Komarov

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