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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Crystal filter for SSB. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Knots of amateur radio equipment. Quartz filters As you know, the SSB exciter is greatly simplified if you use a quartz filter in it, tuned to a frequency above 1 MHz. On fig. 1 shows a filter circuit with four quartz. When operating at frequencies up to 2-3 MHz, such a filter allows you to obtain suppression of the second sideband up to 40-50 dB. The filter circuit is extremely simple, and if an amateur has five or six quartzes at the same frequency, anyone can make it.
Before making a filter, it is necessary to select quartz for it. To select quartz, you need to assemble a device, the block diagram of which is shown in Fig. 2. In this device, the frequency of the crystal oscillator, in which one of the quartz intended for the filter is installed, is compared with the frequency of the range oscillator, and the suitability of the quartz is determined by the received beats.
The GSS-6 device is used as a range generator. It is possible to assemble a special narrow-range generator for this purpose, which overlaps the bandwidth of the future filter with some margin. It may not have graduations at all, but rather high frequency stability is required. The crystal oscillator can be assembled according to any scheme. To obtain beats, any converter stage on a multi-grid lamp is used. Voltage is supplied to the heterodyne grid of the converter lamp from a quartz oscillator, and to the control grid - the output voltage of the range generator. A resistor of about 200 kΩ is included in the anode circuit of the lamp. If there is an oscilloscope, the beat frequency is determined from Lissajous figures using a graduated sound generator. If there is no oscilloscope, you can use another converter and determine the equality of the beat frequency and the frequency of the sound generator by zero beats between them. The manufacture of the filter must begin with measuring the separation of the frequencies of series and parallel resonances in each of the available quartz. Measurements must be done several times, trying to determine the resonant frequencies with an accuracy of 10-20 Hz. In order to follow the scheme shown in Fig. 1, it was possible to make a filter with a bandwidth sufficient for SSB, the frequency separation of both resonances for all quartz should be more than 1000 Hz. Usually this condition is satisfied. Otherwise, it is necessary to reduce the capacitance of the quartz holder, if possible, or assemble the filter according to a different scheme. Then, using the same device (Fig. 2), it is necessary to check the absence of noticeable parasitic resonances in all quartz closer than 20-30 kHz from the main one. If there are spurious resonances, but they are weaker than the main one by 20 dB or more, and also do not coincide with different quartz crystals in frequency, they will not degrade the filter performance. Now you need to set aside two quartzes that have equal frequencies of series resonances, and tune the other two to a higher or lower frequency. There is no good way to lower the frequency of quartz in amateur conditions. One of them is sawing grooves in the side faces. In this case, however, the temperature stability of quartz deteriorates and parasitic resonances may appear. It is better to increase the frequency of quartz. If they are metallized, this is achieved by carefully erasing part of the metal coating (evenly over the entire area) using an ordinary red (so-called ink) gum. In order not to break the quartz when erasing the metal coating, it must be placed on a flat, hard surface. If quartz is not metallized, it is necessary to increase the frequency by grinding their plastic with the smallest (micron) sandpaper. You need to grind by moving the plate along the sandpaper, but not vice versa. It should be remembered that sometimes 2-3 movements of the plate over the skin are enough to increase the frequency of quartz by 1000 Hz. In the process of tuning quartz resonators, it is necessary to control the frequency of their series resonance as often as possible. To obtain a filter with an optimal bandwidth (2600 Hz) for operation on SSB, it is necessary to rebuild the series resonances of two quartz crystals at 1800 Hz. In this case, before the restructuring, the quartz must have a spacing of series and parallel resonances of at least 2000 Hz. If, as a result of the measurements made at the beginning, it turned out that the resonance spacing is less than 2000 Hz, but more than 1000 Hz, the quartz is rebuilt by 0,9 of the frequency spacing. The filter bandwidth in this case will be less than 2500 Hz, but still sufficient so that the intelligibility of the transmitted speech is not affected. The filter coil L1 is placed in the SB-3 type core and has a tap from the midpoint. In order for both halves of the winding to be as equal as possible, and this is very important, the winding is carried out in two wires, and then the end of one of them is connected to the beginning of the other, thus obtaining an average output. The value of the inductance L1 should be such that when the capacitance of the capacitor C3 is equal to 15-20 pF, the resulting circuit is tuned to the middle frequency of the filter passband. It is not possible to indicate the exact winding data of the L1 coil, since the average frequency may be different. The filter is assembled on a plate of insulating material, placing the quartz so that to the left and to the right of the L1 coil there is one quartz with a higher frequency (in Fig. 1, quartz Kv1 and Kv4 with a higher, and Kv1 and Kv3 - with a lower frequency). Trimmer capacitors C1 and C2, shown in Fig. 1, are not connected at the first stage of filter tuning. Setting assembled filter is produced as follows. Voltage is supplied to the filter input from a range generator (GSS-6 or another), and a sensitive tube voltmeter or receiver is connected to the output as shown in Fig. 3. If a receiver is used as an indicator, then in order to be able to take the frequency response of the filter, you need to put a step attenuator at the input of the receiver and calibrate its S-meter.
When used as a range generator of the GSS-6 device, it is possible to determine the attenuation by its attenuators, maintaining a constant signal level at the receiver input. In any case, it should be possible to measure attenuations from 0 to 60 dB with an accuracy of 1-2 dB. The crystal filter must be matched to both the oscillator and the indicator. Resistors R1 and R2 are used for matching (see Fig. 3). The resistance value of the resistor R2 must be equal to the characteristic impedance of the filter. If the output impedance of the range generator is low enough, resistors R1 and R3 should be set with the same resistance, otherwise the resistance R1 should be correspondingly less than R2. Since the characteristic impedance of the filter is not known in advance, R2 = 2 com is initially taken. Resistor R3 is decoupling, so its resistance should always be significantly greater than that of R2. By attaching devices to the filter, the frequency response of the filter is taken point by point in the range of ± 5 kHz from the middle of the passband. By alternately selecting the filter capacitor C3 and the resistors R1 and R2, the passband response is as flat as possible. Small dips of 1-2 dB are acceptable. The bandwidth slopes at the atom tuning stage will be quite flat. To increase their steepness, small capacitors are connected in parallel with higher-frequency quartz. In this case, however, "tails" appear on both sides of the passband of the filter - gentle rises in its frequency response, which reduce the suppression of the second sideband. In order to get the steepest possible slopes with an acceptable "tails" value, first connect only one of the capacitors shunting the quartz, for example, C1. The value of the capacitance of the capacitor is selected such that the attenuation in the "tails" is 40-45 dB greater than in the passband. This is usually achieved with a capacitance Ci equal to 5-10 pF. Then turn on the capacitor C2, achieving a decrease in the size of the "tails". Capacitance C2 should be about 3-5 pF less than C1. A properly tuned filter should have four "infinite" attenuation points on the characteristic: two above and two below the passband. "Tails" located above the bandwidth in frequency must be of equal size. If, after selecting capacitors C1 and C2, the filter characteristic in the passband becomes less flat, it is necessary to select resistors R1 and R2 again. This completes the filter setup. It remains to enclose it in the screen and once again check the frequency response. The high-frequency slope of the passband of the filter, in which the same quartz is used, turns out to be steeper, therefore it is better to form the lower sideband using such a filter, obtaining the upper one during frequency conversion in subsequent stages. The attenuation of the filter in the passband is about 10 dB. This should be taken into account when designing the exciter. Figure 4 shows the characteristic of the filter for a frequency of 2 MHz, tuned according to the described method. Its characteristic resistance turned out to be 1000 ohms, inductance L1 - 265 μH, capacitance C3 - 56 pF, C1 - 12 pF, C2 - 9 pF. The frequency spacing of quartz Kv2, Kv3 and Kv1, Kv4 - 1800 Hz.
In conclusion, it must be recalled that in the exciter in which the manufactured filter will operate, the output impedance of the balanced modulator and the input impedance of the cascade following the filter must be purely active and equal to the characteristic impedance of the filter. Literature 1. Plonsky A.F. Piezoquartz in communication technology, Gosenergoizdat, M-L., 1951. Author: G.Zverev; Publication: N. Bolshakov, rf.atnn.ru
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