Adjustment of quartz filters. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Knots of amateur radio equipment. Quartz filters

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Radio amateurs have repeatedly approached me with requests to share their experience in setting up quartz filters, but I was in no hurry with this, because many sensible articles on this topic have already been published in the periodical press. After reading several of them again, you come to the conclusion that you should pay tribute to the work of their authors and thank you, because in each article there is something after reading which you can say: live a century - learn a century. However, along with gratitude, several questions remain unsolved each time.

You often come across the phrase in articles: “A quartz filter is easier to set up using curve tracers (for example, X1-38, X-1-48, SK-4-59, etc.). Of course, if they are, then the filter setting is simple. But this if you have the appropriate device, and even the instructions for it.Otherwise, the word "simple" will quickly turn into its opposite "difficult."Therefore, this article focuses on setting up a quartz filter using the simplest devices.

Some articles omit information about the type of filter to be adjusted (ladder, bridge, monolithic), describing general tuning rules. However, I came to the conclusion that each of them has, along with common ones, also its own characteristics.

Let's start by setting up the ladder type filter (Fig. 1).

Fig.1 (click to enlarge)

Experience shows that:

- the filter is obtained with the best parameters if all quartzes have the closest possible series resonance frequencies (±10 Hz). However, do not be upset if this condition is not feasible, because a good filter is obtained even with a frequency spacing of up to 1 kHz [1];

- it is best to select quartzes by including them in the reference oscillator of the device in which this filter is supposed to be used, and use the lowest frequency of them directly in the reference oscillator. In this case, the tuning elements of the generator should not be touched;

- the filter should be adjusted directly in the "native" device;

- if the quartzes have different frequencies, they should be arranged in the following sequence: set the highest frequency first at the input, and all subsequent ones - in turn from left to right, in rank, with decreasing frequency;

- containers should be used small-sized, with a minimum temperature coefficient of capacity (TKE) with an accuracy of no worse than ± 1,5%. But do not despair if there are none, because in the process of setting them up you still have to pick them up. In most cases, during the setup process, up to 90% of the containers are replaced with other (albeit close) denominations;

- it is better to use filter quartz (taken, for example, from disassembled factory filters).

So, from four filters for a frequency of 10,7 MHz (type FP2P-325-10700M-15), you can assemble four eight-crystal ladder filters (these filters have four pairs of quartz with the same frequencies) with different, but close to 10,7 MHz frequencies. Usually, several radio amateurs do this (usually 4 people) with one filter each. The most experienced of them selects four sets of quartz of the same frequency, then quartz with a minimum. keeps the scatter for himself, and gives the rest back to his friends (or vice versa?!). With somewhat less success, generator quartz can also be used.

At home, a quartz filter can be adjusted in three ways.

In the first case, you should use (except for the tuned device) as an auxiliary device another transceiver with a digital scale, in the second case - GSS (standard signal generator) and a frequency meter (with a limiting frequency exceeding at least the lowest frequency of your tuned device, for example 1,9 MHz). The frequency meter measures either the frequency of the GSS or the frequency of the GPA of the device under study.

In the third case, a quartz local oscillator is used for one of the operating frequencies (either GSS or another transceiver without a digital scale), and a digital scale is required in the device being tuned.

In all three cases, an RF signal of the operating range is fed to the input of the tuned device. In the first two cases, the supplied frequency is slowly changed in the transparency band of the quartz filter, while taking the readings of the S-meter in relative units, and every 200 Hz they are recorded in a table. Then, according to the table, graphs (frequency response) are built. The S-meter readings are plotted vertically, and the frequency is plotted horizontally. By connecting the points marked on the graph with an interpolation (averaging) line, the frequency response is obtained - the amplitude-frequency characteristic of the newly made filter.

In the third case, everything is done in the same way, only the tuned device itself is tuned in frequency, taking readings directly from its digital scale and S-meter at the same time.

In this case, the "newly made" filter, as a rule, has:

- a different lane than required;

- unevenness in the upper part of the frequency response;

- gentle (and sometimes with emissions) lower slope of the frequency response.

In the future, the filter is adjusted in the three above directions in order of priority.

At the first stage of tuning (coarse tuning), you should obtain a filter bandwidth of up to 2,4 kHz by alternately replacing the capacitances, starting from the filter input, and then taking the frequency response. In doing so, keep in mind the following:

- if you install additional capacitances parallel to quartz (especially extreme ones) and increase their value (up to a certain limit), then the filter bandwidth will decrease. A similar effect will be observed with an increase in the capacitances of the capacitors going to the case. With a decrease in the values ​​of these capacitances, the opposite effect will be observed. This property is used to narrow the bandwidth of the crystal filter in CW mode. Thus, the bandwidth can be reduced to 0,8 kHz. With further narrowing of the band, the attenuation of the filter in the transparency band sharply increases (to obtain low attenuation in the CW filter, one should use resonators with a Q factor at least an order of magnitude greater than the Q factor of the filter);

- the magnitude of "humps" and dips in the upper part of the frequency response (linearity of the characteristic) will depend not only on the value of the selected capacitances, but also on the resistance value of the load resistors installed at the input and output of the filter. With a decrease in their resistance, the linearity of the characteristic improves, but the attenuation in the passband of the filter increases;

- if it is not possible to obtain sufficient steepness of the lower slope, quartz similar to those used in the filter should be installed in parallel with the load resistors, while from all available quartz, the lowest frequency should be selected or its frequency should be reduced by connecting the inductance in series. By selecting the number of turns of this inductance, you can change the steepness of the lower slope;

- the filter setting must be repeated several times. If at the last stage of tuning it is not possible to obtain an acceptable frequency response, it is necessary to try to adjust the frequency of the series resonance of individual quartz. To do this, a capacitor is installed in series with the quartz, and by selecting this capacitor, generation is achieved at the frequency of the remaining quartz. If this does not help (and this may be with a small separation between the frequencies of the parallel and series resonances of the quartz), the quartz should be replaced. Quartz in the filter should be placed in a chain, carefully shielding the input from the output. Figure 2 shows the frequency response of the KF receiver "TURBO-TEST", taken at different values ​​of the capacitances of the capacitors. -

Fig.2- For greater clarity, the frequency values ​​were taken without observing the received sideband and the actual IF value. Figure 3 shows the frequency response of the final filter setting. -

Ris.3

Now some practical tips for setting up a bridged crystal filter. Such a filter is shown in Figure 4. Coils L1 and L2 contain 2x10 turns of wire with a diameter of 0,31 mm; ferrite rings from the FP2A-325-10,700 M-15 filter are used as cores. The filter bandwidth is 2,6 kHz.

Fig.4 (click to enlarge)

If you have a low-pass filter (2...6 MHz), it usually turns out to be narrower than required, and if a high-pass filter (8...10 MHz) is too wide-band. In the first case, it is necessary to expand the bandwidth by connecting to the upper or lower (Fig. 4) quartz inductors, which should be selected experimentally. In the second case, in order to reduce the bandwidth, it is necessary to connect trimmer capacitors in parallel with the resonators (similar to coils). The quartz in the filter must be selected with an accuracy of 50 Hz (series resonance frequency), and the frequencies of all upper resonators must be the same and differ from the lower ones (also the same) by 2 ... 3 kHz.

If only quartzes of the same frequency are available, you can change the frequency of the quartzes by erasing the silver layer from the crystal (increase the frequency) or by shading with a pencil (decrease). But practice shows that the stability of the parameters of such a filter over time leaves much to be desired.

More stable results are obtained by adjusting the frequency by connecting a tuning capacitor in series with quartz. After tuning, it is advisable to replace the capacitor with a constant capacitance of the same value.

With a large filter bandwidth, a dip (attenuation) may appear in the middle of its frequency response. It should be said that its depth largely depends on the resistance of the resistors R1 and R2. Their value can be from hundreds of ohms (with a bandwidth of 3 kHz) at frequencies of 8 ... 10 MHz to several kilo-ohms at lower frequencies and with a smaller filter bandwidth. In the manufacture of a bridge filter, great attention should be paid to the symmetry of its shoulders, as well as the windings of the transformers included in it, and, of course, careful screening of the input from the output. More details about bridge filters can be found in [2].

Literature

1. Goncharenko I. Ladder filters on unequal resonators. - Radio, 1992, No. 1, S. 18.
2. Bunin S.G., Yaylenko L.P. Handbook of a radio amateur-shortwave. - K .: Technique, 1984, S. 21 ... 25.

Author: V.Rubtsov (UN7BV), Kazakhstan, Astana, "Tselinny" area; Publication: N. Bolshakov, rf.atnn.ru

See other articles Section Knots of amateur radio equipment. Quartz filters.

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