DIY quartz filter. Quartz filters "Desna" - Document. Homemade water filter
When implementing frequency filters, it is necessary to take into account the peculiarities of their application. Earlier, we have already considered that active filters (most often) are convenient to use for the implementation of relatively low-pass filters. it is convenient to use in the frequency range from hundreds of kilohertz to hundreds of megahertz. These implementations of filters are quite convenient in manufacturing and in some cases can be tuned in frequency. However, they have low parameter stability.
The resistance value of the resistors in the filter is not constant. It changes with temperature, humidity, or as the elements age. The same can be said about the value of the capacitance of the capacitor. As a result, the tuning frequencies of the filter poles and their quality factor change. If there are zeros of the filter gain, then their tuning frequencies also change. As a result of these changes, the filter changes its . They say about such a filter that it "falls apart"
A similar situation occurs with passive LC filters. True, in LC filters, the dependence of the frequency of the pole or zero depends less on the value of the inductance and capacitance. This dependence is proportional to the square root, in contrast to the linear dependence in RC circuits. Therefore, LC circuits have greater parameter stability (approximately 10 −3).
By applying some measures (such as the use of capacitors with positive and negative TKE, thermal stabilization), the stability of the parameters of the described filters can be improved by an order of magnitude. Nevertheless, this is not enough when creating modern equipment. Therefore, since the 1940s, more stable solutions have been sought.
In the process of research, it was found that mechanical vibrations, especially in vacuum, have lower losses. Filters were developed on musical tuning forks, strings. Mechanical oscillations were excited and then removed by inductors using a magnetic field. However, these designs turned out to be expensive and cumbersome.
Then, the conversion of electrical energy into mechanical vibrations began to be done using magnetostrictive and piezo effects. This made it possible to reduce the size and cost of filters. As a result of the research, it was found that the plates of quartz crystals have the greatest stability of the oscillation frequency. In addition, they have a piezoelectric effect. As a result, by now, quartz filters are the most common type of high-quality filters. The internal design and appearance of the quartz resonator are shown in Figure 1.
Figure 1. Internal structure and appearance of a quartz resonator
Single crystal resonators are rarely used in crystal filters. This solution is usually used by radio amateurs. Currently, it is much more profitable to buy a ready-made quartz filter. Moreover, the market usually offers filters for the most common intermediate frequencies. Manufacturers of quartz filters use a different solution to reduce the size. On one quartz plate, two pairs of electrodes are deposited, which form two resonators interconnected acoustically. The appearance of a quartz plate with a similar design and a drawing of the case where it is placed is shown in Figure 2.
Figure 2. The appearance of a quartz plate with two resonators, a drawing of the housing and the appearance of a quartz filter
Such a solution is called a quartz deuce. The simplest quartz filter consists of one deuce. Its conventional graphic designation is shown in Figure 3.
Figure 3. Conditionally-graphic designation of a quartz two
Quartz 2 is electrically equivalent to the bandpass filter circuit with two coupled circuits shown in Figure 4.
Figure 4. Two-loop filter circuit equivalent to a quartz two
The difference lies in the achievable quality factor of the circuits, and, consequently, the bandwidth of the filter. The gain is especially noticeable at high frequencies (tens of megahertz). Quartz filters of the fourth order are performed on two deuces connected to each other with a capacitor. The input and output of these twos is no longer equivalent, therefore it is denoted by a dot. The scheme of this filter is shown in Figure 5.
Figure 5. Fourth order crystal filter circuit
Filters L1C1 and L2C3, as usual, are designed to transform the input and output resistance and bring them to a standard value. Quartz filters of the eighth order are built in a similar way. For their implementation, four quartz twos are used, but unlike the previous version, the filter is made in one housing. A schematic diagram of such a filter is shown in Figure 6.
Figure 6. Schematic diagram of the eighth order crystal filter
The internal design of the eighth order quartz filter can be studied from the photograph of the filter with the cover removed, which is shown in Figure 7.
Figure 7. Internal construction of the eighth order crystal filter
The photograph clearly shows four quartz deuces and three surface mount capacitors (SMD). A similar design is used in all modern filters, both penetrating and surface mounting. It is used by both domestic and foreign manufacturers of quartz filters. Of the domestic manufacturers, one can name OJSC "Morion", LLC NPP "Meteor-Kurs" or the Piezo group of enterprises. The list of references lists some of the foreign manufacturers of quartz filters. It should be noted that the design shown in Figure 7 is easily implemented in surface mount packages (SMD).
As we can see, now there is no problem to buy a ready-made quartz filter with minimal dimensions and at an affordable price. They can be used to design high quality receivers, transmitters, transceivers, or other types of radio equipment. In order to make it easier to navigate the types of quartz filters offered on the market, we present a graph of typical dependences of the amplitude-frequency characteristic on the number of resonators (poles), given by SHENZHEN CRYSTAL TECHNOLOGY INDUSTRIAL
Figure 8. Typical shape of the frequency response of a quartz filter depending on the number of poles
Literature:
Together with the article "Quartz filters" they read:
http://site/Sxemoteh/filtr/SAW/
http://site/Sxemoteh/filtr/piezo/
http://website/Sxemoteh/filtr/Ceramic/
http://site/Sxemoteh/filtr/Prototip/
The crystal filter is known to be “half a good transceiver”. The proposed article presents the practical design of a twelve-crystal quartz filter of the main selection for a high-quality transceiver and set-top box for a computer, allowing you to configure this and any other narrow-band filters. Recently, in amateur designs, quartz eight-crystal ladder-type filters made on the same resonators are used as the main selection filter. These filters are relatively easy to manufacture and do not require large material costs.
Computer programs have been written for their calculation and simulation. The characteristics of the filters fully meet the requirements for high-quality signal reception and transmission. However, with all the advantages, these filters also have a significant drawback - some asymmetry in the frequency response (flat low-frequency slope) and, accordingly, a low squareness factor.
The workload of the amateur radio determines rather stringent requirements for the selectivity of a modern transceiver in the adjacent channel, so the main selection filter must provide attenuation outside the passband of at least 100 dB with a squareness factor of 1.5 ... 1.8 (at levels of -6 / -90 dB ).
Naturally, the losses and uneven frequency response in the passband of the filter should be minimal. Guided by the recommendations set out in, a ten-crystal ladder filter with a Chebyshev characteristic was chosen as the basis with a frequency response unevenness of 0.28 dB.
To increase the steepness of the slopes, additional circuits were introduced parallel to the input and output of the filter, consisting of series-connected quartz resonators and capacitors.
The calculations of the parameters of the resonators and the filter were carried out according to the method described in. For a filter bandwidth of 2.65 kHz, the initial values were obtained C1,2 = 82.2 pF, Lkv = 0.0185 H, Rn = 224 Ohm. The filter circuit and the calculated values of the capacitor ratings are shown in fig. one.
The design uses quartz resonators for television PAL decoders at a frequency of 8.867 MHz, manufactured by VNIISIMS (Alexandrov, Vladimir Region). The stable repeatability of crystal parameters, their small dimensions and low cost played their role in the choice.
The selection of the frequency of quartz resonators for ZQ2-ZQ11 was carried out with an accuracy of ±50 Hz. The measurements were carried out using a self-made self-oscillator and an industrial frequency meter. Resonators ZQ1 and ZQ12 for parallel circuits are selected from other batches of crystals with frequencies respectively below and above the fundamental frequency of the filter by about 1 kHz.
The filter is assembled on a printed circuit board made of double-sided foil fiberglass 1 mm thick (Fig. 2).
The top layer of metallization is used as a common wire. The holes on the side of the resonator installation are countersunk. The cases of all quartz resonators are connected to a common wire by soldering.
Before parts are installed, the filter PCB is soldered into a tin-plated box with two removable covers. Also, on the side of the printed conductors, a screen-partition is soldered, passing between the leads of the resonators along the central axial line of the board.
On fig. 3 shows the wiring diagram of the filter. All capacitors in the filter are KD and KM.
After the filter was made, the question arose: how to measure its frequency response with maximum resolution at home?
A home computer was used with subsequent verification of the measurement results by plotting the frequency response of the filter by points using a selective microvoltmeter. I, as a designer of amateur radio equipment, was very interested in the idea proposed by DG2XK, to use a computer program of a low-frequency (20 Hz ... 22 kHz) spectrum analyzer to measure the frequency response of narrow-band amateur radio filters.
Its essence lies in the fact that the high-frequency spectrum of the frequency response of a quartz filter is transferred to the low-frequency range using a conventional SSB detector, and a computer with an installed spectrum analyzer program makes it possible to view the frequency response of this filter on the display.
As a source of high-frequency signal DG2XK, a noise generator based on a zener diode was used. My experiments have shown that such a signal source allows you to view the frequency response up to a level of no more than -40 dB, which is clearly not enough for a high-quality filter setting. In order to view the filter's frequency response at -100 dB, the oscillator must have
the side noise level is below the specified value, and the detector has good linearity with a maximum dynamic range of no worse than 90 ... 100 dB.
For this reason, the noise generator was replaced by a conventional sweep generator (Fig. 4). The circuit of a quartz oscillator is taken as a basis, in which the relative power spectral density of the noise is -165 dB / Hz. This means that the noise power of the generator at a detuning of 10 kHz in a bandwidth of 3 kHz
less than the power of the main oscillation of the generator by 135 dB!The source code has been slightly modified. So instead of bipolar transistors, field-effect transistors are used, and a circuit consisting of an inductor L1 and varicaps VD2-VD5 is connected in series with a quartz resonator ZQ1. The oscillator frequency is tuned relative to the quartz frequency within 5 kHz, which is quite enough to measure the frequency response of a narrow-band filter.
The quartz resonator in the generator is similar to the filter one. In the oscillating frequency generator mode, the control voltage to the VD2-VD5 varicaps is supplied from a sawtooth voltage generator made on a unijunction transistor VT2 with a current generator on VT1.
For manual tuning of the generator frequency, a multi-turn resistor R11 is used. Chip DA1 works as a voltage amplifier. The originally conceived sinusoidal control voltage had to be abandoned due to the uneven speed of the passage of the MCF in different sections of the frequency response of the filter, and in order to achieve maximum resolution, the generator frequency was reduced to 0.3 Hz. Switch SA1 selects the frequency of the "saw" generator - 10 or 0.3 Hz. The frequency deviation of the GKCH is set by a tuning resistor R10.
The schematic diagram of the detector block is shown in fig. 5. The signal from the output of the quartz filter is applied to the X2 input if the L1C1C2 circuit is used as a filter load.
If measurements are carried out on filters loaded with active resistance, this circuit is not needed. Then the signal from the load resistor is applied to the X1 input, and the conductor connecting the X1 input to the circuit is removed on the detector printed circuit board.
A source follower with a dynamic range of more than 90 dB on a powerful field-effect transistor VT1 matches the load resistance of the filter and the input impedance of the mixer. The detector is made according to the scheme of a passive balanced mixer based on field-effect transistors VT2, VT3 and has a dynamic range of more than 93 dB.
The combined gates of the transistors through the P-circuits C17L2C20 and C19L3C21 receive anti-phase sinusoidal voltages of 3 ... 4V (rms) from the reference oscillator. The reference oscillator of the detector, made on the DD1 chip, has a quartz resonator with a frequency of 8.862 MHz.
The low-frequency signal formed at the output of the mixer is amplified by about 20 times by an amplifier on the DA1 chip. Since the sound cards of personal computers have a relatively low-impedance input, a powerful K157UD1 op amp is installed in the detector. The amplifier frequency response has been adjusted so that below 1 kHz and above 20 kHz there is a gain rolloff of approximately -6 dB per octave.
The oscillator is mounted on a printed circuit board made of double-sided foil fiberglass (Fig. 6). The top layer of the board serves as a common wire, the holes for the leads of the parts that do not have contact with it are countersunk.
The board is soldered in a 40 mm high box with two removable covers. The box is made of tin plate. Inductors L1, L2, L3 are wound on standard frames with a diameter of 6.5 mm with trimmers made of carbonyl iron and placed in screens. L1 contains 40 turns of PEV-2 0.21 wire, L3 and L2 - 27 and 2+4 turns of PELSHO-0.31 wire, respectively.
Coil L2 is wound on top of L3 closer to the "cold" end. All chokes are standard - DM 0.1 68 μH. Fixed resistors MLT, tuning R6, R8 and R10 type SPZ-38. Multi-turn resistor - PPML. Permanent capacitors - KM, KLS, KT, oxide - K50-35, K53-1.
The establishment of the GKCH begins with setting the maximum signal at the output of the sawtooth voltage generator. By controlling the signal at pin 6 of the DA1 microcircuit with an oscilloscope, the trimming resistors R8 (gain) and R6 (bias) set the amplitude and shape of the signal shown on the diagram at point A. By selecting the resistor R12, stable generation is achieved without entering the signal limiting mode.
By selecting the capacitance of the capacitor C14 and adjusting the L2L3 circuit, the output oscillatory system is tuned to resonance, which guarantees a good load capacity of the generator. The L1 coil trimmer sets the oscillator tuning limits within 8.8586-8.8686 MHz, which marginally covers the frequency response band of the tested quartz filter. To ensure maximum restructuring of the GKCh
(not less than 10 kHz) around the connection point L1, VD4, VD5 the top layer of foil is removed. Without load, the output sinusoidal voltage of the generator is 1V (rms).
The detector unit is made on a printed circuit board made of double-sided foil fiberglass (Fig. 7).
The top layer of foil is used as the common wire. Holes for the conclusions of parts that do not have contact with a common wire are countersinked.
The board is soldered in a tin box 35 mm high with removable covers. Its resolution depends on the quality of manufacture of the attachment.
Coils L1 -L4 contain 32 turns of wire PEV-0.21, wound round to round on frames with a diameter of 6 mm. Trimmers in coils from armor cores SB-12a. All chokes type DM-0.1. Inductance L5 - 16 μH, L6, L8 - 68 μH, L7 - 40 μH. Transformer T1 is wound on an annular ferrite magnetic circuit 1000NN of size K10 x 6 x 3 mm and contains 7 turns in the primary winding, 2 x 13 turns of PEV-0.31 wire in the secondary.
All tuning resistors - SPZ-38. During the pre-tuning of the block, a high-frequency oscilloscope controls the sinusoidal signal at the gates of transistors VT2, VT3 and, if necessary, adjusts the coils L2, L3. Trimmer coil L4 the frequency of the reference oscillator is removed below the filter bandwidth by 5 kHz. This is done in order to reduce the number of various interferences that reduce the resolution of the device in the working area of the spectrum analyzer.
The oscillator is connected to a quartz filter through a matching oscillatory circuit with a capacitive divider (Fig. 8).
During tuning, this will allow you to get low attenuation and ripple in the passband of the filter.
The second matching oscillatory circuit, as already mentioned, is located in the detector attachment. Having assembled the measurement circuit and connected the output of the set-top box (X3 connector) to the microphone or line input of the sound card of a personal computer, we launch the spectrum analyzer program. There are several such programs. The author used the program SpectraLab v.4.32.16, located at: http://cityradio.narod.ru/utilities.html. The program is easy to use and has great features.
So, we launch the “SpektroLab” program and, by adjusting the frequencies of the GKCH (in manual control mode) and the reference oscillator in the detector attachment, set the peak of the GKCh spectrogram to around 5 kHz. Further, by balancing the mixer of the detector attachment, the peak of the second harmonic is reduced to the noise level. After that, the GKCh mode is turned on and the long-awaited frequency response of the filter under test appears on the monitor. First, the swing frequency of 10 Hz is turned on and, by adjusting the center frequency using R11, and then the swing band R10 (Fig. 4), we set an acceptable “picture” of the filter's frequency response in real time. During measurements, by adjusting the matching circuits, minimal passband ripple is achieved.
Further, to achieve the maximum resolution of the device, we turn on the swing frequency of 0.3 Hz and set the maximum possible number of Fourier transform points (FFT, the author has 4096 ... 8192) and the minimum value of the averaging parameter (Averaging, the author has 1) in the program.
Since the characteristic is drawn in several passes of the GKCh, the storage peak voltmeter mode (Hold) is switched on. As a result, on the monitor we get the frequency response of the filter under study.
Using the mouse cursor, we obtain the necessary digital values of the obtained frequency response at the required levels. In this case, one must not forget to measure the frequency of the reference oscillator in the detector attachment, in order to then obtain the true values of the frequencies of the frequency response points.
After evaluating the initial “picture”, the frequencies of the series resonance ZQ1n ZQ12 are adjusted, respectively, to the lower and upper slopes of the filter's frequency response, achieving a maximum squareness of -90 dB.
In conclusion, using the printer, we get a full-fledged “document” for the manufactured filter. As an example, in fig. 9 shows the spectrogram of the frequency response of this filter. The spectrogram of the GKCH signal is also shown there. The visible unevenness of the left slope of the frequency response at the level of -3 ... -5 dB is eliminated by rearranging the ZQ2-ZQ11 quartz resonators.
As a result, we obtain the following filter characteristics: bandwidth by level - 6 dB - 2.586 kHz, frequency response unevenness in the passband - less than 2 dB, squareness factor by levels - 6/-60 dB - 1.41; by levels - 6/-80 dB 1.59 and by levels - 6/-90 dB - 1.67; attenuation in the band - less than 3 dB, and behind the band - more than 90 dB.
The author decided to check the results obtained and measured the frequency response of the quartz filter point by point. For measurements, a selective microvoltmeter with a good attenuator was required, which was a microvoltmeter of the HMV-4 type (Poland) with a nominal sensitivity of 0.5 μV (at the same time, it well fixes signals with a level of 0.05 μV) and an attenuator of 100 dB.
For this measurement option, the scheme shown in Fig. 1 was assembled. 10. Matching circuits at the input and output of the filter are carefully shielded. Shielded connecting wires are of good quality. The “earth” chains are also carefully made.
By smoothly changing the frequency of the GKCH with resistor R11 and switching the attenuator by 10 dB, we take the readings of the microvoltmeter, passing through the entire frequency response of the filter. Using the measurement data and the same scale, we build a graph of the frequency response (Fig. 11).
Due to the high sensitivity of the microvoltmeter and the low side noise of the GKCH, signals at the level of -120 dB are well fixed, which is clearly reflected in the graph.
The results of the measurements were as follows: bandwidth level - 6 dB - 2.64 kHz; uneven frequency response - less than 2 dB; -6/-60 dB squareness ratio is 1.386; by levels - 6 / -80 dB - 1.56; by levels - 6/-90 dB - 1.682; by levels - 6/-100 dB - 1.864; attenuation in the band - less than 3 dB, behind the band - more than 100 dB.
Some differences between the measurement results and the computer version are explained by the presence of accumulating digital-to-analogue conversion errors when the analyzed signal changes in a large dynamic range.
It should be noted that the above graphs of the frequency response of the quartz filter were obtained with a minimum amount of tuning work and with a more careful selection of components, the filter characteristics can be noticeably improved.
The proposed oscillator circuit can be successfully used for the operation of AGC and detectors. By applying the signal of the oscillator to the detector, at the output of the set-top box to the PC we get the signal of the low-frequency oscillator of the oscillating frequency, with which you can easily and quickly adjust any filter and cascade of the low-frequency path of the transceiver.
It is no less interesting to use the proposed detector attachment as part of the panoramic indicator of the transceiver. To do this, connect a quartz filter with a bandwidth of 8...10 kHz to the output of the first mixer. Further, the received signal is amplified and applied to the input of the detector. In this case, you can observe the signals of your correspondents with levels from 5 to 9 points with good resolution.
G. Bragin (RZ4HK)
Literature:
1. Usov V. SSB quartz filter. - Radio amateur, 1992, No. 6, p. 39, 40.
2. Drozdov VV Amateur KB transceivers. - M.: Radio and communication, 1988.
3. Klaus Raban (DG2XK) Optimizierung von Eigenbau-Quarzfiltern mit der PC-Soundkarte. - Funkamateur, No. 11, 2001, S. 1246-1249.
4 Frank Silva Shmutzeffekte vermeiden und beseitig. - FUNK, 1999, 11, S. 38.
Simple and cheap filter for SSB
Vorontsov A. RW6HRM proposes, as an alternative to EMFs, to use a simple and, most importantly, cheap quartz filter circuit. The article is relevant due to the scarcity and high cost of these elements.
Recently, very often in Internet publications there are “tears” of novice radio amateurs, they say, it is difficult to get an EMF, it is expensive, it is difficult to make a quartz filter, devices are needed, etc. Indeed, it is rather problematic to get a good new EMF now, what is offered on the market is a deep used one without a guarantee of normal operation, and to pile a quartz filter even on commercially available quartz at 8.86 MHz without having the appropriate instrumentation, “on peephole, impossible. At first glance, the situation is not so hot ...
However, there is an option to make a simple crystal filter for a low-frequency SSB transmitter or transceiver quite simple and, most importantly, inexpensive. It is enough to go through the radio stores and see the sale of "two-legged" quartz for remote controls at frequencies from 450 to 960 kHz. These details are made with sufficiently large tolerances for the generated frequencies, which gives us the right to choose both the intermediate frequency used and the bandwidth of the filter being made. I’ll make a reservation right away: the idea is not mine, it was previously tested by the Swedish radio amateur HARRY LYTHALL, SM0VPO, and I just inform you about it (after having made several filters for myself).
So, what we need for the selection of quartz is a simple three-point generator and a frequency meter or a radio receiver with a frequency meter that covers the amateur range of 160 meters. From a bunch of quartz, we need to choose two with a spacing of generated frequencies of 1 - 1.5 kHz. If we use quartz at a frequency of 455 kHz, then it is most convenient to tune in to their fourth harmonic (about 1820 kHz, achieving a spacing of 4 - 4.5 kHz), and if 960 kHz, then to the second (1920 kHz, spacing 2 - 2, 5 kHz).
The CL1 circuit in this example is the load of the previous IF stage, this is a standard 455 kHz circuit from any foreign AM receiver. You can also use data from amateur radio literature for homemade circuits at a frequency of 465 kHz, reducing the number of turns by 5%. The dots indicate the beginning of the coupling coils L2 and L3, 10 - 20 turns are enough for them. It is quite possible to put a filter right after the mixer, for example, an annular one with four diodes. In this case, you will already get a 1: 1: 1 transformer, which can be made on the F600 ring with an outer diameter of 10 - 12 mm, the number of turns of the twisted triple wire PEL-0.1 - 10 - 30. Capacitor C in the case of a transformer, of course, does not needed. If the second stage of the IF is made on a transistor, then a 10 kΩ resistor can be used in the current-setting base circuit, then a 0.1 μF isolation capacitor is not needed. And if this filter is used in a simple radio path circuit, then the resistor can be excluded.
Now, from the remaining pile of quartz, we need to choose the right one for the reference oscillator. If we select quartz at 455 kHz to the values \u200b\u200bspecified in the diagram, then at the filter output we will get the lower side band, if at 454 kHz - the upper one. If there are no more quartz left, then it is quite possible to assemble the reference oscillator according to the capacitive three-point circuit and, by selecting its frequency, adjust the resulting filter. In this case, the generator must be made with increased measures in terms of its thermal stability.
Tuning can be done even by ear, according to the carriers of radio stations, but we will leave this pleasure for more or less experienced "musicians". For tuning it would be nice to have a sound generator and an oscilloscope. We feed a signal from a sound generator with a frequency of 3 - 3.3 kHz to a microphone amplifier (assume that the filter is already in the transmitter circuit), connect the oscilloscope to the filter output and shift the frequency of the reference generator until the output level of the signal after the filter decreases minimally . Next, we check the lower limit of the filter's transmission by applying a frequency of 300 Hz from the sound generator to the microphone input. By the way, to increase the lower limit of the bandwidth of the microphone amplifier in terms of audio frequencies, it is enough to install transition capacitors with a capacity of about 6800 pF or less, and for the upper limit, in any case, it would be good to install at least a single-link low-pass filter.
That's all. As you can see, you will not incur large costs in the manufacture of this filter, and the signal will turn out to be quite presentable. Of course, due to its simplicity, it is already undesirable to use it in second category transmitters, but for 1.8 - 7 MHz it will be more than enough. According to the results of measurements, this classical construction completely coincides with that described in the reference books (for example, Bunin and Yailenko's Handbook of the Shortwave) - the lower part of the characteristic is somewhat tightened. Attenuation in the passband is about 1 - 2 dB, it depends on the quality of the resonators used. But if you find an even cheaper way to get on the air with SSB (other than phase) - let me know
Improving the frequency response of the "Leningrad" quartz filter
S. Popov RA6CS
Fig.1 Quartz filters with "parallel" capacitances
Arrows AA and BB show the second option for switching on KPI. Resistors R1, R4 (0 ... 300 Ohm) are installed in the presence of large emissions in the frequency response. Capacitor C4 * is selected in the range from 0 to 30 pF.
In order to minimize the number of capacitors, filter circuits containing only parallel capacitances were chosen, Fig.1. Since the filters are symmetrical (with respect to their input-output), it turned out to be possible to use dual KPIs from broadcast receivers with a capacitance of 12 - 495 pF. In addition, you will need one more, pre-calibrated in pF, single-section variable capacitor.
The filter setup is as follows.
For tuning, you may need a device for measuring the amplitude-frequency characteristics X1-38 or similar. I use an oscilloscope and a homemade prefix (see below).
Initially, all capacitors are set to a position corresponding to a capacitance of 30 ... 50 pF. By controlling the frequency response of the filter on the screen of the device, by rotating the capacitors within small limits, we achieve the required bandwidth. Then, by adjusting the variable resistors (use only non-inductive ones, for example, SP4-1) at the input and output of the filter, we try to equalize the top of the frequency response. The above operations are repeated several times until the desired frequency response is obtained.
Further, instead of each individual section of the KPI, we solder a pre-calibrated capacitor, with which we try to optimize the frequency response of the filter. On its scale, we determine the capacitance of a constant capacitor and make a replacement. Thus, all sections of the KPI, in turn, are replaced by capacitors of constant capacitance. We do the same with variable resistors, which we will later replace with constant ones.
The final "finishing" of the filter is made directly in place, for example, in the transceiver. After installing the filter in the transceiver, it may be necessary to correct the values of these resistors, while for optimal matching of the filter with the mixer output and the input of the IF, the GKCH and the oscilloscope must be connected according to the diagram shown in Fig. 2.
Fig.2 Connecting a quartz filter for final adjustment
Several filters were made according to the described method. I would like to note the following. Setting up three or four crystal filters with some skill takes no more than an hour, but with 8 crystal filters the time is much higher. At the same time, attempts to pre-configure first two separate 4-crystal filters, and then dock them - turned out to be fruitless. The slightest scatter of their parameters (and this always takes place) leads to distortion of the resulting frequency response. It is also interesting to note that theoretically equal capacitances (for example, С1=СЗ, in Fig. 1a; С1=С7; СЗ=С5, in Fig. 1b) after tuning with a graduated KPI according to the optimal frequency response, had a noticeable spread.
In my opinion, the advantage of this technique is its visibility. On the screen of the device, you can clearly see how the frequency response of the filter changes depending on the change in the capacitance of each capacitor. For example, it turned out that in some cases it is quite enough to change the capacitance of one capacitor (with the help of a relay) in order to change the filter bandwidth without much deterioration in its squareness.
As noted above, an S1-77 oscilloscope and a converted attachment for measuring the frequency response are used to set up the filter.
Why C1-77? The fact is that on its side wall there is a connector on which there is a sawtooth voltage of the sweep generator. This allows you to simplify the attachment itself and exclude the sawtooth voltage generator (SPG) from its circuit. Therefore, there is no need for additional synchronization and it becomes possible to observe a stable frequency response at various sweep times. Obviously, other types of oscilloscopes can be adapted, maybe with a little refinement.
Since the simplified prefix is used only when working with quartz filters near the frequency of 8 MHz, all other subbands were excluded from it.
Also, in the used set-top box, you will need to slightly increase the output voltage. To do this, it is enough to convert the output stage into a resonant one. It must be tuned to resonance every time a new filter is connected to its output.
Fig.3 Attachment to the oscilloscope for setting up quartz filters
Literature.
- V.Zalneruskas. A series of articles "Quartz filters" Magazine "Radio" No. 1, 2, 6 1982, No. 5, 7 1983
- S. Bunin, L. Yailenko "Handbook of the shortwave", ed. "Technique" 1984
- V. Drozdov "Shortwave transceivers" ed. "Radio and Communications" 1988
- Magazine "Radio" No. 5 1993 "Sweeping frequency generator"