Transceiver quartz filter. Quartz filters "Desna" - Document Connection diagram for a four-crystal erasing filter
When implementing frequency filters, it is necessary to take into account the specifics of their application. We have already discussed earlier that active filters (most often) are convenient to use for implementing relatively low-pass filters. It is convenient to use in the frequency range from hundreds of kilohertz to hundreds of megahertz. These filter implementations are quite convenient to manufacture 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 depending on temperature, humidity or when elements age. The same can be said about the capacitance value of the capacitor. As a result, the tuning frequencies of the filter poles and their quality factors change. If there are filter gain zeros, 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 pole or zero frequency depends less on the value of 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 certain 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. However, when creating modern equipment, this is not enough. Therefore, starting from the 40s of the 20th century, more stable solutions were searched for.
During the research, it was found that mechanical vibrations, especially in a vacuum, have lower losses. Filters were developed on musical tuning forks and strings. Mechanical vibrations 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 research, it was found that quartz crystal plates have the greatest stability of vibration frequency. In addition, they have a piezoelectric effect. As a result, quartz filters are by far the most common type of high-quality filter. The internal structure 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 another solution to reduce dimensions. Two pairs of electrodes are deposited on one quartz plate, which form two resonators interconnected acoustically. The appearance of a quartz plate with a similar design and a drawing of the housing where it is placed are shown in Figure 2.
Figure 2. Appearance of a quartz plate with two resonators, drawing of the housing and appearance of the quartz filter
This solution is called a quartz pair. The simplest quartz filter consists of one pair. Its graphical designation is shown in Figure 3.
Figure 3. Graphic designation of a quartz pair
The quartz double is electrically equivalent to the bandpass filter circuit with two coupled circuits shown in Figure 4.
Figure 4. Double-circuit filter circuit equivalent to a quartz twin
The difference lies in the achievable quality factor of the circuits, and therefore the filter bandwidth. The gain is especially noticeable at high frequencies (tens of megahertz). Quartz filters of the fourth order are made on two pairs connected to each other using a capacitor. The input and output of these twos are no longer equivalent, so they are denoted by a dot. The diagram of this filter is shown in Figure 5.
Figure 5. Fourth order quartz filter circuit
Filters L1C1 and L2C3, as usual, are designed to transform the input and output resistance and bring them to the standard value. Eighth-order quartz filters are constructed in a similar way. To implement them, four quartz twins 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 an eighth-order quartz filter
The internal design of an 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 design of an eighth order crystal filter
The photo clearly shows four quartz duals and three surface mount capacitors (SMD). A similar design is used in all modern filters, both penetrating and surface mounted. It is used by both domestic and foreign manufacturers of quartz filters. Among domestic manufacturers, we can name JSC Morion, LLC NPP Meteor-Kurs or the Piezo group of enterprises. The list of references shows some of the foreign manufacturers of quartz filters. It should be noted that the design shown in Figure 7 can easily be implemented in surface mount (SMD) packages.
As we can see, now there is no problem buying 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. 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 response 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:
Along with the article "Quartz filters" read:
http://site/Sxemoteh/filtr/SAW/
http://site/Sxemoteh/filtr/piezo/
http://site/Sxemoteh/filtr/Ceramic/
http://site/Sxemoteh/filtr/Prototip/
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 beginning radio amateurs, they say, it is difficult to get an EMF, it is expensive, a quartz filter is difficult to make, instruments are needed, etc. Indeed, it is now quite problematic to get a good new EMF, what is offered on the market is deep used without a guarantee of normal operation, and to build a quartz filter even on commercially available quartz at 8.86 MHz without having the appropriate control and measuring equipment, “at peephole,” impossible. At first glance, the situation is not so great...
However, there is an option to make a simple quartz filter for a low-frequency SSB transmitter or transceiver quite simple and, most importantly, inexpensive. It’s enough to walk through radio stores and see “two-legged” quartz crystals for sale for remote controls for frequencies from 450 to 960 kHz. These parts are made with fairly large tolerances on the generated frequencies, which gives us the right to choose both the intermediate frequency used and the bandwidth of the filter being made. Let me make a reservation right away: the idea is not mine, it was previously tested by the Swedish radio amateur HARRY LYTHALL, SM0VPO, and I’m just letting you know about it (after making several filters for myself).
So, what we need to select quartz is a simple three-point generator and a frequency meter or a radio receiver with a frequency meter covering the amateur band of 160 meters. From a bunch of quartzes, we need to select 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 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).
Circuit CL1 in this example is the load of the previous amplifier stage; this is a standard 455 kHz circuit from any foreign manufactured 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 communication coils L2 and L3; 10–20 turns are enough for them. It is quite possible to install a filter immediately after the mixer, for example, a ring one with four diodes. In this case, you will already get a 1:1:1 transformer, which can be made on an F600 ring with an outer diameter of 10 - 12 mm, the number of turns of twisted triple wire PEL-0.1 - 10 - 30. Capacitor C in the case of a transformer, of course, is not needed. If the second stage of the amplifier is made on a transistor, then a 10 kOhm 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 circuit circuit, then the resistor can be eliminated.
Now from the remaining pile of quartz we need to select one suitable for the reference oscillator. If we select quartz at 455 kHz to the values indicated in the diagram, then at the filter output we will get a lower sideband, if at 454 kHz we will get an upper one. If there are no more quartz left, then it is quite possible to assemble a reference oscillator using a three-point capacitive circuit and, by selecting its frequency, adjust the resulting filter. In this case, the generator must be made with increased measures regarding its thermal stability.
Tuning can be done even by ear, using radio station carriers, but we will leave this pleasure for more or less experienced “musicians”. For setup it would be nice to have a sound generator and an oscilloscope. We feed a signal from the sound generator with a frequency of 3 - 3.3 kHz to the microphone amplifier (assuming that the filter is already in the transmitter circuit), connect an oscilloscope to the output of the filter and shift the frequency of the reference oscillator until the output signal level after the filter decreases minimally . Next, we check the lower limit of the filter’s transmission by applying a frequency of 300 Hz from a sound generator to the microphone input. By the way, to increase the lower limit of the transmission bandwidth of a microphone amplifier at 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 be quite presentable. Of course, due to its simplicity, it is no longer advisable to use it in transmitters of the second category, but for 1.8 - 7 MHz it will be more than enough. According to the measurement results, this classical design completely coincides with that described in reference books (for example, the Shortwave Handbook of Bunin and Yaylenko) - 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 go on the air with SSB (except phase) - let me know
Improving the frequency response of the "Leningrad" quartz filter
S. Popov RA6CS
You often come across the phrase in articles: “A quartz filter is easier to tune using curve tracers (for example, X1-38, X-1-48, SK-4-59, etc.). Of course, if they are available, then setting up the filter is simple. But this if you have the appropriate device, and even instructions for it. Otherwise, the word “easy” 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 being configured (ladder, bridge, monolithic), describing general configuration rules. However, I came to the conclusion that each of them, along with common ones, also has its own characteristics.
Let's start by setting up a ladder-type filter (Fig. 1).
Fig.1
Experience shows that:
The filter is obtained with the best parameters if all crystals have as close as possible successive resonance frequencies (±10 Hz). However, you should not be upset if this condition is not fulfilled, because a good filter is obtained even with a frequency spacing of up to 1 kHz;
It is best to select quartz 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 configured directly as part of the “native” device;
If quartzes have unequal frequencies, they should be placed in the following sequence: the highest frequency should be installed first at the input, and all subsequent ones - alternately from left to right, by rank, with a decrease in frequency;
Small-sized containers should be used, with a minimum temperature coefficient of capacity (TKE) with an accuracy of no worse than ±1.5%. But don’t despair if you don’t find any, because you will still have to select them during the setup process. 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 at 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. Typically, this is what several radio amateurs do (usually 4 people), each having one filter. The most experienced of them selects four sets of quartz of the same frequency, then quartz with the minimum. he keeps the scatter for himself, and gives the rest back to his friends (or vice versa?!). Generator quartz can also be used with somewhat less success.
At home, a quartz filter can be configured in three ways.
In the first case, you should use (in addition to the device being tuned) another transceiver with a digital scale as an auxiliary device, in the second case - a GSS (standard signal generator) and a frequency meter (with a limit frequency exceeding at least the lowest frequency of your device being tuned, for example 1.9 MHz). A frequency meter measures either the GSS frequency or the GPA frequency of the device under study.
In the third case, a quartz local oscillator is used for one of the operating frequencies (either the GSS or another transceiver without a digital scale), and the presence of a digital scale in the device being tuned is required.
In all three cases, an RF signal of the operating range is supplied to the input of the device being tuned. In the first two cases, the supplied frequency is slowly changed in the transparency band of the quartz filter, while taking S-meter readings in relative units, and recording them in the table every 200 Hz. Then, according to the table, graphs (frequency response) are constructed. 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, we obtain the frequency response - the amplitude-frequency characteristic of the new filter.
In the third case, everything is done in the same way, only the tuned device itself is adjusted 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 configured in the three above directions in order of priority.
At the first stage of tuning (rough tuning), you should obtain a filter bandwidth of up to 2.4 kHz by replacing the capacitors one by one, starting from the filter input, and removing the frequency response. Please keep the following in mind:
If you install additional capacitors parallel to the quartz (especially the outermost ones) and increase their nominal value (up to a certain limit), then the filter bandwidth will decrease. A similar effect will be observed when increasing the capacitance of the capacitors going to the housing. When the values of these capacities are reduced, the opposite effect will be observed. This property is used to narrow the bandwidth of a quartz filter in telegraph mode. In this way, 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 a CW filter, resonators with a quality factor at least an order of magnitude higher than the quality factor of the filter should be used);
The magnitude of the “humps” and dips in the upper part of the frequency response (linearity of the characteristic) will depend not only on the size of the selected capacitances, but also on the resistance value of the load resistors installed at the input and output of the filter. As their resistance decreases, the linearity of the characteristic improves, but the attenuation in the filter passband increases;
If it is impossible to obtain a sufficient steepness of the lower slope, quartz should be installed parallel to the load resistors, similar to those used in the filter, and from all available quartz, the lowest frequency should be selected or its frequency should be lowered 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 settings must be repeated several times. If at the last stage of tuning it is not possible to obtain an acceptable frequency response, you must try to adjust the frequency of the sequential resonance of individual quartzes. 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 happen if the difference between the frequencies of the parallel and series resonances of the quartz is small), the quartz should be replaced. The 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 CF receiver "TURBO-TEST", taken at different values of capacitor capacitances. -
Fig. 2 - For greater clarity, the frequency values are taken without respecting the received sideband and the actual IF value. Figure 3 shows the frequency response of the final filter setup. -
Fig.3
Now some practical tips for setting up a bridge quartz filter. Such a filter is shown in Fig. 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
If you have a filter for low frequencies (2...6 MHz), it usually turns out to be more narrowband than required, and if a filter for high frequencies (8...10 MHz) is too broadband. In the first case, the bandwidth should be expanded by connecting inductors to the upper or lower (Fig. 4) quartz, which should be selected experimentally. In the second case, in order to reduce the bandwidth, it is necessary to connect trimming 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 identical) by 2...3 kHz.
If only crystals of the same frequencies are available, you can change the frequency of the crystals by erasing the silver-plated layer from the crystal (increase the frequency) or by shading with a pencil (lower). 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 adjustment, it is advisable to replace the capacitor with a constant capacitance of the same size.
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 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. When manufacturing a bridge filter, great attention should be paid to the symmetry of its arms, as well as the windings of the transformers included in it, and, of course, careful shielding of the input from the output. You can read more about bridge filters in.
Literature
1. Goncharenko I. Ladder filters on unequal resonators. - Radio, 1992, No. 1, P. 18.
2. Bunin S.G., Yaylenko L.P. Shortwave Radio Amateur's Handbook. - K.: Technology, 1984, P.21...25.
Quartz filters "Desna"
Eight-crystal quartz filter “Desna”. Assembled, configured, without housing (shielded box). Quartz filter at a frequency of 8.865 MHz. The filter is assembled on a 75x19 mm printed circuit board. The kit includes 2 reference quartz (SSB, CW). Squareness coefficient at levels 6 and 60 dB – 1.5; attenuation beyond the passband more than 80 dB; unevenness in the passband no more than 3 dB; 6 dB bandwidth – 2.4 kHz; Rin and Rout. from 200 to 280 Ohm (indicated in the passport). It is possible to produce several CFs for one frequency with a spread of no more than 20 Hz.
Four-crystal quartz filter "Desna". Assembled, configured, without housing (shielded box). Quartz filter at a frequency of 8.865 MHz, Kp. 2.1; bandwidth 2.4 kHz. The kit includes 2 reference quartz (SSB, CW). The filter is assembled on a 35x19 mm printed circuit board. It is possible to produce several CFs for one frequency with a spread of no more than 20 Hz.
Four-crystal (cleanup) quartz filter “Desna”. Assembled, configured, without housing (shielded box). Manufactured at the frequency of the main HF. Possibility of changing the band from 2.7 to 0.7 kHz. The filter is made on a 30x15 mm printed circuit board. The kit includes 3 varicaps KV-127.
Radio amateur set "Desna"
The Desna set is intended for the manufacture of quartz filters: an eight-crystal main selection and a four-crystal eraser with a variable bandwidth (0.7 - 2.7 kHz) for devices with one frequency conversion used in amateur radio communications.
For the manufacture of quartz ladder filters, identical quartz resonators from PAL/SECAM television set-top boxes are used. As measurements have shown, these quartzes have a high quality factor, the resonant gap is about 12 - 15 kHz. The manufactured eight-crystal quartz filter from such resonators has the following parameters:
squareness coefficient at levels 6 and 60 dB ~ 1.6;
attenuation beyond the passband more than 80 dB;
unevenness in the passband – 1.5 - 2 dB;
6 dB bandwidth – 2.4 0.15 kHz;
input and output resistance - 20210 Ohm.
The set includes:
selected quartz resonators « NEW» (C = 5 Pf) – 12 pcs.;
quartz resonators of reference oscillators (marked – G) – 2 pcs.
Capacitor KM-12-15pF – 2 pcs. Capacitor KM-91pF – 2 pcs.
Capacitor KM-39pF – 2 pcs. Capacitor KM-110pF – 2 pcs.
Capacitor KM-47pF – 2 pcs. Capacitor KM-120pF – 2 pcs.
Capacitor KM-56pF – 2 pcs. Printed circuit board – 2 pcs.
Varicap KV-127A (B) – 3 pcs.
* Capacitor ratings are given for quartz resonators only of this “NEW” type.
P 
Conceptual diagrams of CF and PKF:
C1, C7-39pF, C2, C6-12-15pF, C3, C5-47pF, C4-91pF, C8, C11-120pF, C9, C10-110pF. C1, C3-56pF, C2-91pF.
Filters are implemented on printed circuit boards. One of the terminals (marked *) of the outer resonators, and in the PKF and all four, cannot be cut off on the boards; they will be the input/output of the KF and PKF, as well as for connecting additional capacitors in the PKF.
Connection diagram for a four-crystal cleanup filter
The quartz filter is, as we know, “half of a good transceiver”. This article presents a practical design of twelve basic selection crystal quartz filters for a high-quality transceiver and computer attachment, allowing you to configure this and any other narrow-band filters. In amateur designs, quartz eight-crystal ladder-type filters made on identical resonators have recently been used as the main selection filter. These filters are relatively simple to manufacture and do not require large material costs.
Computer programs have been written for their calculation and modeling. The characteristics of the filters fully satisfy the requirements for high-quality signal reception and transmission. However, with all the advantages, these filters also have a significant drawback - some asymmetry of the frequency response (flat low-frequency slope) and, accordingly, a low squareness coefficient.
The congestion of amateur radio broadcasts determines quite stringent requirements for the selectivity of a modern transceiver on an adjacent channel, therefore the main selection filter must provide attenuation outside the passband of no worse than 100 dB with a squareness factor of 1.5... 1.8 (at levels -6/-90 dB ).
Naturally, the losses and unevenness of the frequency response in the filter passband should be minimal. Guided by the recommendations set out in, a ten-crystal ladder filter with a Chebyshev characteristic with an uneven frequency response of 0.28 dB was chosen as the basis.
To increase the steepness of the slopes parallel to the input and output of the filter, additional circuits were introduced, consisting of series-connected quartz resonators and capacitors.
Calculations of the parameters of the resonators and filter were carried out according to the method described in. For a filter passband of 2.65 kHz, the initial values were obtained: C1,2 = 82.2 pF, Lkv = 0.0185 Hn, Rn = 224 Ohm. The filter circuit and the calculated values of the capacitor values are shown in Fig. 1.
The design uses quartz resonators for television PAL decoders at a frequency of 8.867 MHz, produced by VNIISIMS (Aleksandrov, Vladimir region). The stable repeatability of crystal parameters, their small dimensions and low cost played a 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 homemade self-oscillator and an industrial frequency meter. Resonators ZQ1 and ZQ12 for parallel circuits were selected from other batches of crystals with frequencies respectively lower and higher than the main filter frequency by approximately 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 where the resonators are installed are countersunk. The housings of all quartz resonators are connected to a common wire by soldering.
Before installing the parts, the filter circuit board is sealed in 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.
In Fig. Figure 3 shows the installation diagram of the filter. All capacitors in the filter are CD 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, followed by checking the measurement results by constructing the frequency response of the filter point by point using a selective microvoltmeter. As a designer of amateur radio equipment, I was very interested in the idea proposed by DG2XK to use a computer program for a low-frequency (20 Hz...22 kHz) spectrum analyzer to measure the frequency response of narrowband 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 a spectrum analyzer program installed makes it possible to view the frequency response of this filter on the display.
A Zener diode noise generator is used as a source of the DG2XK high-frequency signal. The experiments I carried out showed that such a signal source allows one to view the frequency response to a level of no more than 40 dB, which is clearly not enough for high-quality filter tuning. In order to view the frequency response of a filter at a level of -100 dB, the generator must have
the level of side noise 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 traditional sweep generator (Fig. 4). The basis is the circuit of a quartz oscillator, in which the relative spectral noise power density is equal to -165 dB/Hz. This means that the generator noise power at 10 kHz detuning in a 3 kHz bandwidth
less than the power of the main oscillation of the generator by 135 dB!The layout of the original source is slightly modified. So, instead of bipolar transistors, field-effect transistors are used, and a circuit consisting of inductor L1 and varicaps VD2-VD5 is connected in series with the quartz resonator ZQ1. The generator frequency is tunable relative to the quartz frequency within 5 kHz, which is quite sufficient for measuring the frequency response of a narrow-band filter.
The quartz resonator in the generator is similar to a filter. In the sweep frequency generator mode, the control voltage to the varicaps VD2-VD5 is supplied from a sawtooth voltage generator made on a unijunction transistor VT2 with a current generator on VT1.
To manually adjust the generator frequency, a multi-turn resistor R11 is used. The DA1 chip works as a voltage amplifier. The originally conceived sinusoidal control voltage had to be abandoned due to the uneven speed of passage of the frequency response of different sections of the frequency response of the filter, and 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 MFC is set by trimming resistor R10.
The schematic diagram of the detector block is shown in Fig. 5. The signal from the output of the quartz filter is supplied to input X2 if circuit L1C1C2 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 input X1, and the conductor connecting input X1 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 resistance of the mixer. The detector is made according to a passive balanced mixer circuit using 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 antiphase sinusoidal voltages of 3...4V (rms) from the reference generator. The detector's reference oscillator, made on the DD1 chip, contains a quartz resonator with a frequency of 8.862 MHz.
The low-frequency signal formed at the output of the mixer is amplified approximately 20 times by an amplifier on the DA1 chip. Since personal computer sound cards have a relatively low-impedance input, the detector is equipped with a powerful K157UD1 op-amp. The amplifier's frequency response is adjusted so that below 1 kHz and above 20 kHz there is a gain rolloff of approximately -6 dB per octave.
The swing frequency generator 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 parts that do not have contact with it are countersunk.
The board is sealed in a 40 mm high box with two removable covers. The box is made of tinned sheet metal. Inductors L1, L2, L3 are wound on standard frames with a diameter of 6.5 mm with carbonyl iron trimmers and placed in screens. L1 contains 40 turns of PEV-2 0.21 wire, L3 and L2 - respectively 27 and 2+4 turns of PELSHO-0.31 wire.
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 resistors 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 MCC begins with setting the maximum signal at the output of the sawtooth voltage generator. By monitoring the signal at pin 6 of the DA1 microcircuit with an oscilloscope, using trimming resistors R8 (gain) and R6 (offset) set the amplitude and shape of the signal shown on the diagram at point A. By selecting resistor R12, stable generation is achieved without entering the signal limiting mode.
By selecting the capacitance of capacitor C14 and adjusting the circuit L2L3, the output oscillatory system is tuned to resonance, which guarantees good load capacity of the generator. Using the L1 coil trimmer, the oscillator tuning limits are set within the range of 8.8586-8.8686 MHz, which overlaps the frequency response band of the quartz filter under test with a margin. To ensure maximum restructuring of the GKCH
(at least 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 block is made on a printed circuit board made of double-sided foil fiberglass (Fig. 7).
The top layer of foil is used as a common wire. The holes for the leads of parts that do not have contact with the common wire are countersinked.
The board is sealed in a tin box 35 mm high with removable covers. The resolution of the set-top box depends on the quality of its manufacture.
Coils L1 - L4 contain 32 turns of PEV-0.21 wire, wound turn to turn on frames with a diameter of 6 mm. Trimmers in coils from SB-12a armor cores. All chokes are type DM-0.1. Inductance L5 - 16 µH, L6, L8 - 68 µH, L7 - 40 µH. Transformer T1 is wound on a 1000NN ring ferrite magnetic core of standard size K10 x 6 x 3 mm and contains 7 turns in the primary winding, and 2 x 13 turns of PEV-0.31 wire in the secondary winding.
All trimming resistors are SPZ-38. During preliminary setup of the unit, a high-frequency oscilloscope is used to monitor the sinusoidal signal at the gates of transistors VT2, VT3 and, if necessary, adjust the coils L2, L3. By adjusting coil L4, the frequency of the reference oscillator is lowered below the filter passband by 5 kHz. This is done so that in the working area of the spectrum analyzer there is less interference that reduces the resolution of the device.
The sweeping frequency generator is connected to a quartz filter through a matching oscillatory circuit with a capacitive divider (Fig. 8).
During the tuning process, this will allow you to obtain low attenuation and unevenness in the filter passband.
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 (XZ connector) to the microphone or linear input of the sound card of the personal computer, we launch the spectrum analyzer program. There are several such programs. The author used the SpectraLab v.4.32.16 program, located at: http://cityradio.narod.ru/utilities.html. The program is easy to use and has great capabilities.
So, we launch the “SpektroLab” program and, by adjusting the frequencies of the MCG (in manual control mode) and the reference oscillator in the detector attachment, we set the peak of the spectrogram of the MCG at around 5 kHz. Next, by balancing the mixer of the detector attachment, the peak of the second harmonic is reduced to the noise level. After this, the GCH 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, using R11, adjusting the central frequency, and then the swing band R10 (Fig. 4), we establish an acceptable “picture” of the frequency response of the filter in real time. During measurements, by adjusting the matching circuits, we achieve minimal unevenness in the passband.
Next, to achieve the maximum resolution of the device, we turn on the sweep frequency of 0.3 Hz and set in the program 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).
Since the characteristic is drawn in several passes of the GKCh, the storage peak voltmeter mode (Hold) is turned on. As a result, we get the frequency response of the filter under study on the monitor.
Using the mouse cursor, we obtain the necessary digital values of the resulting frequency response at the required levels. In this case, you must not forget to measure the frequency of the reference oscillator in the detector attachment, in order to then obtain the true frequency values of the frequency response points.
Having assessed the initial “picture”, they adjust the frequencies of the sequential resonance ZQ1n ZQ12, respectively, to the lower and upper slopes of the filter’s frequency response, achieving maximum squareness at a level of - 90 dB.
In conclusion, using the printer, we obtain a full-fledged “document” for the manufactured filter. As an example in Fig. Figure 9 shows the spectrogram of the frequency response of this filter. A 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: level passband - 6 dB - 2.586 kHz, frequency response unevenness in the passband - less than 2 dB, level squareness factor - 6/-60 dB - 1.41; by levels - 6/-80 dB 1.59 and by levels - 6/-90 dB - 1.67; attenuation in the band is less than 3 dB, and attenuation beyond the band is 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 the measurements, a selective microvoltmeter with a good attenuator was required, which was a microvoltmeter type HMV-4 (Poland) with a nominal sensitivity of 0.5 μV (at the same time, it records signals well at a level of 0.05 μV) and an attenuator of 100 dB.
For this measurement option, the diagram shown in Fig. 1 was assembled. 10. The matching circuits at the input and output of the filter are carefully shielded. The connecting shielded wires are of good quality. The “earth” circuits are also carefully executed.
Smoothly changing the frequency of the high frequency frequency resistor R11 and switching the 10 dB attenuator, we take microvoltmeter readings, passing through the entire frequency response of the filter. Using measurement data and the same scale, we build a frequency response graph (Fig. 11).
Thanks to the high sensitivity of the microvoltmeter and low side noise of the GKCh, signals are well recorded at a level of -120 dB, which is clearly reflected in the graph.
The measurement results were as follows: level passband - 6 dB - 2.64 kHz; frequency response unevenness - less than 2 dB; the squareness coefficient for levels -6/-60 dB 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 is less than 3 dB, behind the band is more than 100 dB.
Some differences between the measurement results and the computer version are explained by the presence of accumulating errors in digital-to-analog conversion when the analyzed signal changes over a large dynamic range.
It should be noted that the above graphs of the frequency response of a quartz filter were obtained with a minimum amount of setup work and with a more careful selection of components, the characteristics of the filter can be significantly improved.
The proposed generator circuit can be successfully used to operate AGC and detectors. By applying a sweep frequency generator signal to the detector, at the output of the set-top box to the PC we receive a signal from a low-frequency sweep frequency generator, with which you can easily and quickly configure 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. Next, the received signal is amplified and fed to the detector input. 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. Quartz filter SSB. - Radio Amateur, 1992, No. 6, p. 39, 40.
2. Drozdov V.V. 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.