DIY DVB-T2 (UHF) antenna amplifier. A simple active filter for a two-way amplifier Adding a bandpass filter for a UHF amplifier
Make your own subwoofer filter
Making your own filter for a subwoofer is not as difficult as it seems at first glance. The decision to make it yourself does not come easily.
Sooner or later, all car audio enthusiasts become professionals and try in every way to improve the audio system. The simplest low-pass filter for a subwoofer and its manufacture will become one of the modernization solutions.
Purpose
Beyond the boundaries of the “native” band (effectively reproduced), the sound pressure coming from the speaker noticeably decreases and at the same time the level of distortion increases. In this case, it is simply stupid to talk about some kind of sound quality and, therefore, in order to solve the problem, you have to use several speakers in the audio system (see).
This is the reality: this happens in both home acoustics and car audio. That's not news.
Typical speaker layouts in cars and the role of filters
Regarding car acoustics, I would like to highlight two typical schemes for constructing a sound system, which are probably familiar to everyone who is not very familiar with car audio.
We are talking about the following schemes:
- The most popular scheme involves three speakers. This is a woofer (aimed exclusively at the lows), a midrange speaker and low frequencies(midbass) and tweeter responsible for HF reproduction.
Note. This scheme is used mostly by amateurs and in any car where the acoustic circuit, you can meet her.
- The following scheme is for more professionals and participants in car audio competitions. Here, a separate speaker is responsible for each of the frequency ranges.
Note. Despite the significant differences, both schemes obey the same rule: each speaker is responsible for reproducing its own frequency band and does not affect others.
It is precisely in order not to violate this requirement that electrical filters are designed, the role of which is to isolate specific “native” frequencies and suppress “foreign” ones.
Filter types
- A notch filter is the exact opposite of a bandpass filter. Here, the band that the PF passes through without changes is suppressed, and bands outside this interval are enhanced;
- FINCH or infra-low frequency suppression filter stands apart. The principle of its operation is based on the suppression of high frequencies with a low cutoff rate (10-30Hz). The purpose of this filter is to directly protect the bass player.
Note. The combination of several filters is called a crossover in acoustics.
Options
In addition to the types of filters, it is customary to separate their parameters.
For example, a parameter such as order indicates the number of coils and capacitors (reactive elements):
- 1st order contains only one element;
- 2nd order two elements, etc.
Another, no less important indicator is the slope of the frequency response, which shows how sharply the filter suppresses “alien” signals.
For subwoofer
In principle, any filter, including this one, is a combination of several elements. These components have the property of selectively transmitting signals of certain frequencies.
It is customary to separate three popular schemes for this separator for the bass player.
They are presented below:
- The first scheme involves the simplest separator (which is not difficult to make with your own hands). It is designed as an adder and uses one transistor.
Of course, serious sound quality cannot be achieved with such a simple filter, but due to its simplicity, it is perfect for amateurs and novice radio enthusiasts;
- The other two schemes are much more complex than the first. Elements built according to these circuits are placed between the signal output point and the input of the bass amplifier.
Whatever the separator, simple or complex, it must have the following technical characteristics.
A simple filter for a 2-way amplifier
This separator does not require any special setup and assembly is as easy as pie. It was performed using available op-amps.
Note. This filter circuit has one slight advantage over the others. It lies in the fact that when the low-frequency channel is overloaded, its distortions are well masked by the mid/high-frequency link and, therefore, the negative load on hearing is noticeably reduced.
Let's get started:
- We apply the input signal to the input of the operational amplifier MC1 (it performs the function of an active low-pass filter);
- We also feed the signal to the input of the MC2 amplifier (in this case, we are talking about a differential amplifier);
- We now apply the signal from the output of low-pass filter MS1 to the input of MS2.
Note. Thus, in MS2, the low-frequency part of the signal (input) is subtracted from the spectrum, and the high-frequency part of the signal appears at the output.
- We provide the specified cutoff frequency of the low-pass filter, which will become the crossover frequency.
The process of making a filter with your own hands will require familiarization with the thematic video review. In addition, it will be useful to study detailed photos - materials, diagrams, other instructions and much more.
The cost of making and installing a filter yourself is minimal, because there are practically no expenses required.
Ph.D. ROSOV Andrey Valentinovich
(LLC "Technical Center ZhAiS")
Today you can find a fairly large number of different antenna amplifiers on sale. If you look at their passports, everything looks quite convincing, and most importantly, they claim quite good characteristics. However, when it comes to the practical use of these “toys”, there is either no effect, or vice versa - the use of an amplifier only worsens the quality of the television image. The fact is that the development of a truly high-quality antenna amplifier is a rather serious matter and requires the simultaneous solution of many problems: minimizing the noise figure, ensuring the required gain in the operating frequency band for a given frequency response unevenness, the required dynamic range of the input signal, high temperature stability (in the case of , if the amplifier is directly located on the antenna (which is where it should be for normal and efficient operation), high manufacturability and repeatability of parameters, and many others.
So let's go back to the amplifier. In Fig. 1 shows it circuit diagram.
Rice. 1 Schematic diagram of the UHF antenna amplifier.
The elements C1, L1, C2 are equipped with a third-order high-pass filter (HPF), which has a cutoff frequency of 360...400 MHz. This high-pass filter performs the following functions: it ensures matching of the input impedance of the amplifier stage on VT1 with the characteristic impedance of the antenna, reduces the effective noise bandwidth of the amplifier and largely eliminates the effect of “clogging” the amplifier with powerful stations operating in the meter wavelength range. The amplifier consists of three amplification stages made of microwave transistors VT1...VT3, connected according to a circuit with an OE. Stabilization of operating modes of transistors by DC carried out through negative feedback(OOS) through resistors R1, R3, R5. This stabilization circuit allows the emitter terminals of the transistors to be directly grounded, which ensures a high stable gain of each stage. The load of each of the stages is the corresponding inductances (L2, L4, L6). The inductive nature of the load makes it possible to increase the cascade gain in the high-frequency region by compensating for the frequency dependence of the transistor transconductance. The high transmission coefficient of each stage is also achieved due to the elimination of negative feedback at high frequencies by installing blocking capacitors C4, C7, C10. The required amplitude-frequency response of the amplifier is formed by high-pass filter elements, inductances L2, L4, L6 and capacitors C5 and C8, which perform the function of coupling between stages. Capacitor C11 provides output matching.
The amplifier can be powered in two ways: either from a separate external power supply, or through a reduction cable from the corresponding supply voltages of the TV. The supply voltage must be within +8...16V. The amplification stages are directly powered from an external stabilizer with a voltage of +4.7V, made using a zener diode VD1 and a quenching resistor R7. All amplifier stages are isolated from each other via power circuits using filters L3C3, L5C5, as well as elements R2C4, R4C7, R6C10. All this allows us to ensure high stability of the main parameters of the amplifier under the influence of various destabilizing factors.
Diode VD2 prevents DC voltage from entering the input of the television receiver when using a separate power supply. The first stage of the amplifier (on transistor VT1) is optimized to minimize noise figure and its emitter current is 2...3 mA, which is achieved by appropriate selection of R1. The current consumption of the second and third cascades (on VT2 and VT3) is about 5...7 mA, which makes it possible to achieve maximum cascade gains. A typical amplifier frequency response is shown in Fig. 2.
Rice. 2 antenna amplifier frequency response
Structurally, the amplifier is made on a printed circuit board made of single-sided foil fiberglass laminate with dimensions of 48x60 mm (in microwave technology, standard sital substrates with the same dimensions were used) with a thickness of 1.5 mm. A distinctive feature of the printed circuit board is the installation of all attachments on it according to option U 1. b. (OST 4GO.010.030-81), i.e. from the side of the current-carrying tracks, which eliminates drilling holes in the board and increases the manufacturability of the amplifier as a whole in small-scale and mass production. High-frequency inductors are made by printing, which also makes it possible to improve the manufacturability of the amplifier and the stability of the parameters of these coils both within one amplifier and within a production batch. The developed amplifier topology allows you to completely get rid of tuning elements and achieve high repeatability of the main amplifier parameters from instance to instance. An amplifier assembled from known good parts immediately provides output characteristics after power is applied.
The circuit and topology of the amplifier allow the use of many microwave transistors (KT372, KT3115, etc.) that have the same pinout.
Rice. 3 PCB topology
Figure 3 shows the printed circuit board of the amplifier. The area marked in black is the tinned foil layer, white is the etched part. Board dimensions - 48x60mm. The printed circuit board in Fig. 3 is made on a scale of 1:1.
The arrangement of elements is shown in Fig. 4
Fig.4 Location of elements
The amplifier housing at home can be easily made from double-sided foil fiberglass laminate with a thickness of 1.5-2 mm.
In Fig. Figure 5 shows the appearance of such an amplifier (without the top cover).
Rice. 5 Appearance antenna amplifier. Rice. 6. Fragment of inductor L1
Now a little about the details. Resistors are the most affordable: either C2-33 or MLT-0.125. The only requirement is that during installation the resistor leads should be as short as possible. Blocking capacitors are preferably unframed ones (they take up less space. Well, if you don’t have them at hand, use the ones you have. Just make your conclusions shorter!). There are quite a wide variety of them now available. Capacitors C1, C2, C5, C8, C11 are high-frequency, and their capacitance should be exactly the same as indicated on the circuit diagram. Inductor L1 - 3-4 turns of PEV -1.0 wire. The inner diameter of the winding is 4 mm. Chokes L3, L5 - either standard type DM-0.1, for example, with an inductance of 50 μH, or 18-20 turns of PEV-0.1 wire with the same internal winding diameter as L1. After installation, you need to check the functionality of the amplifier (if you did everything correctly and used known-good radio components, then there will be no problems). To do this, it is necessary to measure the voltage drop across resistors R2, R4, R6, and then, using the well-known Ohm’s law, calculate the collector current of transistors VT1...VT3. If they correspond to the numbers indicated above, then everything is fine and you can safely solder the top cover to your amplifier, thereby ensuring its complete tightness.
UHF television reception has a number of features:
1. UHF practically does not bend around the earth's surface and have low penetrating power, so the area of reliable reception is limited to the direct line of sight between the transmitting and receiving antennas.
2. At the same time, the UHF is well reflected from the earth's surface and from the ionized layers of the atmosphere. This makes reception possible at a considerable distance (300-500 km) from the television center. At the same time, the passage of UHF is quite stable and does not have fading characteristic of meter waves (MB).
3. Characteristic feature UHF is the so-called wave propagation, in which the signal can be received at a distance of up to several thousand km from the television center. It occurs over the sea surface on clear days in the spring and summer months.
4. UHF receiving antennas have significantly smaller geometric dimensions than MB antennas. At the same time, their effective area is small, and therefore the signal power supplied to the input of the television receiver is small.
5. The sensitivity of television receivers in the UHF range is significantly lower than in the MB range, which is due to the poor noise parameters of the UHF selector.
Analysis of the listed features shows the fundamental possibility of long-range and ultra-long-range television reception in the UHF range and two main ways of its implementation. This is an increase in the efficiency of the antenna system and the real (noise-limited) sensitivity of the television receiver. The possibilities of increasing the gain of UHF antennas in practice are limited by the complexity of their design and coordination with the feeder.
Increasing the sensitivity of a television receiver requires altering the UHF selector and usually does not give the desired results. The fact is that in the UHF range the signal attenuation in the cable is high, and when using antennas with low gain it is not possible to obtain a significant gain in the signal-to-noise ratio at the input of the television receiver.
The most optimal way is to use a structurally simple antenna with an amplifier located in close proximity to it. In this case, it is possible to simultaneously increase the efficiency of the antenna and the sensitivity of the television receiver without modifying it.
Antenna amplifier must have a high gain, low noise figure, and a wide range of operating temperatures. At the same time, it should be simple in design, assembled from available parts, easy to set up and not prone to self-excitation.
As a result of many years of theoretical and experimental research, we were able to create an optimal circuit and design for the UHF amplifier according to the listed requirements. has no industrial or amateur analogues
1. UHF antenna amplifier.
Amplifier parameters and circuit:
The amplifier has the following parameters:
Gain coefficient Ku and noise figure Fsh in the range
470-630 MHz (21-40 channels) - Ku? 30 dB, Fsh? 2.0 dB;
630-790 MHz (41-60 channels) - Ku? 25 dB, Fsh? 2.5 dB;
790-1270 MHz (61-100 channels) - Ku? 15 dB, Fsh? 3.5 dB.
Input and output impedance - 75 Ohm
- supply voltage - 9-12 V
- operating temperature range - (-30...+40) °C.
The amplifier circuit is shown in Fig. 1. It contains two cascades on transistors VT1 and VT2, connected according to a circuit with a common emitter. To obtain maximum gain, the emitters of the transistors are connected directly to the common wire. The loads of the cascades are broadband circuits L2, R2, L3, C4 and L4, R6, L5, C10, which ensure matching of their input and output impedances. Circuit L1, C1 is a high-pass filter (cutoff frequency 400 MHz), which is used to eliminate interference from MB band TV transmitters. Capacitors SZ, C5, C7, C8 are blocking. The amplifier is powered via a coaxial cable connecting it to the TV, through a low-pass filter L6, R8, C11. Directly in front of the TV, the UHF signal and the supply voltage are separated by filter C12, L7, C13.
The DC modes of the transistors are set by resistors R1 and R5 so as to obtain the optimal values of the collector currents I1 and I2 of transistors VT1 and VT2. Current I1 is selected from the condition of obtaining the minimum noise figure of the first stage, and I2 - from the condition of obtaining the maximum gain of the second stage.
Amplifier parts and design.
All amplifier resistors are MLT-0.125. Capacitors C1, C2, C4-C7, C9, C10 - small-sized disk capacitors (types KD, KD-1, etc.); SZ, S8 and S11 - type KM-5b, KM-6, etc.
All amplifier coils are frameless. Coil L1 contains 2.75 turns of silver-plated wire with a diameter of 0.4-0.8 mm, its outer diameter is 4 mm, the interturn distance is 0.5 mm. Coils L2-L5 are the leads of resistors R2 and R5, wound on a mandrel with a diameter of 1.5 mm, so that the interturn distance is 0.5 mm, and contains 1.5 turns each. The directions of windings L2, L3 and L4, L5 must be the same (i.e., for example, L2 and L3 are a coil of 3 turns, in the gap of which resistor R2 is connected). Coil L6 contains 15-20 turns of enameled copper wire with a diameter of 0.3 mm, wound turn to turn on a mandrel with a diameter of 3 mm. Choke L7 is a standard type DM-0.1 with an inductance of more than 20 μH. Zener diode VD1 - any with a stabilization voltage of 5.5-7.5 V.
The amplifier can use microwave low-noise transistors with cutoff frequency fgr. more than 2 GHz. If the amplifier operates in the range of 21-60 channels, then transistors with fgr can be used. more than GHz, and if - only in the range of 21-40 channels, then - with fgr. more than 800 MHz. in this case, it is necessary to install a transistor with a lower noise figure in the first stage, and in the second - with a higher gain. In table 1 shows the parameters of transistors that can be used in the amplifier. Transistors are arranged in order of worsening parameters.
It is not recommended to use transistors KT372 due to their tendency to self-excitation and GT346 due to poor noise parameters. If used p-p-p transistors, then it is necessary to change the polarity of the amplifier's power supply.
The amplifier is assembled on a printed circuit board made of foil fiberglass laminate with a thickness of 1-1.5 mm. A drawing of the printed circuit board and a diagram of the installation of parts on it are shown in Fig. 2. The board is designed to use transistors with planar leads (KT3132, KT3101, KT391, etc.), which are soldered directly to the contact pads on the foil side. However, it also allows the installation of transistors with a different pin arrangement (KT399, KT3128, etc.), but from the installation side, for which it is necessary to drill the corresponding holes in the board for the pins (see below).
The transistor leads must have a minimum length, especially the emitter lead, which should not exceed 4 mm. The terminals of capacitors C4, C5, C7 and C10 should be no more than 4 mm, and capacitors C1, C2, C6 and C9 should be 4-6 mm (they are additional inductances included in the circuits). Some of the terminals of capacitors C1 and C2 are soldered into the board, while others are soldered directly to the central core of the input coaxial cable. Capacitors C6 and C9 are soldered at one end to the heads of resistors R2 and R6, cleared of paint. The other end of C6 is in the board, and C9 is soldered to the central core of the output coaxial cable. Capacitor C2 is soldered at one end to the board, and at the other end it is soldered to coil L1 at a distance of 3/4 of a turn from the top end according to the diagram. Resistors R3, R4, R7 and R8 are installed vertically.
The printed circuit board is placed in a rectangular sealed case, divided into 4 parts by shielding partitions (Fig. 2, 4). Drawings of the housing parts are shown in Fig. 3. It consists of a side wall 1, a sleeve 2, a partition 3, 4 and covers 5. Parts 1, 3, 4 and 5 are made of sheet brass (it is convenient to use a photo-glazing plate annealed over a gas burner), parts 2 are machined from a brass rod. Bushings 2 are designed so that the input and output of the amplifier are made of a 75-ohm coaxial cable with an outer insulation diameter of 4 mm. You can use another 75-ohm cable, but in this case it is necessary to change the diameters of bushings 2 and holes in the housing wall 1 accordingly.
The power supply filter L7, C12, C13 is mounted in a separate box of any design, on which the input antenna socket and output antenna plug are installed.
The amplifier can be powered from any stabilized 9-12 V source, for example, from commercially available power supplies for transistor receivers BP9V, D2-15, etc.
You can also mount filter elements inside the TV next to the UHF antenna input, and use 12 V voltage from the UHF selector to power the amplifier.
Installation and adjustment of the amplifier.
The amplifier is assembled in the following sequence. Mount all elements on the board except resistors R1 and R5. If transistors with non-planar terminals are used, then holes are drilled in the board for them, and rectangular cutouts are made in partitions 4 (shown with a dashed line in Fig. 3). The partitions 3 and 4 are soldered into the board with the corresponding protrusions. The side wall of the housing 1 is bent and soldered. The bushing 2 is hermetically sealed into it. Input 7 and output 8 coaxial cables 80 cm long are inserted into the holes of the bushings, the braid is divided into 2 parts and soldered to the housing from the inside. The central core of the cables should protrude 3-4 mm into the housing. Insert the board into the case so that the edges of the partitions 3, 4 and the edge of the wall 1 lie in the same plane (Fig. 4), and solder the joints of the partitions between themselves and the case. In addition, the odd board is soldered to wall 1 at 10 points. The soldering locations are shown in Fig. 2 and fig. 4. Elements C1, L1 and C9, L6 are soldered to the central cores of the cables. Check the rice carefully. 1, 2 and 4 correct installation.
Next, the amplifier is configured. To do this, power is supplied to the amplifier via output cable 8. By measuring the voltage U1 on resistor R3 by selecting resistor R1, set the value of current I1 (I1 = U1/R3) in accordance with table. 1 for the transistor of the first stage. Solder the selected resistor R1 into the board. A similar procedure is performed for the second stage, measuring the voltage U2 across resistor R7 and setting the current I2 = U2/R7 in accordance with table. 1. Solder in resistor R5. In Fig. 1, the values of R1 and R5 are approximate; in reality, they may differ significantly from those indicated. Check the absence of self-excitation of the amplifier. To do this, connect a voltmeter in parallel with R3 and touch the collector output of transistor VT1 with your finger. If the first stage is not excited, the voltmeter reading will not change. The second cascade is checked in the same way. Self-excitation can be eliminated (its presence is indicated by a sharp decrease in the transistor current when it is touched with a finger) only by replacing the transistor. It should be noted that the amplifier is not prone to self-excitation - out of several dozen amplifiers manufactured, only one, assembled on KT372A transistors, was excited. Check the current consumed by the amplifier, which should be equal to: I1 + I2 = 10 mA; if necessary, select resistor R8 so that the current through the zener diode VD1 is about 10 mA. The final operation is to seal the amplifier. To do this, covers 5 are soldered around the perimeter of the case, and the places where the coaxial cable is inserted are additionally coated with some kind of sealant, waterproof glue, etc. The amplifier is then attached to the antenna mast.
2. UHF antenna
As mentioned above, it makes no sense to achieve a very high gain of the UHF antenna, since this leads to unjustified complication of its design. However, you can’t count on long-range reception with an ineffective antenna either.
Experience in the design and use of UHF antennas shows that the simplest and at the same time very effective is the Z-antenna with a reflector. Her distinctive features is wide-bandwidth, high gain, good matching directly with a 75-ohm coaxial cable and non-critical dimensions.
The antenna design for 21-60 channels is shown in Fig. 5. If the antenna will be used in the range of 61-100 channels, then all its dimensions must be reduced by 1.5 times. The active canvas 1 of the antenna is made of aluminum strips and is fastened “overlapping” with screws and nuts. There must be reliable electrical contact at the points of contact of the plates. At match 6 (it can be metal or wood), the canvas is fixed with the help of support posts 2 at points C and D. Since these points have zero potential relative to the ground, the posts 2 can be metal. Cable 3 is connected to points A and B (the braid to one point, and the core to the other) and is laid along the fabric along the lower post 2 and along match 6 to amplifier 7. The cable is secured with wire clamps. The web 1 can itself be used as an antenna. Its gain is 6-8 dB. However, it is better to equip the canvas with a reflector.
The simplest reflector 4 (Fig. 5b) is a flat screen made of tubes or pieces of thick wire. The diameter of the reflector elements is not critical and can be 3-10 mm. An antenna with a flat reflector has a gain of 8-10 dB. The gain factor can be increased to 15 dB (equivalent to a 40-element “wave channel” antenna) using a complex “dilapidated box” type reflector (Fig. 5c). The design of such a reflector can be very different, depending on your capabilities.
The spatial orientation of the antenna, shown in Fig. 5 corresponds to receiving signals with horizontal polarization. To receive vertically polarized signals, the blade and reflector must be rotated 90°.
The UHF amplifier is located in close proximity to the antenna (see Fig. 5). The amplifier input is connected to the antenna surface using the same cable that is embedded in the amplifier. The amplifier input cable is extended with a reduction cable. It is desirable that it be as large in diameter as possible (losses in the cable depend on this); a cable with a diameter of 4 mm can be used only if its length does not exceed 10 m.
Cable connections should be made “vetically”, so that the coaxial structure of the feeder is minimally disrupted.
If it is not possible to produce the antenna described, then the amplifier can be used with slightly worse results with industrial outdoor broadband UHF antennas, for example, type ATNG(V)-5.2.21-41 (trade name “GAMMA-1”).
Antenna installation is determined by what type of UHF transmission you are counting on. If it is necessary to receive reception directly outside the service area of the television center (60-200 km), then the antenna should be installed so that in the direction of arrival of the signals there are no obstacles between it and the horizon line (houses, hills, etc.). If you are focusing on ultra-long-range reception with tropospheric or wave propagation (in this case, the signal comes “from the sky” at an angle of 5-10° to the horizon), then obstacles that are not very close are usually not an obstacle.
CONCLUSION
In conclusion, a few words about the practical results of taking UHF. An antenna with an amplifier manufactured according to the attached description was used for several years in Odessa for regular reception of signals from the Chisinau television center (distance - 160 km). Outside the city, in the radio shadow zone for the MB television center, signals from low-power UHF repeaters located on the opposite side of the Odessa Bay (distance - 60-80 km) are confidently received. On clear days in the spring and summer months, the Bulgarian program BT2 from Varna (distance - 500 km) and the Turkish program TV2 from Istanbul (distance more than 600 km) are received with good quality.
It was already noted above that installing an antenna amplifier near the TV between the feeder and the antenna input of the television receiver increases the gain of the receiving path, i.e. improves the sensitivity limited by the gain.
It has been shown that when using modern televisions, this method does not lead to improved images in long-distance reception conditions, since it requires an improvement in sensitivity, which is limited not by gain, but by noise. The antenna amplifier, having approximately the same noise level as a television receiver, does not improve noise-limited sensitivity.
Nevertheless, the use of an antenna amplifier in some cases makes it possible to improve reception, but for this it should be installed not near the TV, but near the antenna, on the mast between the antenna and the feeder, or in the feeder gap, in the immediate vicinity of the antenna. What's the difference?
The fact is that the signal, passing to the feeder, undergoes attenuation and its level decreases. Attenuation depends on the brand of cable from which the feeder is made. In addition, the greater the attenuation, the greater the length of the feeder and the greater the frequency of the signal, i.e., the number of the channel through which the transmission is received.
When an antenna amplifier is installed near the TV, its input receives a signal that has already been weakened by passing through the feeder, and the ratio of the signal level to the noise level at the input of the antenna amplifier is less than if the antenna amplifier was installed near the antenna when the signal is not attenuated by the feeder. In this case, of course, passing through the feeder, the signal is also weakened, but by the same amount. noise is also reduced. As a result, the signal to noise ratio does not deteriorate.
TV cables different brands characterized by the dependence of specific attenuation on frequency. The specific attenuation of a coaxial cable is usually called the attenuation that a signal of a certain frequency experiences when passing through a cable 1 m long.
Specific attenuation is measured in dB/m and is given in reference books in the form of graphical dependences of specific attenuation on frequency or in the form of tables. In Fig. 1 shows such curves for some brands of 75-ohm coaxial cable.
Using them, you can calculate the signal attenuation in a cable for a certain length, on any frequency channel in the meter or decimeter range. To do this, you need to multiply the specific attenuation value obtained from the figure by the length of the feeder, expressed in meters. The result is signal attenuation in decibels.
Rice. 1. Specific attenuation curves of coaxial cables.
The most common type of cable for the feeder is RK 75-4-11, its specific attenuation is 0.05...0.08 dB/m in the range of channels 1-5, 0.12...0.15 dB/m in the range of 6-12 channels and 0.25...0.37 dB/m in the range of 21-69 channels. Hence, with a feeder length of 20 m, the signal attenuation in the feeder on the 12th channel will be only 3 dB, which corresponds to a decrease in signal voltage by 1.41 times, and with a feeder length of 50 m, the attenuation on the 12th channel will be 7.5 dB (decrease I 2.38 times).
In the decimeter range, with a feeder length of 20 m, the attenuation will be equal to 5.0...7.4 dB V, depending on the channel number, which corresponds to a decrease in signal voltage1 by 3.78...2.34 times^, and with a length feeder 50 m - 12.5... 18.5 dB (signal reduction by 4.22...8.41 times).
Thus, with a feeder length of 50 m, given to channel 12, the signal passing through the feeder is reduced by more than half, and the signal-to-noise ratio at the TV input will also be reduced by more than half. If you install an antenna amplifier before the signal enters the feeder, at the same level of input noise of the antenna amplifier as the TV, the gain in signal-to-noise ratio will be more than doubled.
An even more significant gain will be obtained with a longer feeder length or when receiving a signal in the UHF range. The necessary and sufficient gain of the antenna amplifier must be equal to the signal attenuation in the feeder. There is no point in using antenna amplifiers with a gain greater than required.
Several types of antenna amplifiers are available. The most widely used antenna amplifiers for the meter range are the UTDI-1-Sh type (individual range television amplifier for frequencies of ranges 1-1II).
They are designed for all 12 channels" of the meter range and contain a built-in power supply from an AC mains voltage of 220 V. The design of the amplifier allows it to be installed on a mast near the antenna with power supply through the feeder without laying additional wires. The gain of the UTDI-1-Sh amplifier is no less than 12 dB (4 times the voltage), and its noise level is slightly lower than the noise floor of black-and-white and color television receivers.
If the UTDI-1-III amplifiers are band and are designed to amplify a television signal on any of the 12 channels of the meter range, then antenna amplifiers of the UTKTI type (individual channel transistor television amplifier) are single-channel and are designed to amplify the signal of only one, very specific frequency channel of the meter range.
The channel number is indicated after the amplifier type designation. Thus, UTKTI-1 means that the amplifier is designed to amplify the signal on the first frequency channel, and UTKTI-8 is designed to amplify the signal on the eighth channel. Amplifiers of the UKTI type also have a built-in power supply from an alternating current network with a voltage of 220 V.
The gain of UTKTI-1 - UTKTI-5 is not less than 15 dB, and UTKTI-6 - UTKTI-12 is not less than 12 dB. The self-noise level of amplifiers of this type is somewhat lower than that of the UTDI-1-Sh type. The power consumed from the alternating current network UTDI-1-Sh does not exceed 7 W, and UTKTI - 4 W.
Due to the fact that television broadcasting in the UHF range is now becoming increasingly widespread, and the signal attenuation in the feeder in this range is increased, the use of antenna amplifiers designed for this range is becoming relevant. For example, an amplifier type UTAI-21-41 (individual television antenna amplifier, designed for 21-41 channels) with a gain of at least 14 dB in the frequency range 470...638 MHz.
Previously, despite the production of industrial antenna amplifiers, in the magazines "Radio" and in the collections "To Help the Radio Amateur" a large number of descriptions and diagrams of antenna amplifiers for self-production were given, B last years Such publications have become rare. So, in the collection “To help the radio amateur,” issue 101, p. 24-31 provides a very detailed description of a narrowband antenna amplifier with a tunable amplitude-frequency response by O. Prystaiko and Yu.
Pozdnyakova. The amplifier is tuned to one of the channels of the meter range using a tuning capacitor, the amplifier's bandwidth is 8 MHz, and the gain is 22...24 dB. The amplifier is powered by a constant voltage of 12 V. It makes sense to use such an amplifier only in the case when transmissions are received via one specific channel, since it is not possible to rebuild the amplifier installed on the mast.
Wideband antenna amplifier MV
Much more often there is a need for a broadband antenna amplifier that can amplify the signals of all television programs received by the antenna. In Fig. 2 shown circuit diagram of antenna amplifier, designed to amplify all 12 meter channels, developed by I. Nechaev.
Rice. 2. MV antenna amplifier circuit.
At a voltage of 12 V, the gain is 25 dB with a current consumption of 18 mA. The amplifier is assembled using low-noise transistors with a noise figure of about 3 dB. Back-to-back diodes connected at the input protect the amplifier transistors from damage by lightning discharges. Both cascades are assembled according to a common emitter circuit.
Capacitor C6 provides correction of the frequency response of the amplifier in the higher frequencies.
The output of the amplifier is connected to the feeder going to the TV. The central core of this part of the feeder supplies the amplifier with supply voltage through inductor N. Through the same inductor, a voltage of +12 V is supplied to the central conductor of the antenna socket of the TV. The signal from the antenna socket on the TV to the input of the channel selector must be supplied through an isolation capacitor with a capacity of 3000 pF.
The chokes are wound on ferrite cylindrical cores with a diameter of 3 mm and a length of 10 mm using PEL or PEV wire with a diameter of 0.2 mm turn to turn. Each inductor contains 20 turns. Before winding, the core must be wrapped in two layers of lavsan film, and after winding, the turns are secured with polystyrene varnish or enamel.
A more detailed description of the amplifier, a drawing of the printed circuit board and the placement of parts on it are given in the magazine "Radio", 1992, No. 6, p. 38-39.
Another antenna amplifier, designed for the UHF range 470...790 MHz (21...60 channels), was proposed by A. Komok. Its circuit diagram is shown in. rice. 3. The passband gain of this amplifier is 30dB when powered at 12V, and the current consumption does not exceed 12mA.
Rice. 3. UHF antenna amplifier circuit.
The high-pass filter coil L1 is wound with PEV-2 wire with a diameter of 0.8 mm and contains 2.5 turns.
Winding is carried out on a mandrel with a diameter of 4 mm turn to turn, after which the coil is removed from the mandrel. Power, as for the Nechaev amplifier, is supplied through the feeder through the chokes of the design described above. The author used unpackaged transistors in the amplifier, which require careful sealing.
We can also recommend the use of KT399A packaged transistors, which are more affordable and resistant to changes in climatic conditions. Detailed description of this amplifier was published in the magazine "Radio Amateur 11, 1993, No. 5, p. 2.
As noted, the main purpose of the antenna amplifier is to compensate for signal attenuation in the feeder. When using an antenna amplifier, noise-limited sensitivity, i.e., the ability to receive a weak signal, is determined by the signal-to-noise ratio not at the input of the television receiver, but at the input of the antenna amplifier. Therefore, when installing an antenna amplifier near an antenna, to obtain a certain sensitivity value limited by noise, a lower input signal level will be required than when installing it near a TV. Thus, it is possible to receive a weaker signal with better quality.
Application of antenna amplifier allows the deliberate use of feeders of such great length that, in the absence of an amplifier, would weaken the signal level to an unacceptable level. The need to use a long feeder sometimes arises in closed areas, when the television receiver is located in a hollow and the receiving antenna installed near the house is obscured by hills on the way to the transmitter.
At the same time, television antennas installed at a distance of 100...200 m from this building provide quite reliable reception with good image quality due to the fact that they are not covered by a local obstacle. In such conditions, normal reception can be achieved in one of two ways: either by increasing the height of the antenna mast, which is usually a very difficult task, or by installing the antenna in an open area, at a distance of 100...200 m from the house. Then to connect the antenna to the television receiver you will need to use a long feeder.
It is easy to calculate that with a feeder length of 200 m, a cable of the RK 75-4-11 brand at the frequency of the 12th channel creates an attenuation of 30 dB, which corresponds to a decrease in signal voltage by 31.6 times, which, as a rule, is below the sensitivity threshold of a television receiver . Installing an antenna amplifier with at least the same gain at the antenna output will compensate for signal attenuation in a long feeder and ensure normal operation of the TV.
If the gain of one amplifier is not enough, you can connect two amplifiers in series one after the other. In this case, the resulting gain will be equal to the sum of the gains of the amplifiers, if they are expressed in decibels.
If the feeder length is very long and the signal needs to be amplified by more than 30 dB, when it is necessary to use two or more antenna amplifiers, in order to avoid overload or self-excitation, all amplifiers should not be installed in one place. Under these conditions, the first amplifier is installed at the antenna output, i.e., at the input of the feeder, and the subsequent ones are installed in the feeder gap at approximately equal distances from one another. These distances are chosen so that the signal attenuation in the feeder section between the two amplifiers is approximately equal to the gain of the amplifier.
From the dependences of specific attenuation on frequency for coaxial cables of different brands (Fig. 1), certain conclusions can be drawn. Cables of brands RK 75-2-13 and RK 75-2-21 have a fairly high specific attenuation even in the meter wavelength range; they should not be used in the decimeter wavelength range. Cables of brands RK 75-7-15, RK 75-9-13, RK 75-13-11 and RK 75-17-17 have lower specific attenuation compared to RK 75-4-11, especially in the decimeter range.
If, with a feeder length of 50 m at a frequency of 620 MHz (channel 39), the RK 75-4-11 cable introduces an attenuation of 16 dB (attenuation of the signal voltage by 6.3 times), then under the same conditions the RK 75-9 cable -13 introduces an attenuation of 9.5 dB (attenuation by 3 times), and RK 75-13-1.1 - 7.25 dB (attenuation by 2.3 times). Thus, a successful choice of cable brand for a feeder in the UHF range can increase the signal level at the TV input several times, even without using an antenna amplifier.
We can offer fairly simple advice on cable selection: the larger the cable diameter, the less attenuation it introduces. A coaxial cable with a characteristic impedance of 75 Ohms is always used as a television feeder.
Nikitin V.A., Sokolov B.B., Shcherbakov V.B. - 100 and one antenna designs.
The article will talk about active filter For two-way amplifier. The filter does not require time-consuming setup and is made using available op-amps.
The first time I assembled this circuit was about 10 years ago, I needed to pump up the speakers Radio engineering S90 not very powerful homemade amplifier(Watts 25-30 offhand), the goal is to find out what these speakers are generally capable of.
But the amplifier power was clearly not enough. And in one interesting book I came across a diagram of this filter. I decided to try to power up the S90 with a two-way amplifier.
One of the advantages is that when the low-frequency channel is overloaded, its distortions are well masked by the mid-high frequency link, therefore the maximum undistorted power to the ear becomes noticeably greater.
In the end, I managed to swing one column so much that the slate on the garage began to crack.
Scheme
Pay
The input signal is fed to the non-inverting input of the operational amplifier MC1, which serves as an active low-pass filter with a frequency response slope of 18 dB/octave, and to the non-inverting input of the operational amplifier MC2, which functions as a differential amplifier with a voltage gain Ku=1.The inverting input MS2 is supplied with a signal from the output of the low-pass filter MS1. In the differential amplifier MC2, its low-frequency part is subtracted from the spectrum of the input signal, and only the high-frequency part of the input signal appears at the output of MC2.
Thus, you only need to provide a given cutoff frequency of the low-pass filter, which will be the crossover frequency. The values of the filter elements are found from the relations C1 = C2 = C3; R1=R4; R5=R1/6.8; R1C1=0.4/Fp, where Fp is the crossover frequency.
I took R1 22 kOhm, and then everything is calculated using formulas depending on the required crossover frequency.
As operational amplifiers I tried K157UD2 (dual op-amp - 2 housings) and K1401UD2 (quadruple op-amp - signet for it), both showed good results.
Of course, you can use any quad imported op-amp.
Source
Book "High-quality low-frequency amplifier", G.L. Levinzon, A.V. Loginov, 1977Files
Attached is a drawing of the printed circuit board for K1401UD2, with a jumper under the chip.▼ 🕗 08/10/11 ⚖️ 6.41 Kb ⇣ 420