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 with a not very powerful homemade amplifier (Watt 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.
As a result, 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 acts as an active filter low frequencies with a frequency response slope of 18 dB/octave, and to the non-inverting input of the operational amplifier MC2, which serves 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. printed circuit board for K1401UD2, there is a jumper under the chip.▼ 🕗 08/10/11 ⚖️ 6.41 Kb ⇣ 420
Selective antenna amplifiers UHF
When receiving television signals in the UHF range, many TV owners are forced to use several different antennas, which can sometimes give rise to specific problems associated with summing the signals. Antenna amplifiers will help solve them, providing not only signal amplification, but also their filtering.
One of the problems that TV viewers have to deal with when watching television programs is the need to receive signals from different directions and at different levels. This forces them to use two or more directional antennas, and if the signal level is low - active antennas or antenna amplifiers, they have to turn on adders or TV signal splitters. Unfortunately, all this often does not provide the desired quality of reception. The reason for this does not necessarily lie in a bad feeder or unsuccessful coordination. If, for example, you have several antennas designed to operate in the same range, then reception of the same signal, especially a powerful one, will be possible with two or more antennas. However, in this case, due to different signal propagation times in the feeders, multiple contours or blurred images appear, although the signal level is quite sufficient for high-quality reception.
This disadvantage can be eliminated by using bandpass filters or selective amplifiers, which isolate one or more signals received by one of the antennas and suppress interfering ones. And so - after each antenna, while filtering different channels. Then all signals are summed. For the MB range, this problem is solved using amplifiers and filters discussed in. There are almost no descriptions of such structures for the UHF range. Therefore, options for selective amplifiers specifically for the UHF range are described here.
However, you should pay attention to the fact that the use of filters is not always advisable (although acceptable). The fact is that, firstly, filters introduce attenuation, and when receiving weak signals this can affect the image quality. Secondly, the frequency response of filters, especially narrowband ones, significantly depends on their coordination with the connecting cables. Therefore, even small changes in load resistance can greatly change the frequency response and reduce the quality of reception. To eliminate this undesirable effect, amplification stages must be installed at the input and output of the filter.
The schematic diagram of a selective amplifier for isolating one or more closely spaced signals is shown in Fig. 1.
The device uses a bandpass filter consisting of two connected circuits L2C7 and L3C9. At the input of the filter there is an amplification stage on transistor VT1, and at the output there are two stages on transistors VT2 and VT3. Overall coefficient the gain reaches 20...23 dB, and the passband is determined by a bandpass filter.
The signals received by the antenna are fed to the C1L1C2 filter, which suppresses signals with a frequency of less than 450 MHz. Diodes VD1, VD2 protect transistor VT1 from powerful signals and electrical interference from lightning discharges. From the input stage the signal passes to the first circuit L2C7. To obtain the required quality factor, partial switching is applied (to the tap of coil L2). For communication with circuit L3C9, capacitor C8 is included (capacitive coupling). The signal from part of the turns of coil L3 comes to the base of transistor VT2, and after amplification - to the base of transistor VT3. Frequency response of the output amplifier for the purpose additional increase its selectivity can be adjusted by adjusting the L4C11 circuit in the feedback circuit.
Diodes VD3, VD4 protect the amplifier from electrical discharges from the TV. They may arise due to the fact that pulse block The power supply of modern devices is connected to a 220 V network through small capacitors. The amplifier is powered by a stabilized voltage source of 12 V and consumes a current of about 25 mA. The VD5 diode will protect the amplifier when a power source is connected to it in the wrong polarity. If it is planned to be powered via a separate wire, then the voltage is supplied directly to the diode VD5, and if via a reduction cable, decoupling elements L5, C16 are introduced into the amplifier.
All amplifier parts are placed on one side of the printed circuit board made of double-sided foil fiberglass, shown in Fig. 2.
The second side of the board is left almost completely metallized. There are only cut out areas for the input, output and supply voltage (they are shown in the figure with a dashed line). The metallization of both sides is connected to one another along the contour of the board with soldered foil. After setting up the amplifier, the board is covered from the parts side with a metal cover and soldered to it.
The amplifier can use KT382A.B transistors, and if high sensitivity is not required, KT371A is also suitable; diodes KD510A, KD521A.
Capacitors C7, C9, C11 - KT4-25, the rest - K10-17, KM, KLS; resistors - MLT, S2-10, S2-33, R1-4. The leads of all parts must be of a minimum length.
Coil L1 is wound with PEV-2 0.4 wire on a mandrel with a diameter of 2.5 mm and contains 2.8 turns. Coils L2, L3 are made of PEV-2 0.7 wire on a mandrel with a diameter of 3 mm. Winding length - 7 mm. They have three turns with a tap from the middle of the first turn. Coil L4 is wound with the same wire and contains two turns, and coil L5 is wound with PEV-2 0.4 wire and has 15 turns, both on a mandrel with a diameter of 4 mm.
The design of capacitor C8 is shown in Fig. 3. It is made of two plates of tin or thick foil, which are soldered to the contact pads of the board. By changing the distance between the plates, the capacitance of the capacitor is changed.
Setting up an amplifier begins with installing and checking the necessary modes according to DC. By selecting resistor R1, a voltage of 4...5 V is achieved at the collector of transistor VT1. The mode of transistors VT2, VT3 is obtained automatically.
To adjust the frequency response of the amplifier, use a panoramic indicator. Capacitors C7 and C9 tune the circuits to the desired frequencies. With the specified ratings, the central frequency of the filter can be changed from 500 to 700 MHz. The bandwidth is set by adjusting the capacitance of capacitor C8. At the same time, the gain of the amplifier also changes within small limits. By adjusting capacitor C11, the maximum gain is obtained at the required frequency.
By changing the capacitance of capacitor C8, you can achieve a minimum amplifier bandwidth of 10...12 MHz with a single-hump frequency response. This is necessary to isolate the signal of only one television channel. If you need to select two adjacent channels, then the passband is increased to 40...50 MHz (the plates of capacitor C8 are brought closer together) with a double-humped frequency response with slight unevenness. In addition, the frequency response of the filter is also influenced by the location of the taps of the coils L2, L3.
However, the broadcast environment can be difficult. For example, in Kursk in the UHF range, broadcasting is carried out on channels 31 and 33 from one place and with high power, and on channels 26 and 38 - from another place and with less power. This option is quite typical for most cities in the country. Therefore, to receive and select signals from the 31st and 33rd channels, you can use the already described amplifier. Such an amplifier is not suitable for receiving signals from the 26th and 38th channels (or two others with a large frequency separation). Here we need another one, which has two passbands, i.e. contains two filters.
The schematic diagram of such an amplifier is shown in Fig. 4.
The signal from the antenna through filter C1L1C2 is supplied to the first amplifier stage on transistor VT1. From its output, the signal is divided and sent to two independent stages on transistors VT2 and VT3, each of them loaded with its own bandpass filter: L2C10-C12L3 and L4C13-C15L5. The filters are connected to amplifier stages on transistors V4 and VT5, the outputs of which operate on the same load. The overall gain of this device is 18...20 dB, and the current consumption is approximately 40 mA.
This amplifier uses the same parts as those discussed above. A drawing of its printed circuit board with the placement of parts is shown in Fig. 5.
The setup is carried out in the same way. By selecting resistors R11 and R12, a constant voltage of about 5 V is established on the collectors of transistors VT4 and VT5. Filters are adjusted to the desired frequencies. By adjusting capacitors C6 and C7, maximum gain is obtained at selected frequencies.
If it is necessary to narrow the passband and increase the selectivity of the filter, increase the quality factor of the circuits by using thicker silver-plated wire in the coils and tuned capacitors with an air dielectric, or increase the number of circuits.
Literature
- Nechaev I. Active antenna of the MB range. - Radio, 1997, No. 2, p. 6, 7.
- Nechaev I. Active antenna MV-UHF. - Radio, 1998, No. 4, p. 6 - 8.
- Nechaev I. Television antenna amplifier. - Radio, 1992, No. 6, p. 38.39.
- Nechaev I. Combined amplifiers of TV signals. Radio, 1997, No. 10, p. 12, 13.
- Nechaev I. UHF antenna amplifier on a microcircuit. - Radio, 1999, No. 4, p. 8, 9.
- Nechaev I. TV signal adders. - Radio. 1996, no. 11, p. 12, 13.
- Nechaev I. Correcting antenna amplifier. - Radio, 1994, No. 12, p. 8 -10.
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 both the efficiency of the antenna and the sensitivity of the television receiver without modifying it.
The 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), 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 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 pnp transistors are used, 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 bushing 2, a partition 3, 4 and covers 5. Parts 1, 3, 4 and 5 are made from sheet brass(it is convenient to use a photogloss 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 are used that do not have planar terminals, then holes are drilled in the board for them, and rectangular cutouts are made in the 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. The 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 between 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 manufacture 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 good quality The Bulgarian BT2 program is being received from Varna (distance - 500 km) and the Turkish TV2 program from Istanbul (distance more than 600 km).
Digital terrestrial television within a 100 kilometer zone from Moscow.
When it finally started snowing and real winter arrived, I remembered the snow on the screen of my country TV while watching UHF channels. It's time to prepare for summer.
It's time to implement digital television at the dacha.
Starting this year, digital broadcasting is carried out in two multiplex packages at frequencies of 498 MHz and 546 MHz. 20 television programs in the DVB - T 2 standard are broadcast freely (free of charge, without a subscription fee). All that remains is to buy a set-top box (costing from 1200 rubles) for an old TV or a new TV with this new standard.
I have already bought a set-top box - DVB - T 2 receiver. I advise you to hurry up too. Knowledgeable people they simply sweep them off the shelves a few at a time. I also need to buy another one for my son, for his Squirrel House. I checked the operation of the receiver (receiver) in a city apartment. Everything is super cool! In dense urban areas, in the absence of line of sight (which is 15 km, Moscow, Vostok), away from windows - excellent reception quality with a conventional 2-antennae folded antenna. All 20 channels pass through as if from a disk, without a multi-circuit picture and noise. These are: Channel 1, Russia - 1, Russia - 2, NTV, Channel 5, Culture, Russia - 24, Karusel, OTR, TVC, REN TV, Spas, STS, Home, TV - 3, Sport +, Star, World, TNT , Muz TV.
All I have to do is make an antenna amplifier and an antenna, because I have to do something myself. And although the old broadband antenna can handle digital reception quite well, there is a desire to make a homemade mobile active antenna, because for this range it will be no larger than a book, and then you can watch TV in the gazebo. I think that the design of the antenna itself will be simplified, because now it will be narrowband, and it will be easier to match it without loss of gain at a fixed frequency.
For now, all that remains is to be proud of yourself that, due to your sluggishness, you did not manage to transform country house to interplanetary spacecraft, strewing the roof with multi-band multi-story antennas and satellite dishes.
Antenna amplifier for digital television reception.
From the beginning, I just wanted to make a homemade amplifier for the UHF broadcast range 470 - 870 MHz, to receive an analogue television signal, in order to sweep snow off the screen and increase noise immunity. You can’t imagine how difficult it is to suppress cellular communications that tear up the TV screen in stripes, since in terms of frequencies it is located close to the boundaries of the decimeter range of television channels. When receiving a digital signal, such stripes will be converted into a square mosaic. But now the task has been simplified and instead of a wide band of 400 MHz (this is exactly the bandwidth included in the amplifiers of active decimeter antennas), only 50 - 80 MHz have to be amplified, and in this case it will be easy to suppress interference outside the range. And the amplifier itself, having a smaller gain band, will have less noise, which means the range of reliable reception will increase. For me, this is especially important, since when the weather is transmitted in a region, I have to additionally subtract 5 degrees, due to the fact that the region with summer cottages is located in a low area, therefore, the likelihood of high-quality radio reception is in question, since the receiving antenna is below the level of this reception. There are two solutions: a high-elevation antenna or an antenna amplifier, perhaps both together. The latter symbiosis is necessary at the maximum reception boundaries, which are about 100 kilometers from the television center.
But in any case, an amplifier is necessary, since at this frequency there are significant losses in the cable.
The amplifier itself consists of one active element - a transistor and two filters that limit the gain band and suppress interference. Inductances L 1 – L 5 are components of the high-pass filter (high-pass), with additional rejection near the passband, and L 8 – L 9 are parts of the low-pass filter (low-pass). Inductances L 6 – L 7 are correcting links that level the frequency response.
The amplifier is powered by a separate stabilizer with an output voltage of 3 - 3.3 volts. The amplifier itself is powered via cable. The set-top boxes I know use a program (from the remote control) to supply power to the antenna input of 5 or 12 volts. If necessary, the amplifier can be powered from a separate mains power supply.
Amplifier parameters.
Bandwidth 490 – 600 MHz.
Gain 15 dB.
Rejection at 900 MHz is more than 25 dB.
Current consumption 13 mA.
I checked the amplifier for noise at the average gain frequency, connecting it to the input of the measuring receiver, having previously measured its signal-to-noise ratio at the level of its sensitivity in the WFN wide-band mode. After connecting the amplifier, the ratio at the receiver output increased by 2 times, that is, together with the amplifier Its sensitivity almost doubled.
So far I have tested the amplifier in urban conditions, in a place where there was no reception of the second multiplex package. When it was connected, reception was restored. Power supply with a voltage of 5 volts was provided from a regular telephone charger.
Amplifier design.
In an educational institution I would have been given a bad mark for using double-sided foil fiberglass laminate with a thickness of 1.2 -1.5 mm as a printed circuit board. At microwave frequencies, this material has losses, so the parameters of the active elements will differ from the tabulated data. However, modern transistors have a high gain at this frequency, so a loss of a few decibels will not greatly affect the operation of the amplifier. I cut out the conductive tracks on the board using a gravel (a semicircular chisel made from a sewing needle), adjusting to the dimensions of the CHIP capacitors and resistors, if possible reducing the area of the conductive tracks and increasing the distance between them. The edges of the board are soldered with tinned copper tape connecting the top side to the bottom. Next to the transistor, I drilled two holes into which a wire is soldered, connecting the two sides of the board and providing double-sided metallization.
The photos come out bad. I'll try to draw a sketch of the printed circuit board.
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| Rice. 2. Installation sketch. |
All coils are wound with enameled copper wire with a diameter of 0.5 mm on a drill with a diameter of 2 mm. L 1 – L 7 – four turns, L 8 – L 9 – two turns. The coils are frameless, winding is stepwise. Chokes L 10 - L 11 with an inductance of 220 μH, are used ready-made or made homemade by winding 15 turns of wire with a diameter of 0.1 mm on a small-sized resistor of 50 -100 kOhm.
Antenna amplifier for digital television reception using a field-effect transistor ATF54143 (analogous to SAV-541+).
Squinting at the instrument readings, we can say that the amplifier based on the field-effect transistor ATF54143 (analogous to SAV-541+) is better. At these frequencies its noise figure is between 0.2 and 0.3 dB, and the gain is 5 dB higher, but in practice you won’t notice much of a difference.
Its power scheme is somewhat more complicated. In a specific case, one of the simple circuits for connecting this transistor was tested. Noise level, linearity and gain will depend on the selected power mode. In the above scheme, a compromise has been found between the listed characteristics. Otherwise, in terms of the purpose of the elements and the design, the scheme does not differ from the previous one.
Amplifier parameters.
Bandwidth 490 – 600 MHz.
Gain 20 dB.
Current consumption 30 mA.
In this article I will talk about the use of my amplifiers in the country and in the city when receiving digital television. In conditions of long-distance reception, the best performance is achieved by a scheme with two field effect transistors Fig 4.
Since the circuit has a fairly high gain (up to 35 dB), additional parts have been added to it to increase resistance to self-excitation.
Photo 6 shows a fragment of a mock-up of a receiver preselector for a cybernetic device operating in conditions of strong interference.
On a similar circuit board, I mounted an amplifier using chip components, replacing the industrial narrow-band filter with discrete coils and capacitors.
This amplifier with a simple homemade antenna coped with the task.
In Fig. Figure 5 shows another diagram of a low-pass filter for an antenna amplifier for on-air digital broadcasting in other regions, where the cutoff bandwidth is 722 MHz. This filter is placed at the outputs of one or two transistors. It can be used separately at the output of a purchased amplifier. The purpose of this filter is to suppress interference from cellular repeaters and mobile phones.
In the case of the lower limit frequency of 650 MHz, I recommend reducing the capacitance values of the high-pass filter capacitors (HPF, which is located at the amplifier input) from 9.1 to 6.2 pF. These capacitors, standing parallel to the coils L 4, L 5, together with them, form plug filters that dampen interference from cellular repeaters at frequencies of about 470 MHz.
The first comment served as an addition to this post.
They brought me two ready-made purchase boards antenna amplifiers, just look at what kind of gain, for example, they have at digital reception frequencies. The owner of these products had problems receiving terrestrial digital television in a city apartment, and the collective antenna worked extremely poorly.
Board in photo 7 provided a gain of just over 20 dB over the entire frequency range from 50 - 800 MHz, but had a dip of 10 dB exclusively in the digital range 500 - 600 MHz. To get rid of the dip, we had to introduce an additional correction of the frequency response. This is a spiral coil in the collector of the first transistor and a P-low-pass filter connected in series with the signal between the transistors. Thus, it was possible to isolate exclusively the area of terrestrial digital reception by gain, which improved the signal / noise in this range. The signal level after this upgrade increased by 20 percent.
The owner of the board in Fig. 7 was pleased, rewarding me with a picture from his TV. Now his amplifier amplifies the signal from the antenna array
Then they brought another amplifier.
I do not recommend using the board in photo 12, since it has a tendency to self-excite. This is due to the design feature of the printed circuit board, where the ground conductors are made of thin tracks, which is unacceptable for microwave installation.