In published here article the author analyzes the circuitry antenna amplifiers Polish production and justifies their conscious approach select it with the standpoint of the coefficients of noise and amplification. It also gives recommendations for repair of such devices, quite often out of order from lightning discharges and the elimination of self-excitation. This will, hopefully, many hams not only to select the amplifier, but also to improve it work.
Active antenna Polish company ANPREL and some other widely distribution in Russia and CIS countries. With little own gain, especially in the MV range, the parameters of this antenna are largely determined by built-in antenna amplifier. This unit is characteristic of a number weaknesses: he is prone to delays, has a pretty high level self-noise, easily overloaded strong signals of MV range, often damaged by lightning discharges. These problems are familiar to many owners of such antennas.
The operation of antenna amplifiers SWA and similar very few lit in the literature. One can only note the publication [1], which indicates an overload amplifier signal MV. Other disadvantages to the owners of antennas have to fight a known manner: replacing amplifiers, choose the best. However, this the method requires much time and effort, as the amplifier, as a rule, remote - located with the antenna on a high mast.
Based on the analysis circuitry, own experience and some materials company ANPREL, offer a more informed approach to the selection of amplifiers, the method of repair that allows you to recover the corrupted block, and in some cases to improve its parameters.
The market is filled with many interchangeable models of antenna amplifiers, produced by ANPREL, TELTAD, etc. under different trademarks and rooms. Despite this diversity, most of them collected by the standard scheme and represent aperiodic two-stage amplifier bipolar microwave transistors included in the circuit with the MA. In support of this consider models of different companies: the simple amplifier SWA-36 firms TELTAD, schematic diagram of which is shown in Fig. 1, and a common amplifier SWA-49 (analogue SWA-9) firms ANPREL - Fig. 2.
Amplifier SWA-36 contains two wideband amplifier transistors VT1 and VT2. The signal from the antenna via a matching transformer (not shown) and the capacitor C1 is supplied to the base of transistor VT1, enabled by the circuit with the MA. The working point of the transistor is set by the bias voltage determined by resistor R1. The current negative feedback (EP) voltage linearizes the characteristic of the first stage, the situation stabilizes operating point, but slightly reduces its gain. The frequency correction in the first the cascade is missing.
The second stage also performed on the transistor on the circuit with the MA and the voltage EP through resistors R2 and R3, but has a current EP via the resistor R4 in emitter circuit, rigidly stabilizing mode of the transistor VT2. Avoid loss gain resistor R4 is shunted alternating current capacitor C3, the capacitance of which is chosen relatively small (10 pF). In the result of the low frequency range of the capacitive resistance of the capacitor C3 is a significant and emerging environmental AC reduces gain, thereby correcting the frequency response of the amplifier.
The disadvantages of the amplifier SWA-36 may include passive losses in the output circuit on the resistor R5, which is included so that it falls as a constant voltage power and signal voltage.
Similarly constructed amplifier SWA-49 (Fig. 2), which also has two cascade is assembled on the circuit with the MA. It differs from SWA-36 best isolation the power supply circuits using l-shaped filters L1C6, R5C4 and a high coefficient amplification due to the presence of the capacitor C5 in the chain EP (R3C5R6) and the second cascade the transition of the capacitor C7 at the output.
Such circuitry inherent to most other amplifiers SWA (see, for example, the amplifier circuit SWA-3, depicted in [1]). Minor differences most often located in the second cascade, which may be provided with different frequency circuits correction, have different depth OOS and, accordingly, the gain. I some models, such as SWA-7, the first and second stages have direct link - the output collector of the transistor VT1 is connected directly with the withdrawal of the base of the transistor VT2. This allows you to cover both the cascade loop on EP DC current and thereby improve the thermal stability of the amplifier.
In the cascade transistors connected in the circuit with the MA, the most important the internal connections and tanks transitions of transistors. It manifests itself in the bandwidth limitation and the tendency of the amplifier to the excitation, the probability of which is greater, the higher the gain. To evaluate aware of the concept of tolerance - limit values of the coefficient gain, above which the amplifier becomes an oscillator. Many antenna amplifiers SWA with a high gain are at the threshold of stability than due to their frequent self-excitation.
As measures to improve the stability of amplifiers firm uses different ANPREL circuit (affecting the capacity of the installation), surface and volume coils, chokes, etc. a More radical way: the inclusion of transistors on cascadei the circuit with the MA-ABOUT - why-that is not used. At a fixed scheme the inclusion of the transistors MA-MA to solve sustainability firm prefers to release adjustable power supplies. Reduced voltage able to eliminate self-excitation of the amplifier while maintaining sufficient gain.
The main parameters (noise figure of CS and the gain KU) base models SWA company catalogue ANPREL are listed in table. 1.
Table 1
Amplifier To The dB To W , dB SWA-1 8 10... 2,5 SWA-1/S (1) 10 2,3 SWA-2, SWA-3 (2) 23...28 3,1 SWA-3/LUX 25 30... 3,0 SWA-4/LUX 28 32... 2,6 SWA-5,CR-6 32 36... 1,9 SWA-7, SWA-7/LUX 32 38... 1,7 SWA-8 28 30... 2,9 SWA-8/S (1) 28 30... 2,8 SWA-8/Zw (3) 28 30... 2,9 SWA-9, SWA-49 32 39... 1,7 SWA-10 32 30... 3,1 SWA-11 30 2,5 SWA-12 36 1,8 SWA-13 32 1,8 SWA-14 32 2,4 SWA-15 34 1,9 SWA-16 34 1,9(1) With balun system on Board. (2) the Amplifiers are different boards. (3) With crossover
Consider the interrelation of the basic parameters with the circuitry of amplifiers and their impact on the quality of reception.
As is known, the gain at high frequencies in the cascades with critical MA to the parameters of the used transistors, especially to the cutoff frequency fГР. In amplifiers SWA, applied to the bipolar microwave transistors of the structure n-p-n, marked as T-67, at least - 415, which determine the maximum achievable the gain KU a two-stage amplifier of about 40 dB. Of course, in such a wide operating frequency gain does not remain constant - it changes reach 10...15 dB due to the uneven frequency response at higher the frequency range and the correction to the lowest. Maximum gain KU is difficult to ensure the stability of the amplifiers, so in the number of models it is limited to values up to 10...30 dB, which is in many cases quite enough (see tab. 1).
Contrary to popular belief, it should be noted that the gain cannot be considered the main parameter of the antenna amplifier. Because TVs themselves have a very large stock of their own gain, i.e., have high sensitivity, limited amplification. Worse they have the sensitivity, limited synchronization. Lastly, most low - sensitivity, limited by noise [2]. Consequently, the determinant of all India radio, should take the noise floor of the electronic tract, not the ratio gain. In other words, limiting the possibility of admission in the first place occurs due to the influence of noise interference and not due to the lack of signal amplification.
The effect of noise is estimated in relation signal/noise, the minimum value of which assumed to be 20 [2]. In this regard, and determine the sensitivity, limited by noise, which is equal to the voltage of the input signal, 20 times higher voltage noise.
For TVs the third to fifth generations, the sensitivity is limited by noise, is $ 50...100 µv. However, when the ratio signal/noise ratio equal to 20, there are very poor picture quality and clarity only the major details. For obtain a good quality image should submit the input on the TV the useful signal is about 5 times greater, thus improving the signal-to-noise about 100 [2].
Antenna amplifier to increase the signal-to-noise, and this should amplifying the signal and not the noise. But any electronic amplifier has inevitably own noises that are amplified along with the desired signal and degrade the ratio signal/noise. Therefore, the most important parameter of the antenna amplifier should to consider the noise factor KSH. If he is not small, the increase gain is useless, because both the signal and the noise are amplified equally least and their attitude improves. As a result, even when a sufficient level the signal on the antenna input on the TV the image will be affected intense noise nuisance (well-known "snow").
For a single noise assessment of multi-path indicator exists given to the input of the noise figure of CS, which is equal to the noise level at the output, divided by the total gain, i.e., CABG=coronary artery bypass grafting.o/KU. As output noise CABG.o depends to the greatest extent from the noise of the first transistor, amplified by all subsequent stages, other noises cascades can be neglected. Then CABG.o=CSCU where X - noise ratio the first transistor. Therefore, get SH=X, i.e. given the noise figure of the amplification path is independent of the number of stages and total of the gain equal to the noise figure of the first transistor.
This implies an important practical conclusion - the use of antenna amplifier to give a positive result when the noise factor of the first transistor of the amplifier is less than the noise figure of the first stage of the TV. In selectors channels TV's the fifth generation of the applied field-effect transistor with CPA noise figure of 4.5 dB at 800 MHz [3]. Therefore, in the first cascade antenna amplifier needs to operate the transistor with X<4.5 dB on the same frequency. Moreover, the smaller this value is compared with the coefficient KS the TV, the more efficient use of the amplifier and the higher the quality of reception.
The noise factor also depends on the quality of matching at the input of the amplifier and the mode of operation of the first transistor. For amplifiers SWA type transistor VT1, mode his work and the quality of the negotiation determines the given coefficient CABG=1,7 3,1...dB (see tab. 1).
From the above it is clear that the choice of the antenna amplifier according to the principle " the more the gain, the better is wrong. That is why many owners, changing amplifiers cannot achieve a good result. The reason for this paradoxically, at first glance, the fact is that the coefficient noise, as a rule, is unknown (it is not in trading information firms), and in he is only slightly different on many models with different reinforcement (see table. 1). The increase of the gain at a constant noise factor does not give a gain in signal-to-noise and thus improve the quality admission. Rare success is only achieved when one accidently gets a low-noise amplifier.
Therefore, when selecting antenna amplifier should focus first turn on the noise floor. Quite good can be considered an amplifier with CABG<2 dB. From table 1 can be considered the best model SWA-7, SWA-9 having CABG=1,7 dB. Information on the noise figure of the new amplifiers can be found in the directory company ANPREL or on the Internet.
As for gain, it is, of course, is also important, but not for the maximum amplification of weak signals, and, primarily, for compensation of losses in the connecting cable matching the splitter devices etc. because of these losses when there is insufficient amplification of the signal level at the input the TV may fall below the threshold, limited sync or even gain, which will make reception impossible. Therefore, for proper selection of the gain is necessary to know the attenuation of the signal in all connective tract. And its estimated value is easy to calculate.
Linear attenuation of the signal in the distributed cable brand RK-75-4-11 is equal to 0,07 dB/m at the first-fifth, of 0.13 dB/m at the sixth-and twelfth of 0.25...of 0.37 dB/m at 21-60-m channels [2]. When the length of the feeder 50 m attenuation at 21-60-m the channels will be 12.5...of 17.5 dB. If there is industrial passive splitter, it introduces additional losses at each exit value which, as a rule, is indicated on the body.
Calculating the attenuation in the cable and instead of the attenuation in the splitter (if it is), get the minimum gain of the antenna amplifier. To him add the stock in 12...14 dB to amplify weak signals, which is necessary because low efficiency compact broadband receiving antennas. On the value of RL is chosen antenna amplifier. Far exceed the resulting value of the gain does not follow, as this increases the probability of excitation and overload strong signals close stations.
Repair of antenna amplifiers basically amounts to replacing the active elements, damaged by lightning discharges. It should be noted that the presence of some models of the diode at the input does not guarantee a complete lightning protection: when powerful atmospheric discharge makes its way as a protective diode, and usually both of the transistor.
Antenna amplifiers SWA collected by the technology of automatic surface Assembly on the micronutrients that requires accuracy in the repair. Soldering should be performed small soldering iron with a sharp sting. In a non-operational amplifier carefully, trying not to damage the delicate traces, desoldering microdentistry VT1, VT2 and protective diode (if any).
The main parameters of the domestic transistors, suitable for installation in amplifiers SWA, are listed in table. 2 [3]. From this it follows that the use in the first the cascade transistors CTA-2, CHA-2, CHA-2, CTB-2, CTV-2 the noise performance of most amplifiers is not worsen, and the use transistors TA-2, TB-2, TB-2, CHA-2 CABG reduces to 1.5 dB, it improves the settings of the amplifier. This allows us to recommend replacement of the first transistor of the amplifier for these last even in good, but "noisy" amplifiers, with the aim of improving the quality of their work. Need note that in table. 2 shows the limit values of the model parameters, as better [3].
Table 2
Transistor f G , GHz To W , dB at f=1 GHz I to , mA h e at T=25°C CTA-2 5 3 10 20...150 CTA-2 4 3 20 35 300... CTA-2, CTB-2,CTV-2 5,8 2 8,5 15 110... TA-2, TB-2, TB-2 6 1,5 7 15 200... CTA-2 5,5 1,5 8,5 15...150Low-noise microwave transistors series T, relatively expensive and CT low-voltage, so it is better to only install in the first stage, while in the second to use cheaper and more powerful transistors CTA-2, CHA-2 (see tab. 2) and even series CT, CT, CT, CT and others with a cutoff frequency of about 2 GHz [3]. However, in the latter case will be slightly less gain on the upper frequency range.
Low-noise microwave transistors series T, relatively expensive and CT low-voltage, so it is better to only install in the first stage, while in the second to use cheaper and more powerful transistors CTA-2, CHA-2 (see tab. 2) and even series CT, CT, CT, CT and others with a cutoff frequency of about 2 GHz [3]. However, in the latter case will be slightly less gain on the upper frequency range.
Case dimensions import microtransaction equal to 1.2(2.8 mm when the length of the conclusions 1...1.5 mm. Respectively, and the distances between the printed circuit Board pads to conclusions transistors are small. The unit produced transistors with a diameter of case 2 mm from the mounting surface, though possible, but difficult: when soldering them can be corrupt. New transistors is better to install on the opposite side of the Board, pre-drilled holes for conclusions drill diameter is 0.5 to 0.8 mm. it is Better to drill not in the printed guide, but the hole touched the edge of the site. If on the side opposite surface mounting, there is a layer of foil, the holes in it should to razzenkovat drill diameter 2...2.5 mm (except the holes for the output emitter transistor VT1).
Then install new transistors so that crystallochemical or corpus device concerned Board. If the findings are significantly on the other hand, after soldering them to bite. Microwave transistors are sensitive to static electricity, so when soldering it is necessary to comply with the relevant protection measures. The soldering time is not more than 3 [3].
The protective diode can not install. The best protection from the weather electricity serves as a good grounding of the antenna.
Amplifiers SWA both transistors operate with a collector current of 10 to 12 mA. After replace this current is acceptable for the second transistor (for example, CTA-2), but exceeds continuously valid for the first, if the series transistors CT, CT and CTA-2 (see tab. 2). Collector current depends on the parameter h21e, whereby the transistors have significant scatter. Therefore, after installation a particular instance it is necessary to set the operating point of the transistor VT1. For this may be given to microreactor R1 and instead, temporarily connect the trimpot resistor (CP3-23, CP3-27, etc.) resistance 68...100 kOhm. Before enabling the power engine of the resistor should be in the position of maximum resistance, to prevent damage to the transistor.
The amplifier serves voltage of 12 V from the power supply and measure the drop the voltage across the resistor R2 (see Fig. 1 and 2). Dividing the measured voltage the resistance of the resistor R2, learn collector current. Adjusting the resistance trimmer downward, making the collector current about 5 mA, which corresponds to the minimum noise characterization of transistors [3]. In this setting finish and instead solder a trimmer permanent same resistance (MLT-0,125 or imported), shortening prior to the minimum of its conclusions.
Then cover the circuit Board and the chip layer transistors radio varnish or compound.
Appearance restored amplifier SWA-36 shown in Fig. 3. In it used transistors (Fig. 3,a) TB-2 (VT1) and CTA-2 (VT2). In connection with the simplest design of the amplifier is taken to eliminate self-excitation: the output collector of the transistor VT1 wearing ferrite microcalc (it is used in the channel selectors SC-M TV USCT and USCT). The collector current of the transistor VT1 is set by resistor R1 (Fig. 3,b) value of 51 ohms (33 ohms).
In the second cascade transistors were tested series CT, CT that continued stability and sufficient gain. It was checked the extra DM capacitor capacitance 150 pF (Fig. 3,b), a shunt resistor R5 (see Fig. 1), to increase the ratio gain. When installing the capacitor self-excitation of the amplifier eliminates lowering the supply voltage.
In the basic version (with transistors TB-2 and CTA-2) amplifier provided better than before the repair, the reception quality which is visually estimated the same reception with a new amplifier SWA-9.
Literature
Author: A. Pakhomov, Zernograd, Rostov region