Mounting inductors on simple printed circuit boards. Coil32 - Thin film printed coil. IC Power Decoupling

10.09.2021

Many elements of the circuit can be made by printing: resistors, capacitors, inductors, multi-turn coils of transformers and chokes, switches and plug connectors.

Printed resistors are made by applying thin varnish films to the surface of the board.

Their configuration (Fig. 35, a) is the most diverse and depends on the possibility of ensuring mechanical strength and heat transfer conditions. Variable resistors are also printed, which consist of a conductive carbon or metal layer of an arcuate shape and a contact slider sliding over the surface of the conductive element. The resistance value of the printed resistor depends on the composition of the suspension, the shape of the pattern and the thickness of the film.

Film composite resistors of the SZ-4 type have received wide application. These resistors are fabricated directly on the surface of the microboard. They can be used in the temperature range from -60 to +125°C, and the power dissipated by microresistors does not exceed 0.25 W.

Printed capacitors are made by applying two conductive plates on both sides of the insulating base (Fig. 35, b). The capacitance of a capacitor is determined by the area of its plates and the thickness of the dielectric (board). On fig. 35, c shows a semi-variable printed capacitor, in which the stator plate is applied directly to the insulating base of the board, and the rotor plate is applied to a ceramic disk, which can rotate around an axis parallel to the board plane, changing the capacitance value. The use of ceramic materials makes it possible to obtain stable capacitors with ratings from a few to several hundred picofarads and an operating voltage of 100V or more.

Printed inductors (Fig. 35, d) are made in the form of flat spiral metallized lines of a round, oval, square or other shape, applied to the board. The value of the inductance of such coils depends on the number of turns of the coil, the distance between them and their diameter. To increase the inductance of the printed coils, they are made multilayer, while one coil is separated from the other by an insulating layer of varnish, and the ends of the coils are connected to each other in series. In some cases, an increase in inductance is achieved by introducing magnetodielectric cores into the center of the spiral or by applying a layer of magnetic paint in the field of the coil. On printed circuits, you can also create a variable inductance, for which a copper or aluminum plate is installed above the printed coil, which can be moved.

To increase the quality factor of the coils, a silver layer with a thickness of 20 ... 50 microns is galvanized on them.

Printing transformers and chokes are made by applying individual helical coils to a flexible base made of fluoroplast, lacquered fabric, bakelized paper, or other insulating materials. Printed windings are connected in series and placed in a special housing or molded into a plastic shell.

Printed switches and plug connectors can be made either directly on the printed circuit board of the radio receiver, or on separate boards. A printed switch, even of the highest complexity, is cheaper than made by any other method. To increase the abrasion resistance of the contacts of the printed switch, they are coated with silver, which ensures reliability in operation up to several hundred thousand switches. To ensure increased resistance, the copper contacts of the switches are coated with a layer of rhodium with a thickness of ... 10 microns.

Printed elements, if necessary, are shielded by applying a layer of insulating varnish to the surface of the pattern, which is then covered with a layer of magnetic material. Shielding of conductors is performed not as a solid, but as mesh or slot-like.

“Iron-laser” technology for the manufacture of printed circuit boards(ULT) in just a couple of years has become widespread in amateur radio circles and allows you to get printed circuit boards of a fairly high quality. “Hand-drawn” printed circuit boards are time consuming and not immune to errors.

Special requirements for the accuracy of the pattern are imposed in the manufacture of printed inductors for high-frequency circuits. The edges of the conductors of the coils should be as even as possible, as this affects their quality factor. It is very problematic to manually draw a multi-turn spiral coil, and here the ULT may well have its say.

Rice. one

Rice. 2

So, everything is in order. We start the computer program SPRINT-LAYOUT, for example, version 5.0. Set in the program settings:

Grid scale - 1.25 mm;

Line width - 0.8 mm;

Board dimensions - 42.5x42.5 mm;

The outer diameter of the “patch” is 1.5 mm;

The diameter of the hole in the “patch” is 0.5 mm.

We find the center of the board and draw a coil conductor template (Fig. 1)along the coordinate grid using the EXPLORER tool, twisting the coil in the desired direction (a mirror image is required for the template, but it can also be obtained later, when printing). At the beginning and at the end of the coil, we install a “patch” to connect the coil to the circuit elements.

In the settings for printing, we set the number of prints on a sheet, the distance between prints and, if necessary, “twist” the spool in the other direction, a mirror print of the pattern. You should print on smooth paper or special transparencies with the printer set to the maximum amount of toner when printing.

Next, follow the standard ULT. We prepare foil fiberglass, clean the surface of the foil and degrease, for example, with acetone. We apply the template with the toner to the foil and iron it with a hot iron through a sheet of paper until the toner adheres to the foil.

After that, under running tap water (cold or room temperature), we soak the paper and carefully remove it with “pellets”, leaving the toner on the board foil. We etch the board and then remove the toner from it with a solvent, for example, acetone. A clear conductor of a high quality “printed” inductor remains on the board.

Printed coils with ULT helical turns are slightly worse quality. The point here is the square shape of the image pixels, so the edges of the spiral coil conductor are jagged. True, these irregularities are quite small, and the quality of the coil, in general, is still higher than with manual execution.

Open the SPRINT-LAYOUT version 5.0 again. In the toolbox, select SPECIAL FORM - a tool for drawing polygons and spirals. Select the SPIRAL tab. Install:

Start radius (START RADIUS) -2 mm;

The distance between the turns (DISTANCE) -1.5 mm;

Conductor width (TRACK WIDTH) -0.8 mm;

The number of turns (TURNS), for example - 20.

The size of the board occupied by such a coil is 65x65 mm (Fig. 2).

Printed coils are usually interconnected in band-pass filters (PF) using small capacitors. However, their inductive coupling is also possible, the degree of which can be changed by changing the distance between the planes of the coils or eccentrically turning one relative to the other. The fixed mounting of the coils relative to each other can be

work with the help of dielectric racks-struts.

Adjustment of the inductance of the coils can be done by closing the turns, breaking the printed conductor or partially removing it. This will increase the tuning frequency of the loop. Frequency reduction can be achieved by soldering small SMD-type capacitors between the turns.

Production of VHF coils in the form of a meander, straight and curved lines, comb filters, etc. with the use of ULT also adds elegance to the final product and, as a rule, increases their quality factor (due to the “smooth” edges of the printed conductors). However, in the manufacture, one should remember the quality of the substrate material (glass fiber), which loses its insulator properties with increasing frequency. In equivalent circuits, the loss resistance in the dielectric should be connected in parallel with the printed coils, and this resistance will be the lower, the higher the operating frequency and the worse the quality of the dielectric.

In practice, foil fiberglass can be fully used for the manufacture of printed resonant circuits up to a 2-meter range inclusive (up to about 150 MHz). Special high-frequency grades of fiberglass can be used in the 70 cm range (up to about 470 ... 500 MHz). At higher frequencies foil RF fluoroplastic (Teflon), ceramic or glass should be used.

The printed inductor has an increased quality factor due to a decrease in the interturn capacitance, which is obtained, on the one hand, due to the small thickness of the foil, and, on the other hand, the winding step of the coil. A closed frame of grounded foil around the printed coil in its plane serves as a shield from other coils and printed conductors, but has little effect on the parameters of the coil if its periphery is under low RF voltage (connected to a common wire), and the center is under high.

Literature

1. G. Panasenko. Manufacture of printed coils. - Radio, 1987, No. 5, S. 62.

Double-sided printed circuit boards, despite all their advantages, are not the best, especially for small-signal or high-speed circuits. In general, the PCB thickness, i.e. the distance between the plating layers is 1.5 mm, which is too much to fully realize some of the advantages of a two-layer printed circuit board, given above. The allocated capacity, for example, is too small due to such a large spacing.

Multilayer printed circuit boards

Responsible circuit design requires multilayer printed circuit boards (MPBs). Some reasons for their use are obvious:

the same convenient as for the common wire bus, power bus wiring; if polygons on a separate layer are used as power rails, then it is quite simple to supply power to each element of the circuit using vias
signal layers are freed from power rails, which makes it easier to route signal conductors
distributed capacitance appears between the ground and power planes, which reduces high-frequency noise

In addition to these reasons for using multilayer printed circuit boards, there are other less obvious ones:

better suppression of electromagnetic ( EMI) and radio frequency ( RFI) interference due to the reflection effect ( image plane effect), known since the time of Marconi. When a conductor is placed close to a flat conducting surface, most of the return high frequency currents will flow in the plane directly below the conductor. The direction of these currents will be opposite to the direction of currents in the conductor. Thus, the reflection of the conductor in the plane creates a signal transmission line. Since the currents in the conductor and in the plane are equal in magnitude and opposite in direction, some reduction in radiated interference is created. Reflection effect works effectively only with non-breaking solid polygons (they can be both ground polygons and food polygons). Any breach of integrity will result in a reduction in interference suppression.
reducing the overall cost in small-scale production. Even though multilayer printed circuit boards are more expensive to manufacture, their possible emission is less than that of single and double layer boards. Therefore, in some cases, the use of only multilayer boards will allow you to meet the requirements for radiation set during the development, and not to carry out additional tests and tests. The use of MFP can reduce the level of radiated noise by 20 dB compared to two-layer boards.

Layer order

For inexperienced designers, there is often some confusion about the optimal order of PCB layers. Take for example a 4-layer chamber containing two signal layers and two polygon layers - a ground layer and a power layer. What is the best layer order? Signal layers between polygons that will serve as screens? Or to make the polygon layers internal to reduce the interference of the signal layers?

It's important to remember in deciding this question that often the placement of layers doesn't really matter, because the components are on the outer layers anyway, and the busses that feed signals to their pins sometimes go through all the layers. Therefore, any screen effects are only a compromise. In this case, it is better to take care of creating a large distributed capacity between the power and ground polygons, placing them in the inner layers.

Another advantage of having signal layers outside is the availability of signals for testing, as well as the possibility of modifying connections. Anyone who has ever changed the connections of conductors located in the inner layers will appreciate this opportunity.

For printed circuit boards with more than four layers, it is a general rule to place high-speed signal traces between the ground and power planes, and leave the outer layers for low-frequency ones.

grounding

Good grounding is a common requirement for a rich, layered system. And it should be planned from the first step of design development.

Basic rule: division of the land.

Dividing the ground into analog and digital parts is one of the simplest and most effective methods of noise suppression. One or more layers of a multi-layer printed circuit board is usually allocated under a layer of ground planes. If the developer is not very experienced or careless, then the ground of the analog part will be directly connected to these polygons, i.e. the analog return current will use the same circuit as the digital return current. Auto breeders work in much the same way and unite all the lands together.

If a previously developed printed circuit board with a single ground polygon that combines analog and digital grounds is subjected to processing, then it is first necessary to physically separate the grounds on the board (after this operation, the operation of the board becomes almost impossible). After that, all connections are made to the analog ground plane of the analog circuit components (an analog ground is formed) and to the digital ground plane of the digital circuit components (the digital ground is formed). And only after that, the digital and analog grounds are combined in the source.

Other land formation rules:

Almost all clock signals are high enough frequency signals that even small capacitances between traces and polygons can create significant coupling. It must be remembered that not only the main clock frequency can cause a problem, but also its higher harmonics.

Figure 4 shows a possible layout of all components on the board, including the power supply. There are three separate and isolated ground/power planes used here: one for the source, one for the digital circuitry, and one for the analog circuitry. The ground and power circuits of the analog and digital parts are combined only in the power supply. High-frequency noise is filtered out in the supply circuits by chokes. In this example, the high frequency signals of the analog and digital parts are separated from each other. Such a design has a very high probability of a favorable outcome, since it ensures good placement of components and adherence to the rules of separation of circuits.

There is only one case where analog and digital signals need to be combined over an analog ground area. Analog-to-digital and digital-to-analog converters are housed in housings with analog and digital ground pins. Considering the previous considerations, it can be assumed that the digital ground pin and the analog ground pin should be connected to the digital and analog ground buses, respectively. However, this is not true in this case.

The pin names (analog or digital) refer only to the internal structure of the converter, to its internal connections. In the circuit, these pins should be connected to the analog ground bus. The connection can also be made inside the integrated circuit, however, it is rather difficult to obtain a low resistance of such a connection due to topological limitations. Therefore, when using converters, an external connection of the analog and digital ground pins is assumed. If this is not done, then the parameters of the microcircuit will be much worse than those given in the specification.

It must be taken into account that the digital elements of the converter can degrade the quality characteristics of the circuit, introducing digital noise into the analog ground and analog power circuits. The design of the converters takes this negative impact into account so that the digital part consumes as little power as possible. In this case, interference from switching logic elements is reduced. If the digital outputs of the converter are not heavily loaded, then internal switching usually does not cause much problems. When designing a printed circuit board containing an ADC or DAC, due consideration must be given to decoupling the converter's digital power to analog ground.

Frequency response of passive components

Proper selection of passive components is essential for the correct operation of analog circuits. Begin your design development by carefully considering the high frequency characteristics of passive components and pre-positioning and arranging them on the board sketch.

A large number of designers completely ignore the frequency limitations of passive components when used in analog circuitry. These components have limited frequency ranges and their operation outside the specified frequency range can lead to unpredictable results. One might think that this discussion is only about high-speed analog circuits. However, this is far from being the case - high-frequency signals affect the passive components of low-frequency circuits quite strongly through radiation or direct connection through conductors. For example, a simple low-pass filter on an op-amp can easily turn into a high-pass filter when high frequency is applied to its input.

Resistors

The high frequency characteristics of the resistors can be represented by the equivalent circuit shown in Figure 5.

Usually three types of resistors are used: 1) wire, 2) carbon composite and 3) film. It doesn't take much imagination to understand how a wirewound resistor can turn into an inductance, since it is a coil of high resistance metal wire. Most electronic device designers have no idea about the internal structure of film resistors, which are also a coil, albeit made of a metal film. Therefore, film resistors also have an inductance that is less than that of wirewound resistors. Film resistors with a resistance of no more than 2 kOhm can be freely used in high-frequency circuits. The terminals of the resistors are parallel to each other, so there is a noticeable capacitive coupling between them. For high resistance resistors, terminal capacitance will reduce the overall impedance at high frequencies.

Capacitors

The high frequency characteristics of capacitors can be represented by the equivalent circuit shown in Figure 6.

Capacitors in analog circuits are used as decoupling and filtering components. For an ideal capacitor, the reactance is determined by the following formula:

Therefore, a 10µF electrolytic capacitor will have a resistance of 1.6Ω at 10kHz and 160µΩ at 100MHz. Is it so?

When using electrolytic capacitors, the correct connection must be observed. The positive terminal must be connected to a more positive DC potential. Incorrect connection leads to DC current flowing through the electrolytic capacitor, which can damage not only the capacitor itself, but also part of the circuit.

In rare cases, the DC potential difference between two points in a circuit can reverse sign. This requires the use of non-polar electrolytic capacitors, the internal structure of which is equivalent to two polar capacitors connected in series.

inductance

The high frequency characteristics of inductors can be represented by the equivalent circuit shown in Figure 7.

The reactance of an inductor is described by the following formula:

Therefore, a 10 mH inductor will have a reactance of 628 ohms at 10 kHz and a reactance of 6.28 MΩ at 100 MHz. Right?

The printed circuit board itself has the characteristics of the passive components discussed above, although not so obvious.

The pattern of conductors on a printed circuit board can be both a source and a receiver of interference. Good wiring reduces the sensitivity of the analog circuit to radiated sources.

The printed circuit board is susceptible to radiation because the conductors and leads of the components form a kind of antenna. Antenna theory is a fairly complex subject to study and is not covered in this article. However, some basics are given here.

A bit of antenna theory

At direct current or low frequencies, the active component predominates. As the frequency increases, the reactive component becomes more and more significant. In the range from 1 kHz to 10 kHz, the inductive component starts to take effect, and the conductor is no longer a low-resistance connector, but rather acts as an inductor.

The formula for calculating the inductance of a PCB conductor is as follows:

Typically, PCB traces have values between 6 nH and 12 nH per centimeter of length. For example, a 10 cm conductor has a resistance of 57 mΩ and an inductance of 8 nH per cm. At 100 kHz, the reactance becomes 50 mΩ, and at higher frequencies the conductor will be an inductance rather than a resistance.

The whip antenna rule states that it begins to noticeably interact with the field at its length of about 1/20 of the wavelength, and the maximum interaction occurs at the length of the pin, equal to 1/4 of the wavelength. Therefore, the 10 cm conductor from the example in the previous paragraph will start to become a pretty good antenna at frequencies above 150 MHz. It must be remembered that despite the fact that the clock generator of a digital circuit may not operate at a frequency higher than 150 MHz, higher harmonics are always present in its signal. If the PCB contains components with pin leads of considerable length, then these pins can also serve as antennas.

The other main type of antenna is the loop antenna. The inductance of a straight conductor increases greatly when it bends and becomes part of an arc. Increasing inductance lowers the frequency at which the antenna begins to interact with the field lines.

Experienced PCB designers who are fairly well versed in the theory of loop antennas know not to create loops for critical signals. Some designers, however, do not think about this, and the return and signal current conductors in their circuits are loops. The creation of loop antennas is easy to show with an example (Fig. 8). In addition, the creation of a slot antenna is shown here.

Consider three cases:

Option A is an example of bad design. It does not use the analog ground polygon at all. The loop circuit is formed by a ground and signal conductor. When a current passes, an electric field and a magnetic field perpendicular to it arise. These fields form the basis of a loop antenna. The loop antenna rule states that for maximum efficiency, the length of each conductor should be equal to half the wavelength of the received radiation. However, one should not forget that even at 1/20 of the wavelength, the loop antenna is still quite effective.

Option B is better than option A, but there is a gap in the polygon, probably to create a specific place for the signal wires to be routed. The signal and return current paths form a slot antenna. Other loops are formed in the cutouts around the chips.

Option B is an example of a better design. The signal and return current paths overlap, negating the efficiency of the loop antenna. Note that this option also has cutouts around the ICs, but they are separated from the return current path.

The theory of reflection and matching of signals is close to the theory of antennas.

When the PCB conductor is rotated through 90°, reflections can occur. This is mainly due to the change in the width of the current path. At the top of the corner, the trace width increases by a factor of 1.414, which leads to a mismatch in the characteristics of the transmission line, especially the distributed capacitance and the intrinsic inductance of the trace. Quite often it is necessary to rotate a trace 90° on a PCB. Many modern CAD packages allow you to smooth the corners of the drawn paths or draw the paths in the form of an arc. Figure 9 shows two steps to improve the corner shape. Only the last example keeps the trace width constant and minimizes reflections.

Tip for experienced PCB layoutrs: leave the smoothing procedure to the last stage of work before creating droplets and pouring polygons. Otherwise, the CAD package will take longer to smooth due to more complex calculations.

PCB traces on different layers are capacitively coupled when they cross. Sometimes this can create a problem. Conductors stacked on top of each other on adjacent layers create a long film capacitor. The capacitance of such a capacitor is calculated according to the formula shown in Figure 10.

For example, a printed circuit board may have the following parameters:

4 layers; signal and ground polygon layer - adjacent
interlayer interval - 0.2 mm
conductor width - 0.75 mm
conductor length - 7.5 mm

Typical ER value for FR-4 is 4.5.

It can be seen that the output signal amplitude doubles at frequencies close to the upper limit of the OS frequency range. This, in turn, can lead to generation, especially at antenna operating frequencies (above 180 MHz).

This effect gives rise to numerous problems, for which, nevertheless, there are many ways. The most obvious of these is the reduction in the length of the conductors. Another way is to reduce their width. There is no reason to use a conductor of this width to feed the signal to the inverting input, since Very little current flows through this conductor. Reducing the trace length to 2.5 mm and the width to 0.2 mm will reduce the capacitance to 0.1 pF, and such a capacitance will no longer lead to such a significant increase in the frequency response. Another way to solve it is to remove part of the polygon under the inverting input and the conductor coming up to it.

The width of PCB traces cannot be reduced indefinitely. The limiting width is determined by both the technological process and the thickness of the foil. If two conductors pass close to each other, then a capacitive and inductive coupling is formed between them (Fig. 12).

Signal wires should not be run parallel to each other, except in the case of differential or microstrip wiring. The gap between the conductors must be at least three times the width of the conductors.

Capacitance between traces in analog circuits can be problematic for large resistor values (several MΩ). The relatively large capacitive coupling between the inverting and non-inverting inputs of an op-amp can easily cause the circuit to self-excite.

For example, with d=0.4 mm and h=1.5 mm (quite common values), the inductance of the hole is 1.1 nH.

Remember that if there are large resistances in the circuit, then special attention should be paid to cleaning the board. Flux residues and contaminants must be removed during the final stages of PCB fabrication. Recently, when mounting printed circuit boards, water-soluble fluxes are often used. Being less harmful, they are easily removed with water. But at the same time, washing the board with insufficiently clean water can lead to additional contamination, which worsens the dielectric characteristics. Therefore, it is very important to clean the PCB with high impedance circuitry with fresh distilled water.

Signal decoupling

As already noted, noise can enter the analog part of the circuit through the power circuits. To reduce such interference, decoupling (blocking) capacitors are used to reduce the local impedance of the power buses.

If you need to separate a printed circuit board that has both analog and digital parts, then you need to have at least a small idea of \u200b\u200bthe electrical characteristics of logic elements.

A typical output stage of a logic element contains two transistors connected in series with each other, as well as between the power and ground circuits (Fig. 14).

These transistors ideally operate strictly in antiphase, i.e. when one of them is open, then at the same time the second one is closed, generating either a logical one or a logical zero signal at the output. In the steady-state logic state, the power consumption of the logic element is small.

The situation changes dramatically when the output stage switches from one logic state to another. In this case, for a short period of time, both transistors can be opened simultaneously, and the output stage supply current increases greatly, since the resistance of the section of the current path from the power bus to the ground bus through two series-connected transistors decreases. The power consumption increases abruptly and then also decreases, which leads to a local change in the supply voltage and the appearance of a sharp, short-term change in current. Such current changes result in the emission of RF energy. Even on a relatively simple printed circuit board, there may be dozens or hundreds of considered output stages of logic elements, so the total effect of their simultaneous operation can be very large.

It is impossible to accurately predict the frequency range over which these current surges will occur, since the frequency of their occurrence depends on many factors, including the propagation delay of switching transistors in the logic element. The delay, in turn, also depends on many random causes that occur during the production process. Switching noise has a broadband harmonic distribution over the entire range. To suppress digital noise, there are several methods, the application of which depends on the spectral distribution of the noise.

Table 2 lists the maximum operating frequencies for common capacitor types.

table 2

From the table it is obvious that tantalum electrolytic capacitors are used for frequencies below 1 MHz, at higher frequencies ceramic capacitors should be used. It must be remembered that capacitors have their own resonance and the wrong choice of them can not only not help, but also exacerbate the problem. Figure 15 shows typical self-resonances of two general purpose capacitors, a 10 µF tantalum electrolytic and a 0.01 µF ceramic.

Actual specifications may vary from manufacturer to manufacturer and even from lot to lot from the same manufacturer. It is important to understand that for the capacitor to work effectively, the frequencies it suppresses must be in a lower range than the self-resonant frequency. Otherwise, the nature of the reactance will be inductive, and the capacitor will no longer work effectively.

Make no mistake that a single 0.1uF capacitor will reject all frequencies. Small capacitors (10 nF or less) can work more efficiently at higher frequencies.

IC Power Decoupling

Integrated circuit power decoupling to suppress high frequency noise consists of one or more capacitors connected between the power and ground pins. It is important that the conductors connecting the leads to the capacitors are kept short. If this is not the case, then the self-inductance of the conductors will play a significant role and negate the benefits of using decoupling capacitors.

A decoupling capacitor must be connected to each chip package, regardless of whether there are 1, 2, or 4 opamps inside the package. If the op-amp is powered by a bipolar supply, then it goes without saying that decoupling capacitors must be located at each power pin. The capacitance value must be carefully chosen depending on the type of noise and interference present in the circuit.

In particularly difficult cases, it may be necessary to add an inductor connected in series with the power output. The inductance should be placed before, not after, the capacitors.

Another, cheaper way is to replace the inductance with a low resistance resistor (10 ... 100 ohms). In this case, together with the decoupling capacitor, the resistor forms a low-frequency filter. This method reduces the supply range of the op-amp, which also becomes more dependent on power consumption.

Usually, to suppress low-frequency noise in power circuits, it is enough to use one or more aluminum or tantalum electrolytic capacitors at the power input connector. An additional ceramic capacitor will suppress high frequency noise from other boards.

Decoupling of input and output signals

Many noise problems result from directly connecting input and output pins. As a result of the high-frequency limitations of passive components, the circuit's response to exposure to high-frequency noise can be quite unpredictable.

In a situation where the frequency range of the induced noise is significantly different from the frequency range of the circuit, the solution is simple and obvious - to place a passive RC filter to suppress high-frequency noise. However, when using a passive filter, one must be careful: its characteristics (due to the non-ideal frequency characteristics of passive components) lose their properties at frequencies that are 100 ... 1000 times higher than the cutoff frequency (f 3db). When using series-connected filters tuned to different frequency ranges, the higher-pass filter should be closest to the interferer. Ferrite inductors can also be used for noise suppression; they retain the inductive nature of the resistance up to a certain specific frequency, and above their resistance becomes active.

The interference on the analog circuit can be so great that it is only possible to get rid of (or at least reduce) it with the help of screens. To work effectively, they must be carefully designed so that the frequencies that cause the most problems cannot enter the circuit. This means that the shield must not have holes or cutouts larger than 1/20 of the wavelength of the shielded radiation. It is a good idea to allow enough space for the intended screen from the very beginning of the PCB design. When using a shield, you can additionally use ferrite rings (or beads) for all connections to the circuit.

Operational amplifier packages

One case usually houses one, two or four operational amplifiers (Fig. 16).

A single op-amp often also has additional inputs, for example to adjust the bias voltage. Dual and quad op amps have only inverting and non-inverting inputs and outputs. Therefore, if you need to have additional adjustments, you must use single operational amplifiers. When using additional outputs, it must be remembered that they are auxiliary inputs in their structure, therefore, they must be managed carefully and in accordance with the manufacturer's recommendations.

In a single op-amp, the output is located on the opposite side of the inputs. This can make it difficult to operate the amplifier at high frequencies due to the length of the feedback wires. One way to overcome this is to place the amplifier and feedback components on opposite sides of the PCB. This, however, results in at least two additional holes and cutouts in the ground polygon. Sometimes it is worth using a dual op-amp to solve this problem, even if the second amplifier is not used (and its outputs must be connected properly). Figure 17 illustrates the shortening of the feedback loop wires for an inverting connection.

Dual op amps are especially common in stereo amplifiers, and quad op amps in multi-stage filter circuits. However, this has a rather significant downside. Although current technology provides decent isolation between the signals of amplifiers located on the same silicon chip, there is still some crosstalk between them. If it is necessary to have a very small amount of such interference, then it is necessary to use single operational amplifiers. Crosstalk does not only occur with dual or quad amplifiers. Their source can be a very close location of the passive components of different channels.

Dual and quad op amps, in addition to the above, allow for tighter mounting. Separate amplifiers are, as it were, mirrored relative to each other (Fig. 18).

Figures 17 and 18 do not show all of the connections required for normal operation, such as a midrange driver with a single supply. Figure 19 shows a diagram of such a driver when using a quad amplifier.

The diagram shows all the necessary connections for the implementation of three independent inverting stages. It is necessary to pay attention to the fact that the conductors of the half-voltage driver are located directly under the integrated circuit package, which makes it possible to reduce their length. This example illustrates not how it should be, but what should be done. The mid-level voltage, for example, could be the same for all four amplifiers. Passive components can be appropriately sized. For example, size 0402 planar components match the pin spacing of a standard SO package. This allows very short conductor lengths for high frequency applications.

When placing operational amplifiers in DIP packages and passive components with wire leads, the presence of vias on the printed circuit board is required for their installation. Such components are currently used when there are no special PCB size requirements; they are usually cheaper, but the cost of the printed circuit board increases during the manufacturing process due to drilling additional holes for component leads.

In addition, when using add-on components, the dimensions of the board and the length of the conductors increase, which does not allow the circuit to operate at high frequencies. The vias have their own inductance, which also imposes restrictions on the dynamic characteristics of the circuit. Therefore, plug-in components are not recommended for high-frequency circuits or for analog circuits located near high-speed logic circuits.

Some designers, in an attempt to reduce the length of the conductors, place the resistors vertically. At first glance, it may seem that this reduces the length of the route. However, this increases the current path through the resistor, and the resistor itself is a loop (coil of inductance). The radiating and receiving capacity increases many times over.

Surface mount does not require a hole for each pin of the component. However, there are problems when testing a circuit, and you have to use vias as test points, especially when using small-scale components.

Unused sections oh

When using dual and quad op amps in the circuit, some of their sections may remain unused and must be connected correctly in this case. Incorrect connection can lead to an increase in power consumption, more heat and more noise used in the same package of op-amps. The outputs of unused operational amplifiers can be connected as shown in fig. 20a. Connecting pins with additional components (Fig. 20b) will make it easy to use this op-amp during commissioning.

Conclusion

Keep the following key points in mind and keep them in mind when designing and wiring analog circuits.

General:

think of the printed circuit board as an electrical circuit component
have an idea and understanding of the sources of noise and interference
model and layout circuits

Printed circuit board:

use printed circuit boards only from quality material (for example, FR-4)
circuits made on multilayer printed circuit boards are 20 dB less susceptible to external noise than circuits made on two-layer printed circuit boards
use separated, non-overlapping polygons for different lands and feeds
place the ground and power polygons on the inner layers of the PCB.

Components:

be aware of the frequency limitations introduced by the board's passive components and traces
try to avoid vertical placement of passive components in high speed circuits
for high frequency circuits, use surface mount components
conductors should be the shorter the better
if a longer conductor length is required, then reduce its width
unused leads of active components must be properly connected

Wiring:

place analog circuit near the power connector
never route logic wires through the analog area of the board, and vice versa
keep the conductors suitable for the inverting input of the op-amp short
make sure that the wires of the inverting and non-inverting inputs of the op-amp are not parallel to each other for a long distance
try to avoid using extra vias, because their own inductance can lead to additional problems
do not run conductors at right angles and smooth the top corners if possible

Interchange:

use the correct types of capacitors to suppress noise in power circuits
use tantalum capacitors at the power input connector to suppress low-frequency interference and noise
use ceramic capacitors at the power input connector to suppress high frequency interference and noise
use ceramic capacitors at each power output of the microcircuit; if necessary, use multiple capacitors for different frequency ranges
if excitation occurs in the circuit, then it is necessary to use capacitors with a smaller capacitance value, and not a larger one
in difficult cases in power circuits, use series-connected resistors of small resistance or inductance
analog power decoupling capacitors should only be connected to analog ground, not to digital ground

Bruce Carter
Op Amps For Everyone, chapter 17
Circuit Board Layout Techniques
Design Reference, Texas Instruments, 2002

Flat printed coils are most often used in the ranges of meter and decimeter waves to reduce the size of the device. Usually they are made with round, square coils or in the form of a meander, although it is also possible in the form of a polygon. Recently, with the advent of multilayer printed circuit board technology, multilayer coils on a printed circuit board have also appeared. The use of a core made of magnetic material is ineffective, since such a core is removed from the turns of the coil and can change its inductance by 3–5%, which is not enough in most cases. Therefore, printed inductors are used in most cases when no adjustment is required and the inductance value does not exceed units of microhenries.

On our website you can use the online calculator for calculating coils on a printed circuit board

In the Coil32 program, starting from version 9.6, flat printed coils with round and square coil shapes are calculated by the general empirical formula:

L- inductance (µH)
D- outer diameter of the spiral (mm)
d- inner diameter of the spiral (mm)
N- number of turns
Davg- average coil diameter (mm)
φ - fill factor

The coefficients c 1 - c 4 are summarized in the table:

The winding step in the figure is indicated as " s". With unchanged " s"if you increase the width of the turn, the quality factor of the coil and its own capacitance increase. Usually, to minimize the size of the coil, the width of the printed conductor is made close to the distance between the conductors, therefore, in the formula, the influence" s" by the value of the inductance is not taken into account. The optimal value d/D = 0.4 for a round coil and its program selects automatically. For a square coil, the optimal value d/D = 0.362 and its program also selects automatically.

The error in calculating the inductance according to this formula does not exceed 8% for s not more than 3w, i.e. if the gap between the strips is not more than twice the width of the strip.

An inductive element in the form of a straight printed conductor is calculated according to the following empirical formula:

, where:

L- inductance (µH)
l- conductor length (mm)
b- conductor width (mm)

Such inductive elements are often used in UHF filters. Since the self-capacitance of such an inductive element is quite large, it must be borne in mind that it is more correct to represent it as a segment of a long line with distributed parameters. However, for approximate calculations, the simplification of the model adopted here is quite acceptable.

In our turbulent age of electronics, the main advantages of an electronic product are small dimensions, reliability, ease of installation and dismantling (equipment disassembly), low energy consumption and convenient usability ( from English- the convenience of use). All these advantages are by no means possible without surface mount technology - SMT technology ( S face M ount T echnology), and of course, without SMD components.

What are SMD components

SMD components are used in absolutely all modern electronics. SMD ( S face M ounted D evice), which is translated from English as “surface-mounted device”. In our case, the surface is a printed circuit board, without through holes for radio elements:

In this case, SMD components are not inserted into the board holes. They are soldered onto the contact tracks, which are located directly on the surface of the printed circuit board. In the photo below, there are tin-colored pads on the board of a mobile phone that used to have SMD components.

Advantages of SMD components

The biggest advantage of SMD components is their small size. In the photo below, simple resistors and:

Due to the small dimensions of SMD components, developers have the opportunity to place a larger number of components per unit area than simple output radio elements. Consequently, the mounting density increases and, as a result, the dimensions of electronic devices are reduced. Since the weight of the SMD component is several times lighter than the weight of the same simple output radio element, the mass of the radio equipment will also be many times lighter.

SMD components are much easier to desolder. For this we need a hair dryer. How to solder and solder SMD components, you can read in the article how to solder SMD correctly. Soldering them is much more difficult. In factories, they are placed on a printed circuit board by special robots. No one welds them manually in production, except for radio amateurs and radio equipment repairmen.

Multilayer boards

Since in equipment with SMD components there is a very dense installation, there should be more tracks in the board. Not all tracks fit on the same surface, so printed circuit boards make multilayer. If the equipment is complex and has a lot of SMD components, then there will be more layers in the board. It's like a layered cake. The printed tracks connecting the SMD components are located right inside the board and cannot be seen in any way. An example of multilayer boards is mobile phone boards, computer or laptop boards (motherboard, video card, RAM, etc.).

In the photo below, the blue board is Iphone 3g, the green board is the computer motherboard.

All radio repairers know that if you overheat a multilayer board, it will swell up with a bubble. In this case, the interlayer connections are torn and the board becomes unusable. Therefore, the main trump card when replacing SMD components is the right temperature.

On some boards, both sides of the printed circuit board are used, while the mounting density, as you understand, is doubled. This is another plus of SMT technology. Oh yes, it is also worth considering the fact that the material for the production of SMD components takes many times less, and their cost in mass production in millions of pieces costs, literally, a penny.

Main types of SMD components

Let's look at the main SMD elements used in our modern devices. Resistors, capacitors, low-value inductors, and other components look like ordinary small rectangles, or rather, parallelepipeds))

On boards without a circuit, it is impossible to know whether it is a resistor, or a capacitor, or even a coil. The Chinese mark as they want. On large SMD elements, they still put a code or numbers to determine their belonging and denomination. In the photo below, these elements are marked in a red rectangle. Without a diagram, it is impossible to say what type of radio elements they belong to, as well as their denomination.

Sizes of SMD components can be different. Here is a description of the sizes for resistors and capacitors. Here, for example, is a rectangular yellow SMD capacitor. They are also called tantalum or simply tantalum:

And this is what SMD looks like:

There are also these types of SMD transistors:

Which have a large denomination, in the SMD version they look like this:

And of course, how could it be without microcircuits in our age of microelectronics! There are a lot of SMD chip package types, but I mainly divide them into two groups:

1) Microcircuits, in which the leads are parallel to the printed circuit board and are located on both sides or along the perimeter.

2) Microcircuits, in which the conclusions are located under the microcircuit itself. This is a special class of microcircuits called BGA (from English ball grid array- an array of balls). The conclusions of such microcircuits are simple solder balls of the same size.

In the photo below, the BGA microcircuit and its reverse side, consisting of ball leads.

BGA chips are convenient for manufacturers in that they greatly save space on the printed circuit board, because there can be thousands of such balls under any BGA chip. This greatly simplifies the life of manufacturers, but does not make life easier for repairmen.

Summary

What do you use in your designs? If your hands are not shaking, and you want to make a small radio bug, then the choice is obvious. But still, in amateur radio designs, dimensions do not particularly play a big role, and soldering massive radio elements is much easier and more convenient. Some radio amateurs use both. Every day more and more new chips and SMD components are being developed. Smaller, thinner, more reliable. The future, unambiguously, belongs to microelectronics.