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AMATEUR RADIO BY GARRY CRATT, VK2YBX Ferrites - how they work & why they are used In many RF applications where large values of inductance are required in physically small areas, air spaced inductors cannot be used because of their size. The solution is to use ferrite-cored inductors. One way of decreasing the size of a coil while maintaining a given inductance is to decrease the number of turns but increase the magnetic flux density. This flux density can be increased by decreasing the reluctance, or magnetic resistance path, between the windings of the inductor. It's possible to do this by adding a magnetic core material, such as iron or ferrite , to the inductor. The permeability (µ) of either of these materials is much greater than of air and thus the magnetic field is not as "reluctant" to flow between the windings when compared to an air-spaced inductor. The net result of adding some -kind Fig.1: typical magnetisation curve for a ferrite core. Note .that once the material is magnetised, it exhibits a degree of hysteresis, as indicated by the dotted curves. of magnetic core to an inductor is the ability to produce a given inductance with less turns. There are several advantages in doing this: (1) smaller size; (2) increased Q (less turns means less DC resistance); (3) variability - this can be obtained by physically moving the core through the windings. However, such an approach requires careful selection of the core for a particular application. For example, if the core permeability is excessively high for the frequency at which the inductor is used , the circuit will be more sensitive to temperature variations (ie, temperature variations will cause excessive variations in the value of the induct- BsATl--------:::=--oi't""',-- ,,, / I I I I I I I I I I 88 SILICO N CHIP _,,..,...,. HsAT H (AMPERE TURNS/METRE) ance). Also, if the permeability is too high for the frequency of operation, saturation of a magnetic core may result, which again changes the value of the inductor. All magnetic core materials tend to introduce loss. The correct material must be chosen for the appropriate frequency. If incorrect material is used, it may make no difference to the realised inductance as the core may appear "transparent" if its permeability approaches that of air. In addition, if the permeability is too high, core saturation may result. Magnetisation curve Fig.1 shows the typical magnetisation curve for a magnetic core. The curve simply indicates the magnetic flux density (BJ that occurs in the inductor with a specific magnetic field intensity (H) applied. As the magnetic field intensity is increased from zero (while increasing the applied signal voltage), the magnetic flux density between the turns of the inductor increases linearly. The ratio of the magnetic flux density to the magnetic field intensity is called the permeability of the material. At this stage, we could branch into a mathematical discussion relating to the calculation of permeability. However, all we need say is that the permeability of material is a measure of how well it transforms an electrical excitation into a magnetic flux. The better it is at this transformation, the higher the permeability. For our application, we need to keep the excitation level low enough to maintain operation in a linear portion of the curve. Any further increase in excitation may cause core saturation, A large range of ferromagnetic cores is available from Stewart Electronics Components Pty Ltd, PO Box 281, Huntingdale, 3166. Phone (03) 543 3733. at which point no further increase in magnetic flux density can occur. Saturation The magnetic flux density at which saturation occurs (BsATl is specified by manufacturers and varies substantially from core to core, depending on the size and shape of the material. It's important to know the BsAT for a particular core as this will determine the suitability for a particular circuit. As the BsAT is a published figure, we need to know the in-circuit operational flux density (Bop). This can be mathematically determined by the formula shown below. Bop= Ex 10 8 / 4.44(fNAel, where Bop = magnetic flux density in Gauss E = maximum RMS volts across the inductor f = frequency in Hz N = number of turns Ae = cross sectional area of the core in cm 2 • If the calculated Bop for a particular application is less than the published specification to be set for a particular core, then the operation will be largely linear and the core will be suitable for the application. There are really no fixed rules governing the use of ferrite cores versus powdered iron cores in RF circuits. In many instances, given the same permeability and type, either type could be used without any change in performance. But there are some exceptions to this rule. Powdered iron cores can typically handle higher RF power without saturation core damage than the same size ferrite cores. For example, ferrite tends to retain its magnetism permanently if driven with a large amount of RF power. This means a permanent change to the characteristics of the permeability. By contrast, powdered iron will eventually return to its initial permeability if overdriven. So in any application where high RF power levels are involved, iron cores might seem to be the best choice. Also, in general, powdered iron cores tend to yield higher Q inductors at higher frequencies than the same size ferrite core. This is due to the inherent core characteristics of powdered iron, which produces much less internal loss than ferrite. This characteristic of powdered iron makes it very useful in narrow band or tuned circuit applications, as. commonly encountered in receivers and transceivers. Table 1 shows various types of powdered iron material and their frnquency classification. However, ferrite cores have a significant advantage and that is that their permeability is much higher than for the same size powdered iron core. This means that a coil of given inductance can usually be wound on a much smaller ferrite core and with fewer turns. And this in turn means that less circuit board area is used. General composition Most readers can imagine the composition of a powdered iron core but few may be aware of the nature and composition of ferrite. The general composition of ferrites used for magnetic cores is a ceramic iron oxide with the general formula MeFe 2 O4 , where Me represents one or several of the divalent transition metals such as manganese, zinc, nickel, cobalt, copper, iron or magnesium. The most popular combinations are manganese and zinc or nickel and zinc. These compounds exhibit good magnetic properties below a defined temperature called the Curie Temperature (CT). These materials can easily be magnetised and have a very high intrinsic resistivity. Such material can Table 1: Powdered Iron Materials Material Properties Applications Carbonyl C Medium Q at 150kHz; high cost AM tuners; low frequency IF transformers Carbonyl E High Q & medium permeability from 1-30MHz; medium cost IF transformers, antenna coils, general purpose designs Carbonyl J High Q at 40-100MHz; medium permeability; high cost FM & TV circuits Carbonyl SF High Q to 50MHz Similar to Carbonyl E Carbonyl TH Higher Q than carbonyl E up to 30MHz, but less than carbonyl SF Similar to carbonyl E Carbonyl W High Q to 100MHz; medium permeability; high cost FM & TV circuits Carbonyl HP Excellent stability & good Q Low frequency applications to 50kHz Carbonyl GS6 Good stability & high Q Commercial broadcast frequencies IRN-8 Good Q from 50-1 S0MHz; medium priced. FM & TV circuits A UGUST 1991 89 2000 F8 1000 ~ i!: :::; ia ~ :lo (a) TYPICAL INDUCTOR (b) TOROIDAL INDUCTOR Fig.2: toroidal inductors radiate far less than conventional inductors since the magnetic flux is contained within the material itself. Fig.3 (right): this graph shows the optimum frequency ranges for various grades of ferrite. be used to very high frequencies without laminating, as would normally be required when using other magnetic metals. Manufacturing process The manufacturing process for ferrite is quite remarkable. The raw materials used are oxide or carbonates of the constituent metals. The final material grade determines the necessary purity of the raw materials to be used. The base materials are weighed in the correct proportions required for final composition and the powders then mixed to obtain a uniform distribution. Finally, the mixed oxides are calcined at approximately 1000°c. This process is called "sintering" and consists of mixing metal powders having different melting points, and then heating the mixture to a temperature equal to the lowest melting point of any of these metals. A solid state reaction then takes place between the constituents and a ferrite is fm;med. Pre-sintering is not essential but provides a number of advantages during the remainder of the production process. Pre-sintered material is milled to a specific particle size, usually in a slurry with water. A small proportion of organic binder is added and then the slurry is spray dried to form granules suitable for forming. Most ferrite parts are formed by pressing. The granules are poured into a suitable die and then compressed. The organic binder acts in a similar way to an adhesive and a so-called "green" product is formed. This is still very fragile and requires sintering to obtain the real ferrite properties. 90 SILICON CHIP 500 F14 200 F16 ffi 100 a. ....< E :!a F25 50 F29 20 10 0.1 0.2 0.3 0.5 For some products (eg, long rods or tubes), a material is mixed into a dough and extruded through a suitable die. The green cores are loaded on refractory plates and sintered at a temperature between 1150° -1300°C, depending on the ferrite grade. A linear shrinkage ofup to 20% takes place (note: the material can be cut to length either before or after sintering). Sintering may take place in tunnel kilns having a fixed temperature and atmosphere distribution, or in box kilns where temperature and atmosphere are computer-controlled as a function of time. The latter type is more suitable for high grade ferrites. After sintering, the ferrite core has the required magnetic properties and dimensions typically within 2% of nominal size (because of variations in shrinkage). Toroids The self-shielding properties of a toroid become evident when Fig.2 is examined. In a typical air-cored inductor, magnetic flux lines linking Large split ferrite shields can be used to suppress noise in computer ribbon cable. The unit shown here is available from Stewart Electronic Components Pty Ltd. 2 3 5 10 20 30 50 100 200 300 FREQUENCY (MHz) the turns of the inductor take the shape shown in Fig.Za. This clearly shows that the air surrounding the indµctor is definitely part of the magnetic flux path. Thus , the inductor tends to radiate the RF signals flowing through it. A toroid on the other hand (see Fig.Zb) completely contains the magnetic flux within the material itself, and thus no radiation occurs. This characteristic of toroids eliminates the need for bulky shields around an inductor. These shields not only reduce available space but they also reduce the Q of the inductor that they are shielding. Ferrite heads Most readers will also be aware of suppression beads which are manufactured from relatively high permeability ferrites and then threaded onto wire leads. At frequencies well beyond the normal operating range, these beads provide a series impedance, the resistive component of which acts as an imaginary resistor in series with the circuit being protected, while the reactive component looks like a series inductance. Suppression beads are used in this manner to prevent high frequency leakage and to prevent parasitic oscillation arising from spurious feedback. They are also used for the suppression of interference. This form of protection is possible because at frequencies far removed from the normal range of application, the losses in ferrites are very high. A ferrite bead threaded onto a lead produces no noticeable direct affect on the operation of equipment because at low frequencies, th e series impedance is very low. But while the bead has no effect at low frequencies, it acts as a suppressor at very high frequencies. This is because the losses in the ferrite become high at high frequencies. At the same time, the reactance generally increases with frequency in spite of a gradual loss of permeability. This decrease in permeability becomes noticeable at frequencies 1020 times higher than the upper limit of the normal range of application. Fig.3 shows the optimum frequency ranges for various grades of ferrite. When using ferrite as a suppression bead, it is important to use a grade where the impedance is high at the frequency we wish to suppress. The series impedance of a wire threaded through a bead is proportional to the length of the bead or the number of beads used. Alternatively, several turns of wire can be wound through the bead to produce a higher impedance. This technique is often used at VHF. Many popular electronics outlets also stock a 6-hole suppression bead which provides even more protection. So it can be seen that ferrites can be used to eliminate all sorts of interfering signals due to the high losses in ferrite material at high frequencies. It has also been demonstrated that ferrites can play a valuable part in the design of modern communications equipment, as they allow a reduction in circuit board area due to the shielding effect. Some communications equipment specialists even stock a range of feed-through capacitors with built-in ferrite beads. This combination forms a re-section filter and is ELECTRONICS WORLD New Universal Remote Control * Replaces up to five separate audio/ These feedthrough capacitors come with built-in ferrite beads & make very effective n:-section filters where space is limited. very effective where space is limited. In addition, "split ferrite shields" are now available for use on computer ribbon cable (see photo). Another simple yet practical use for ferrite is the "magic wand". This uses a ferrite slug attached to one end of a piece of PVC tubing and a brass grub screw at the other end. To determine if a coil in a circuit requires more or less inductance for optimum operation, either slug is inserted into the coil. The ferrite slug will cause an increase in inductance while the brass slug causes a decrease. So the magic wand can be used as a tuning aid, when adjusting tuned circuits. Further reading (1) Neosid Magnetic Components Catalog. (2) "Ferromagnetic Cores", Stewart Electronics Components Pty Ltd .. (3) "Ferrites"; Siemens Databook. (4) "Magnetic Products Data Handbook - Soft Ferrites"; Philips Components. (5) "RF Circuit Design"; Sams Books . (6) "Ferrite Cores-2 For Telecommunications & Industry Fields"; TDK Databook. SC A simple tuning wand can be made by attaching a ferrite slug to one end of a piece of PVC tubing and a brass grub screw to the other. video remote controls. * A total of 85 total commands available. * LCD display shows functions . * Alarm/countdown, timer/clock * Bk memory. Was $79.95 Now $63.95 12 volt DC / 12 watt P.A. Amplifier Was $109.95 Now $89.00 240V AC/ 12 volt DC 15 watt P.A. Amplifier Was $164.95 Now $129.00 Microphone to suit P.A. DM 626 $15.95 100 metere speaker cable $16.95 12 volt Blue strobe light $32. 95 Portasol portable butane powered soldering iron $39.95 Miniscope soldering iron Not $74.95 but only $59.95 Superscope soldering iron Value at $64.95 now only $54.50 Scope 3.3 volt<at> 30A transformer Reduced from $79.95 to $65.00 Full range of Scope parts available for Miniscope and Superscope soldering irons. Mail Orders and Retail Sales Electronics World 30 Lacey St, Croydon VIC, 3136. Telephone: (03)723 3860 (03)723 3094 Fax: (03)725 9443 Disposal bargain store at 27 The Mall Sth. Croydon, Vic, 3136 Telephone: (03) 723 2699 Sorry no transmitting equipment available at the disposal store /\ UGUST 1991 91