Filters and RF Inductors

Filters Basic Concepts

Filters that contain one or more capacitor elements with paper, plastic film, or ceramic dielectric combined with one or more inductive elements in the form of ferrites or chokes are called LC filters. Ceramic capacitors used are of feed-through type, where one terminal consists of the casing soldered or screwed to the chassis. Or there are SMD variants.

It should also be mentioned that there are RC filters where a metalized paper (MP) capacitor is combined with series resistance. Often the resistor element consists of a metallization alloy with higher resistivity. RC filters are mainly used for arc suppression purposes.

Measurements and Concepts

A filter should eliminate interference signals but transmit adequate capacity and signal currents. Usually, the interference signals are located in the RF range, and the true signal/power currents are in the low-frequency range. Thus the filter should transmit low-frequency signals and suppress those of higher frequencies. We call this the low-pass filter. However, there are other filter types, as shown in Figure 1.

The common low pass filter can be described in Figure 2.

Figure 2. The principle function of the filter.

The figure shows how the interference currents are blocked by an inductor and lead away to earth via capacitors. Thus following filter concepts mainly applies to interference signals and interference frequencies.

There is, however, an important exception. Filters connected between phase and earth in the mains contain those Y safety capacitors. Through them will flow a weak current at the net frequency. It is called leakage current and has to be limited to certain values. The following table 1. shows some generally accepted limits.

Table 1. Leakage current limits.

Attenuation

The attenuation, A, is expressed in dB and is determined in a matched system, i.e., where generator, line, filter, and load have the same impedance, as a rule, 50Ω.

The system has no reflection either from the filter or the load. Then the attenuation A is defined as in eq.[1]:

attenuation calculation eq. [1]

Often the minus sign is left out when we speak of attenuation.

Insertion loss, I.L.

It is very seldom that a filter is matched to the system. Then, in a strict specification, we don’t speak of attenuation, but insertion loss abbreviated I.L.

P1 represents the output power without a filter, and P2 the output power from an inserted filter. The difference includes both specified attenuation and returns losses. Less formally, the expression attenuation is used as a measure of the signal reduction of the filter, irrespective of circuit conditions.

insertion lose calculation eq. [2]

Suppose we apply a load in the circuit that differs from the line impedance, the I. L. will be changed. The attenuation of the filter, in its turn, depends, among other things, on the D.C. current. Maximum DC – whole load operation – impairs inductor inductance. The attenuation decreases. Note, therefore, in catalogs and specifications if the I.L. concerns No load or Full load values.

If the attenuation characteristics are correctly established, the generator side must be effectively shielded from the receiver side. Moreover, it is necessary to use shielded cables, correct connectors, and test fixtures supplied with shielding walls.

The manufacturer’s data over I.L. is guidance. In the practical circuit, however, the attenuation generally is poorer.

Filter Types and Their Mounting

The L.C. filters are built up by their elements in specific basic configurations. The following sequence of figures shows which basic style has to be used in different impedance conditions to reduce the mismatch.

The next Figure 7. shows some examples of how the same basic elements added together in different configurations influence the attenuation in practical filters. All attenuation curves usually start leveling out at approximately 70 – 80 dB. Further attenuation is prevented by inductance and series resistance in the capacitor elements and parasitic capacitance and resistances in the inductor elements. A general recommended filter topology depending on source and load impedance requirements is shown in Figure 8.

Filter Construction

HF / π / EMI / RFI filters

This LC filter comprises its most simple design of elements arranged as in Figure 5a or 5b. The most common types look like the one in Figure 5c. They have a feed-through design, i. e.; the capacitor elements are connected directly to the earth when the filters plated outside are soldered to the housing wall. A typical design of an EMI filter of the p-type is shown in the following Figure 9.

Usually, the π filters are supplied with a collar soldered to the silvered surface (Figure 10.).

Note! If the collar in the filter is in Figure 10. is soldered with a high-temperature solder, check carefully that there is no unintentional solder on the top side of the collar. This would increase the outer diameter of the bushing so it will not fit in the intended hole.

EMI filters are often sealed in metal housings encapsulating hermetically (Figure 11.) or epoxy encapsulation. The capacitor elements consist of concentric chips, so-called discoidal, soldered to the metal housing. The cases are supplied with a threaded part. More powerful filters often are provided with a toroidal choke. They usually are built in π, L, or T configurations. The stated capacitance value is often a minimum value, sometimes abbreviated as GMV, i.e., guaranteed min value, because the specified minimum attenuation is of interest.

Figure 11. Schematic of a typical hermetic EMI filter.

Failure modes

The discoidal design should be preferred to the ceramic tube design. The thin ceramic tube is brittle and has to be protected against mechanical damage and temperature gradients at the soldering. A treacherous source of errors may appear at the soldering moment. If, for example, in Figure
9. or 10. the right part of the lead-through wire is soldered, and the solder heat melts the sealing solder around the filter inlet; the filter will be anchored in the lead-through wire only to its left solder joint.

If we then touch the right part of the lead-through wire – and this will happen during the soldering – we get a bending moment that is transferred to the ceramic tube in its fixed position to the wall. Usually, it cracks. If we run the risk of melting internal solder joints, we should specify high-temperature solder for interior construction. The melting point usually is in the vicinity of 300 °C.

Simpler variants of π filter chips have the same buildup as the filter in Figure 8. Other designs utilize a multi-layer technique for both the capacitor and the ferrite elements. The most simple design has a T-circuit configuration with two ferrites and one capacitor connected to the ground terminal.

Another EMI filter, consisting of integrated ceramic capacitors, is built of a matched and balanced pair of Y capacitors (line to ground) together with an X capacitor (between the lines) connected to a four-terminal device4 as shown in Figure 14. The device is called X2Y.

A principle sketch of the construction looks like as shown in Figure 15.

X and Y capacitors in an X2Y device reflect the patented design shown in Figure 15. These EMI filters are mostly used in lower voltage applications, but larger-sized X2Y devices can also be designed for main applications where they, for safety reasons, require test voltages far above 1 kV.

The EMI filter X2Y has many advantages. The inserted ground planes reduce the current loop compared to standard MLCCs, thus decreasing the inductance of the device. A further inductance reduction is achieved by the opposite current direction in opposed electrodes, which is possible by the inserted ground planes. All these measures mean more effective filtering at higher frequencies.

In Figure 15. above, only two electrodes and three ground planes are shown. Depending on the intended capacitance, the number of electrodes and intervened dielectric layers may be much greater. The EMI filter X2Y is produced in sizes 0603 to 1812. Rated voltages for the Y capacitor elements in NP0 dielectric are 100V, in X7R dielectric 10 to 100V, and in Y5V dielectric 16V. Rated voltages for X capacitors are correspondingly greater. The capacitance ranges from 10 pF in NP0 to 10mF in Y5V dielectric. Apart from the above-mentioned advantages, the design offers a size reduction compared to discrete MLCCs by integrating three capacitors in one device.

Power Line Filters

These filters are often supplied with toroidal chokes and usually have considerably more elements than the simple π filter.

Figure 16. Example of a power line filter circuit with a toroidal choke, a choke in the earth line, and an X- and Y capacitor

LTCC Components and RF Inductors

The use of LTCC (Low-Temperature Co-fired Ceramic) material is gaining importance in applications in the field of communications. This multi-layer process produces sophisticated HF components with good performance characteristics. The ceramic-based LTCC technology is an inexpensive substrate technology with up to 50 layers stacked on top of each other. The silver or gold conductor tracks are impressed on the “green” ceramic foil once the holes for the through contacts have been laser or mechanically generated. The various layers are then collected together and pressed into a pressure chamber. Following lamination, the stack is sintered in a furnace, i.e., burned together at around +850 °C in process furnaces.

LTCC can be used to produce multi-layer modules in which cavities, Rs, Ls, and Cs are integrated. Good thermal conductivity and a low TCE (Temperature Coefficient of Expansion) are achieved. The components are hermetically sealed, i.e., stable against mechanical and thermal stress. The dielectric properties (R and thickness) can be controlled very effectively.

The main advantage of this technology is the high degree of miniaturization and the ever-lower space requirement associated with it. The following simple example illustrates this. Rather than discreetly constructing a low-pass filter, there is the option of integrating the inductance and capacitance, e.g., as package type 0603. Integration is an essential aspect in the field of RF applications.

Low-Pass Filter

HF components based on LTCC technology are designed for frequency bands from around 900 MHz to 6 GHz. These components’ application areas are Wireless LAN, Bluetooth®, HomeRF, GPS, PCS, GSM, DECT, and PHS.

The high-precision LTCC technology gives rise to low-loss RF components with reproducible, guaranteed properties and a low space requirement.
LPF low-pass filters for diverse applications are produced in the package types 0603 to 0805. The limit frequencies of these compact low-pass filters range from 900 MHz to 5.5 GHz. The insertion loss of the low-pass filter in the admission range is shallow. The low-pass filters show comparably high attenuation in the blocked content.

To effectively suppress frequencies in the mobile phone range, the attenuation from 900 MHz (GSM900) to 2.1 GHz should be as high as possible. The attenuation in the WLAN/HyperLAN range should also be increased. For this reason, specific poles have been placed at these frequencies for the above filter.

Balun BAL

Symmetry transformers (baluns) with low insertion loss and low amplitude and phase differences can be achieved in LTCC technology. The asymmetrical (unbalanced) impedance is 50 Ω, and the symmetrical (balanced) impedance can be 50 Ω, 100 Ω, or 200 Ω. Two types of baluns are distinguished: LC type and Marchand type. In the case of the latter, no DC bias may be applied. Connecting a capacitor to the ground in front of the pin where the DC voltage is input is recommended. Information on external circuitry and layout recommendations may be found in manufacturers’ datasheets.

Balun with Integrated Band-Pass Filter

A band-pass filter and a balun can be integrated into a component to reduce the space requirement. Besides the high attenuation in the blocking range, low insertion loss is also achieved. The admission range of this component lies typically in the frequency range 2400–2500 MHz. Numerous other features, such as diplexers, couplers, and couplers with an integrated low-pass filter or dual-band baluns, are available in this multi-layer technology on the market.

The electrical parameters of the chip antennas are measured with test boards, as shown in Figure 21. Many factors, e.g., the board type (dielectric constant) and the layout, affect the electrical properties of a chip antenna. The component values for the matching network shown in the datasheet cannot be assumed for this reason. These values only apply to the test boards used; they may be used as starting values for determining the matching network in the application. The return loss can be significantly improved by matching the antenna.

To achieve the performance shown in the datasheet, it is important to ensure sufficient separation between the antenna and the ground plane. The antenna is preferably placed at a corner point of the board to avoid the antenna is completely surrounded by a ground plane. Deviations from the suggested layout instructions can lead to alignment characteristics and input impedance changes. In general, care must be taken that the separation between the long side of the antenna and the ground plane is at least 5 mm. The antenna surface and housing separation must be at least 1 mm.

The microstrip feed lead can be considered as part of the antenna system. Connecting the ground plane surrounding the input lead on the edges to the lower ground with through contacts is recommended. This minimizes the electrical field on the edges and influences the antenna.

The efficiency η is the ratio between the radiant power and the input power. The efficiencies of the chip antennas derived from the numerical calculations are presented in the following table. This includes antennas operating in the frequency range 2400–2500MHz. Essential differences are the antenna dimensions and the gain. The standing wave ratio is a maximum of 2 for all antennas within the bandwidth.

RF Inductors

Air Coil KI; RFH; RFI

The classic inductor is a configuration of one or more wire windings on a magnetically non-conducting coil former. In contrast to coils with magnetic coil formers or cores, this component‘s advantage is in its current independent inductance and the fact that it shows no saturation effect. Further very important advantages are the linear impedance over a wide frequency range, eq. [4] and a very high-quality factor Q (Typ. 50–100). The relationship between an inductance L and the winding turns n can be approximated as shown in eq. [5].

The air coil has a parasitic capacitance dependent on winding diameter, wire thickness, layer, and winding structure, mainly between adjacent windings (Figure 22.). Still, SMD components also occur between both the connection pads.

The self-resonant frequency, above which the component shows capacitive characteristics, results from the inductance and the parasitic capacitance.

The series of KI ceramic SMD inductors are examples of air coils (Figure 23.).

The solder pads on the coil former are designed to be very small (Figure 24.) so that the inductors to achieve a high self-resonant frequency. Self-resonant frequencies of well above 10 GHz are thus attained.

If a higher current carrying capacities are required, the components can be wound on ceramic or ferrite cores depending on the inductance value.

RF Multilayer Ceramic Inductor

The construction of an SMD inductor (Figure 25.) differs from that of a conventional air coil in that the “coil” is printed on a ceramic substrate. This presents the following advantages:

• Very low inductances are achievable
• The very low parasitic capacitance and, therefore, high resonant frequency
• Relatively high current carrying capacity of typ. 300 mA

The specifications of the 0201 multilayer ceramic SMD inductor are shown in Figure 26.

These components’ preferred field of application lies in the high-frequency range, filter circuits, and oscillator circuits.

Source: EPCI