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Understanding How Ferrites Can Prevent and Eliminate RF ...

Understanding How Ferrites Can Prevent and Eliminate RF Interference to Audio Systems Jim Brown (K9YC) Audio Systems Group, Inc. Copyright 2005 Audio Systems Group, Inc. All rights reserved. Over the past few years, we ve been learning about some important mechanisms that can combine to cause radio frequency (RF) interference to sound systems. They are: 1. An audio cable will be excited as an antenna by radio signals that are nearby, and current will flow along its length. Most of this current will flow on the shield. An-tenna action will also cause a voltage to be impressed along the length of the cable, which can appear as a common-mode component on the signal conductor(s).

Extensive laboratory work has shown that ferrite chokes make very effective RFI filters if properly applied. This Tech Topic stud- ... Many impedance analyzers express the impedance between their terminals as Z with a phase angle, and the series equivalent RS, and XS. They could just have easily expressed that same impedance using

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Transcription of Understanding How Ferrites Can Prevent and Eliminate RF ...

1 Understanding How Ferrites Can Prevent and Eliminate RF Interference to Audio Systems Jim Brown (K9YC) Audio Systems Group, Inc. Copyright 2005 Audio Systems Group, Inc. All rights reserved. Over the past few years, we ve been learning about some important mechanisms that can combine to cause radio frequency (RF) interference to sound systems. They are: 1. An audio cable will be excited as an antenna by radio signals that are nearby, and current will flow along its length. Most of this current will flow on the shield. An-tenna action will also cause a voltage to be impressed along the length of the cable, which can appear as a common-mode component on the signal conductor(s).

2 2. Improper termination of cable shields within equipment (the Pin 1 problem) injects RF shield current directly into the equipment, where it is detected. 3. Inadequate balancing of shielded cable converts RF shield current to a differential voltage on the signal pair shield-current-induced noise (SCIN). 4. Inadequate low-pass filtering of signal input and outputs lets RF present on the sig-nal conductors into equipment. Equipment can be susceptible to both differential mode voltage (between the signal conductors) and common mode voltage (an equal voltage on both signal conductors).

3 Analysis of these mechanisms shows that RF shield current is a major contributor to all of them, so eliminating or reducing shield current should be the key to eliminating the inter-ference. Experimental work near WGN s 50 kW AM transmitter confirmed this hypothesis. At WGN, virtually all cases of interference in both microphones and input equipment, many of them quite severe, were either eliminated or greatly reduced when a cable con-necting the mic to the input gear was wound around a toroidal ferrite (Fig 1) to form an RF choke that reduces that current. Fig 1 A toroidal ferrite choke Fig 2 Ferrites are made in many forms To test the effectiveness of ferrite chokes in audio RFI applications, selected product sam-ples were ordered, and a series of tests were run.

4 Extensive laboratory work has shown that ferrite chokes make very effective RFI filters if properly applied. This Tech Topic stud-ies the use of Ferrites to Eliminate RF interference to audio systems. Ferrites are ceramics consisting of various metal oxides formulated to have very high per-meability. Iron, manganese, manganese zinc (MnZn), and nickel zinc (NiZn) are the most commonly used oxides. When a ferrite surrounds a conductor, the high permeability of the material provides a much easier path for magnetic flux set up by current flow in the con-ductor than if the wire were surrounded only by air.

5 The short length of wire passing through the ferrite will thus see its self inductance magnified by the relative permeability of the ferrite. The Ferrites used for suppression are soft Ferrites that is, they are not per-manent magnets. Permeability is the characteristic of a material that quantifies the ease with which it sup-ports a magnetic field. Relative permeability is the ratio of the permeability of the material to the permeability of free space. The relative permeability of non-magnetic materials like Understanding How Ferrites Can Prevent and Eliminate RF Interference to Audio Systems Page 2 air, copper, and aluminum is 1, while magnetic materials have a permeability much greater than 1.

6 Typical values (measured at power frequencies) for stainless steel, steel and mumetal are on the order of 500, 1,000 and 20,000 respectively. Various Ferrites have val-ues from the low tens to several thousand. Fig 3 shows complex permeability S and S for a ferrite material optimized for suppression at UHF. [For the engineers among us, = S + j S. Thus S is the component of permeability defining ordinary inductance, and S defines the loss component.] Fig 3 Permeability of a typical ferrite material (Fair-Rite #61) Fig 4a Data sheet imped-ance Fig 4b Over-simplified equivalent circuit of a ferrite choke Fig 4c A better equivalent circuit of a ferrite choke Fig 5 A UHF material (Fair-Rite #61) Data sheets characterize ferrite chokes by graph-ing their series equivalent impedance, and chokesare usually analyzed as if their equivalent circuithad only a series resistance and inductance, asshown in Fig 4a and 4b.

7 The actual equivalent cir-cuit is closer to Fig 4c. We ll learn more about itas we go along. Fig 5 is the manufacturer s data for a cylindrical bead of 5 mm and 23 mm long, defined in terms of the series R and XL. Interestingly, XL goes off the graph above resonance, but it isn t zero. In fact, there is negative reactance contrib-uted by the capacitors in Fig 4c. Below resonance, the impedance of a ferrite choke is proportional to the length of the wire that is enclosed by the ferrite material. Fig 6 shows the impedance of a family of beads that differ primarily in their length.

8 There are also small differences in their cross section, which is why the resonant frequency shifts slightly. Manufacturers vary the chemical composition (themix) and the dimensions of Ferrites to achieve thedesired electrical performance characteristics. Fig5 is data for a sleeve made of a material useful insuppressing RFI above 200 MHz. The materialused for the beads of Fig 6 is optimized for sup-pression at VHF (30-300 MHz). Like all inductors, the impedance of a ferritechoke below resonance is approximately propor-tional to the square of the number of turns pass-ing through the core.

9 Figure 7 is measured datafor multi-turn chokes wound around the toroid ofFig 1 ( x x ). This ferrite isoptimized for the VHF range (30-300 MHz). Fig 8shows data for chokes wound around the samesize toroid, but using a material optimized forsuppression above 200 MHz. The data of Fig 9 arefor toroids of the same size, but wound on a ma-terial optimized for use below 5 MHz. We'll study the LD CD resonance first. A classictext (Soft Ferrites , Properties and Applicationsby Snelling, published in 1969), shows that thereis a dimensional resonance within the ferrite re-lated to the velocity of propagation within the fer-rite and standing waves that are set up in theUnderstanding How Ferrites Can Prevent and Eliminate RF Interference to Audio Systems Page 3 Fig 6 Small cores of different lengths cross-sectional dimensions of the core!

10 In general,for any given material, the smaller the core, thehigher will be the frequency of this resonance,and to a first approximation, the resonant fre-quency will double if the core dimension ishalved. In Fig 4c, LD and CD account for this di-mensional resonance, and RDfor losses within theferrite. RDis mostly due to eddy currents (andsome hysteresis) in the core. Now it's time to ac-count for RC , LC, and CC. Fig 7 Impedance of multi-turn chokes wound on the core of Fig 1 (Fair-Rite #43). Fig 9 Impedance of multi-turn choke on a core of the size/shape of Fig 1, on a ma-terial optimized for performance above 200 MHz (Fair-Rite #61).


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