Organizing the Engineering Model
To understand the various practical modeling techniques employed with ferrite, it is best to prepare a properly engineered calculation of expected results. An empirical trial and error method may leave the circuit close to borderline performance without adequate safety margins. As indicated previously, a wide range of formulations is possible. The major application factors to be used when defining a specific ferrite solution for a particular interference problem include the following:
- Frequency where maximum attenuation is required.
- Amount of attenuation needed.
- Ferrite permeability formulation characteristics as they relate to the frequency range in question (i.e., initial permeability)
- Ferrite formulation consistency (i.e., expected range of variation in attenuation performance)
- Installation environment and mechanical attachment requirements.
The frequency range requiring attenuation must be matched to the performance of a given ferrite composition (figure 1 on previous page). The optimum profile would be a ferrite in which the highest attenuation level coincides with the disruptive frequency (A). That same ferrite could be used even if the frequency falls in a lower area of its impedance curve (B) but there would be correspondingly reduced attenuation. Conversely, a different ferrite formulation could be employed in the same frequency situation with the intent of using a lower part of its performance curve (C). Space and weight considerations are not normally a concern since good quality ferrites provide high performance per a given cubic volume.
The modeling procedure to calculate impedance characteristics of the source and load coupled with the ferrite suppressor is developed as follows:
Even though the same unit of ferrite is used, the attenuation provided by a ferrite suppressor can differ somewhat as the original circuit impedance varies. The ferrite is more effective when the circuit impedance is low. For example, by using the same 250 ohm ferrite in a 75 ohm circuit, the result will be:
With a high circuit impedance, it may be possible to increase the number of turns or passes through the ferrite (figures 3 and 4), or to use a larger amount of ferrite (cubic volume) in the circuit in order to achieve the same level of insertion loss (fig. 2). By increasing the number of turns (passes) through the ferrite opening, the “effective magnetic path” is increased _ impedance then increases by the square of the number of turns (N2 ) ; i.e., two turns (22) = 4 times the impedance. When additional ferrite volume is added, impedance increases on almost a direct percentage basis; i.e., a 100 percent increase in volume will provide about 100 percent increase in impedance (figure 2) in most situations according to certain prescribed dimensional ratios.
An alternative modeling procedure may also be structured in reverse by solving for a desired insertion loss goal. The result yields an impedance requirement. This can be matched to known performance profiles of existing ferrite configurations in the geometric style best suited for mechanical and packaging requirements.
Next, refer to the Attenuation Properties on page 36. The flat ribbon cable style part that closely matches is #28B2480 with a 250½ impedance at 100 MHz.
Once the ferrite suppressor is installed in the circuit, results should be confirmed by testing. Although these ferrites are “linear,” the term is relative to the common operating range of temperatures. The permeability is different at every degree of temperature. The published initial permeability (µi ) nomenclature applies to standard temperature, 59°F (15°C) only. There are only minor impedance differences, however, throughout normal operational ranges and up to 180° F (82°C). See Material Properties on page 33.