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Pi element

For attenuators used in RF technology, it is important that their impedance matches that of the transmission media. In order not to impair the transmission behavior of the RF line or RF circuit, attenuators used in RF technology must have identical input and output impedances that are the same as the line impedance. If the input impedance or output impedance differs from the characteristic imp edance of the transmission line, reflections and standing waves may occur.

Attenuators are therefore used for impedance matching; they are therefore designed so that their impedances match those of the cable impedances.

There are two basic circuits, each consisting of three resistors, which derive their name from the arrangement of the resistors. It is the Pi element, whose structure can be associated with the letter "Pi", and the T element, which looks similar to the letter "T". This way of looking at it refers to the asymmetrical construction, in the symmetrical one additional resistors are arranged in mirror image.

Structure of a Pi- and T-element

Structure of a Pi- and T-element

The T-element consists of three resistors, two of which are in series with the transmission line, and a third resistor that connects the contact point midway between the two resistors to the second conductor. Decisive for the attenuation is the ratio of the resistors in series (R1) to the resistor in parallel (R2), decisive for the impedance matching is equality of the two series resistors (R1). If they are not equal, the impedance values of the input impedance and the output impedance change.

A pi element can be thought of as a parallel circuit of the load impedance (Rl) with the voltage divider of resistors R1 and R2, with resistor R2 in parallel.

In addition to the pi attenuator, there is also the pi filter, also known as the Collins filter. In the Pi filter, two capacitors and an inductance take the place of the resistors. Such pi filters are used, among other things, as matching filters in KW transmitting stations.

Englisch: Pi element
Updated at: 29.12.2014
#Words: 328
Links: impedance (Z), transmission, media, indium (In), order
Translations: DE

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