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An actual wide-band L pad used to match 50 ohms to 75 ohms. An L pad is a network composed of two impedances that typically resemble the letter capital "L" when drawn on a schematic circuit diagram. It is commonly used for attenuation and for impedance matching.
An attenuator is a passive broadband electronic device that reduces the power of a signal without appreciably distorting its waveform. An attenuator is effectively the opposite of an amplifier, though the two work by different methods. While an amplifier provides gain, an attenuator provides loss, or gain less than unity. An attenuator is often ...
To match the impedances, both cables must be connected to a matching transformer with a turns ratio of 2:1. In this example, the 300-ohm line is connected to the transformer side with more turns; the 75-ohm cable is connected to the transformer side with fewer turns. The formula for calculating the transformer turns ratio for this example is:
A Π-pad attenuator formed from two symmetrical L sections. Because of the symmetry, R 1 = R 3 in this case. For an attenuator, Z and Y are simple resistors and γ becomes the image parameter attenuation (that is, the attenuation when terminated with the image impedances) in nepers. A Π pad can be viewed as being two L sections back-to-back as ...
Characteristic impedance is determined by the geometry and materials of the transmission line and, for a uniform line, is not dependent on its length. The SI unit of characteristic impedance is the ohm. The characteristic impedance of a lossless transmission line is purely real, with no reactive component (see below).
If a quarter-wavelength of 75 Ohm coaxial cable is linked to a 50 Ohm load, the SWR in the 75 Ohm quarter wavelength of line can be calculated as 75Ω / 50Ω = 1.5; the quarter-wavelength of line transforms the mismatched impedance to 112.5 Ohms (75 Ohms × 1.5 = 112.5 Ohms). Thus this inserted section matches a 112 Ohm antenna to a 50 Ohm main ...
In the field of radio frequency (RF) and microwave engineering, by far and away the most common transmission line standard is 50 Ω coaxial cable (coax), which is an unbalanced line. 50 Ω first arose as a nominal impedance during World War II work on radar and is a compromise between two requirements. This standard was the work of the wartime ...
For example, if the DUT has 5 ports and a two port VNA is connected with VNA port 1 to DUT port 3 and VNA port 2 to DUT port 5, the measured VNA results (, , and ) would be equivalent to , , and respectively, assuming that DUT ports 1, 2 and 4 were terminated in adequate 50 Ohm loads . This would provide 4 of the necessary 25 S-parameters.
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