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The term passband is one that refers to the number of different frequencies, called bands, that can pass through the filters located within frequency systems without being altered or attenuated in the process. This term is often used to refer to the range of all of the frequencies, from the highest to the lowest, that are able to flow through the system they are generated from or transmitted from without being disturbed by any signal or frequency filtering devices. The band of frequencies simply passes through the system, remaining unchanged and thus earning the name passband.
In relation to frequency filters, the passband is the section within the entire spectrum of frequencies that make up the bandwidth signal that is allowed to pass through the system filter and become the output signal. In these filters, there is typically a high frequency tolerance and a low frequency tolerance. This means that, depending on the range of the frequencies passing through the filters, there are two bands of frequencies the filter removes from the frequency spectrum before the signal reaches the output. These types of passbands are related directly to the telecommunications industry, as well as to fiber optic applications and recording acoustics.
Passbands are also related to the field of digital transmissions. They are used mainly in the area of wireless communications and wireless signal devices. The different frequencies generated by digital devices that are transmitted to the receiving device have to be filtered so interference with other wireless devices can be minimized.
The incoming signal to wireless digital devices also has to be filtered upon input, so frequencies from other devices aren’t accepted by the receiving device. The allowed frequencies that are transmitted and received by a wireless digital device is also referred to as the passband. It gets this name because it represents the band of frequencies that is allowed to pass into the system to be produced as output signal.
Whether the frequency is transmitted from an analog device or from a wireless digital one, the result is always the same when the input signal or the output signal is filtered through the system it enters or is transmitted from. There is a band of frequencies the device will accept as input signal and a band of frequencies a device will allow to be produced as output signals. These two bandwidths are referred to as the passband.