Ramazan Rıdvan SEKMEN, 030080083, 9th week words# part1

1-Attenuator ( Group: Communication System)

Attenuators are either fixed or variable circuits to reduce signal amplitudes and/or improve return loss, while maintaining proper input and output impedance, of stages they are attached to. Attenuators are used extensively in wireless design.
Attenuation can be varied in discrete steps by a manually turned knob, or by electronic control. Other variable attenuators are actualy inserted between stages on a PCB, and can be either analog voltage or current controlled, with infinite attenuation resolution, while a digital step attenuator will have a limited number of discrete steps. All attenuators are rated for the maximum amount of attenuation they are capable of, along with their maximum frequency and input signal strength levels.

(Complete wireless design, Cotter W. Sayre, p.357)

New and better explanation

An attenuator is a device that reduces the amplitude or power of a signal without distorting its waveforrm. As opposed to an amplifier. an attenuator provides loss instead of gain. Attenuators ale frequently used for gain control. For example, in a multichannel communication system, the gains through different channels need to be equalized. Attenuators are also used in phased array antennas and smart antenna systems.
      Attenuators are usually passive devices made from resistors. The resistive characteristics of the PIN diode and the transistor can he used for this purpose. Attenuators are typically implemented in digital form. A digital attenuator consists of a cascade of attenuation units, each having an on or off state. When all the units arc switched on, the maximum attenuation is achieved; when all the units are off, the system has the minimum attenuation, which is known as the zero-state or reference insertion Ioss of the attenuator. Switches are the most important components in a digital attenuator.
      Attenuators are required to provide the specified attenuation over a certain frequency band. In applications such as phased arrays, the insertion phase is required to he invariant for all possible attenuation states. The GaAs dual-gale FET can be used to achieve equiphase but different gain paths [41], where gate 1 is used for gain level control and gate 2 for on-/off-state control.
      Analog auenuators can provide a continuous amplitude control. This is desirable for correction of any degradation arising from fabrication. Analog attenuators are usually used in AGC control loops and adaptive beamforming systems, and as variable taps in direct implementation of digital filters. Various types of switched attenuators and analog reflection-type attenuators ate described in [41].       
     The resistance characteristic of the forward biased PIN diode, as given by (11.111) can be used in an attenuator . The PIN diode attenuator is extensively used in AGC and RF leveling applications. It can make the form of a simple series or shunt mounted diode acting as a lossy reflective switch. or a more complex structure that maintains a constant matched input impedance across the full dynamic range of the attenuator [47].

( Ke-Lin Du, Swamy, M. N. S.(). Attenuators . Wireless Communication Systems: From RF Subsystems to 4G Enabling Technologies (p. 424). )