Attenuator Schematic Circuit Diagram

Attenuator Schematic Circuit Diagram

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On: 25 Jul, 2014

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VARIABLE attenuators are used to control power transmission, and they have applications in devices modulators, automatic gain control circuits and radar systems. PIN diodes are commonly used as the ...

VARIABLE attenuators are used to control power transmission, and they have applications in devices modulators, automatic gain control circuits and radar systems. PIN diodes are commonly used as the control element in variable attenuators because of their speed and ease of design. Many types of variable attenuators using PIN diodes have been presented; attenuators, resistive line attenuators, bridged-T attenuators, hybrid coupled attenuators, and others [1]. The hybrid coupled attenuator is frequently used when reflected power must be minimized, and a double hybrid coupled attenuator is especially preferred due to its wider bandwidth, though it requires larger area than a single hybrid coupled attenuator.
A PIN diode is a PN junction diode that has an intrinsic layer between its P-type and N-type layers. The PIN diode behaves as an ordinary PN junction diode at low frequencies, but at high frequencies it behaves as a resistor whose value can be controlled by current. Fig. 1 shows a PIN diode high frequency equivalent circuit. The element is the constant capacitance, which is dependent on the geometry of the intrinsic layer. If the reactance value of is much larger than , the PIN diode can be considered as a pure resistor (when the package inductance, can be ignored). In our design, we used a MA4FCP200 PIN
diode manufactured by MA-COM. The total capacitance of this diode is 0.015 pF and its reactance at 1.9 GHz is approximately 5.6 k, which is large enough to consider it as a pure resistor.
attenuator schematic diagram.png
Using this resistive characteristic, we constructed a variable attenuator by combining the PIN diode with a 0-db branch-line coupler [2], [3]. Fig. 2 shows the schematic diagram for the variable attenuator. The 0-dB coupler is composed of 50-quarter-wavelength microstrip lines. Ideally, the resistance of the PIN diodes can be varied from infinity to zero, which means nodes A, B, and C can be floating or, conversely, connected to ground. When the nodes are floating, the whole circuit acts as a 0-dB coupler itself, and the incident power to port 1 is transferred to port 3 without any attenuation or reflection.

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