Voltage Controlled Variable RF Attenuator

Voltage controlled variable attenuators are often needed in RF designs - this design provides a constant impedance and uses PIN diodes.

RF Attenuators Includes:
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Voltage controlled attenuators are often required within radio frequency designs to be able to provide a variable level output.

These voltage controlled attenuators often use PIN diodes and provide a variable level of attenuation dependent upon the voltage applied.

In any RF system, there is a need for a constant impedance and this design provides an acceptable match to 50Ω over a wide range of attenuation levels.

Variable attenuator circuit

The PIN diode variable attenuator is used to give attenuation over a range of about 20 dB and can be used in 50 ohm systems. The inductor L1 along with the capacitors C4 and C5 are included to prevent signal leakage from D1 to D2 that would impair the performance of the circuit.

The maximum attenuation is achieved when Vin is at a minimum. At this point current from the supply V+ turns the diodes D1 and D2 on effectively shorting the signal to ground. D3 is then reverse biased. When Vin is increased the diodes D1 and D2 become reverse biased, and D3 becomes forward biased, allowing the signal to pass through the circuit.

Variable voltage controlled RF attenuator using three PIN diodes — Voltage controlled variable attenuator

Typical values for the variable RF attenuator circuit might be:
+V: 5 volts
Vin: 0 - 6 volts;
D1 to D3: HP5082-3080 PIN diodes
R1: 2k2
R2: 1k
R3: 2k7
L1: is self resonant above the operating frequency, but sufficient to give isolation between the diodes D1 and D2.

These values are only a starting point for an experimental design, and are only provided as such. The circuit may not be suitable in all instances.

Choice of PIN diode

Although in theory almost any diode could be used in variable RF attenuators, PIN diodes have a number of advantages.

Linearity: PIN diodes are more linear than ordinary PN junction diodes. This means that in their action as a radio frequency switch they do not create as many spurious products and additionally as an attenuator they have a more useful curve.
Isolation: When reverse biased and switched off, the depletion layer is wider than that of an ordinary diode and this provides for greater isolation when switching or providing higher levels of attenuation.

This PIN diode based voltage controlled variable attenuator is simple and easy to use. It provides an acceptable match to 50Ω over the operating range and with a little experimentation it can be made to fit many applications.