Question

A variable attenuator that provides 0-15 dB attenuator 0.5 dB steps.

Analyze and describe an appropriate attenuator architecture

and specify the number of control bits required to control this device

Answer 1

Attenuators are used to reduce or weaken the signal strength. This process can be carried out in a controlled manner i.e attenuation can be controlled using various parameters.

There are two different types of variable attenuators:

1. Continuous variable attenuators, which have one control current or voltage and gives attenuation within a specific range.

2. Step-controlled attenuators which are controlled by digital control words and give finite attenuation values.

Variable attenuators are made up of solid state elements such as DIODES and FETs and attenuators can be connected in series or in parallel.

For 0-15dB attenuation, Pin diode attenuator can be used which provides attenuation upto 20dB.

PIN diode attenuator circuit is shown as below.

The inductor L1 along with the capacitors C4 and C5 prevent signal leakage from diodes D1 to D2 which reduces the performance of the circuit. Maximum attenuation is achieved when input voltage Vin is minimum. At this point current from the supply voltage turns the diodes D1 and D2 ON which shorts the signal to ground. Diode 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.

PIN diodes are used in attenuators because they have more linearuty compared to normal PN junction diodes.

Digital attenuators are used to change the attenuation by controlling the number of bits. If n is the number of bits in the digital control signal, then the number of attenuation states is 2ⁿ.