What Is a Step Attenuator?
A step attenuator is a circuit or device that can attenuate a signal by a specified amount without distorting it.
It is a device that combines several types of attenuators with different attenuation levels, and the attenuation level can be adjusted by switching a dial. It is used to protect the input stage in measuring instruments such as spectrum analyzers.
Uses of Step Attenuators
Typical applications of step attenuators include signal level adjustment and impedance matching functions.
1. Level Adjustment Attenuator
If the signal level is too high for the subsequent circuit or device when connected, the signal level is adjusted to an appropriate level before being sent to the subsequent stage.
2. Impedance Matching Attenuator
If the impedances of the sending and receiving sides are not matched when connecting an input signal to a subsequent circuit, it may result in noise generation or signal reflections, causing inconvenience to the expected operation.
The step attenuator is used to serve as an impedance-matching function to prevent such inconveniences.
Principle of Step Attenuators
A step attenuator consists of several attenuators connected in series via switches, and the attenuation can be adjusted by switching these switches. By switching the use of each attenuator, a constant attenuation step can be achieved.
For example, if seven different attenuators are used, there are 128 possible combinations of attenuator use, and the amount of attenuation is adjusted by performing signal control for seven bits.
Construction of Step Attenuators
Step attenuators have a simple structure consisting of a combination of resistors in series and parallel. The resistors are combined in T- and pi-circuits, referred to as T- and pi-circuits respectively. The voltage dividing ratio of the resistors determines the amount of attenuation.
Step attenuators also have multiple stages or subordinate connections of T- and π-shaped circuits respectively to achieve a predetermined amount of attenuation. The attenuation can be adjusted by switching the resistance value with a dial.
Additional Information on Step Attenuators
1. Example of Use in TV Receivers
If the goal is to simply attenuate the signal, this can be achieved by inserting an appropriate resistor. However, consider the case of a TV receiver used to evaluate the reception performance of TV broadcasts. To evaluate the reception quality across the 470-710 MHz frequency range used in digital terrestrial broadcasting, a step attenuator is required to attenuate the signal uniformly at all frequencies while maintaining impedance, making resistors inadequate for this purpose.
2. Example of Use as a Measuring Element
Step attenuators can also serve as measuring elements. Here is an example of how a step attenuator can be used to measure the gain of an amplifier:
A voltmeter is connected to the input of the amplifier, while another voltmeter is connected to the output side of the amplifier through the step attenuator. The step attenuator is adjusted until both voltmeter readings match the input voltage.
When the voltmeter readings at the input and output match, it indicates that the attenuation of the step attenuator is equal to the amplification of the amplifier. Consequently, the amplifier’s gain can be measured based on the step attenuator’s set value.