This device is composed of cells, each having a pair of single-throw, double-pole mechanical switches. Between one pair of switch throw positions is a low-loss through path. Between the alternate pair of throw positions is a precision attenuator. By cascading three or four cells, precision “steps” of attenuation can be switched in or out. A common arrangement includes a 10 dB, 20 dB, and two 40 dB cells. By switching different combinations of cells, 10 dB steps from zero to 110 dB can be had. Since an ALC or AGC network can easily adjust itself over a 10 dB range, it is now possible to continuously adjust power or gain over a 110 dB range.
The mechanical step attenuator is still an appropriate choice where the lowest insertion loss in the “0 dB” state is necessary, or where any degradation in linearity performance is unacceptable. That said, there are drawbacks with a mechanical attenuator. By its very nature, it is prone to mechanical fatigue, and ultimately failure. A great deal of materials and process engineering is dedicated to the high reliability devices found in many of today’s microwave test equipment. The Engineering and Manufacturing costs for these devices are not insubstantial; and it is not unusual to see prices for the highest performance models to top $4,000.
Another inherent drawback for the mechanical step attenuator is the time it takes for the signal amplitude level to settle to its final value. The mechanical switch is spring-loaded, and as such is subject to “chatter”, or it experiences the intermittent closing and opening of the switch as it changes state. Typically, this time interval is measured in milliseconds, and is often specified in the 20 ms range for high performance mechanical step attenuators. However, once the mechanical switch is “closed”, typically the amplitude settling occurs very quickly.