HP 355 Cam Mechanism

Developed by Art Fong, and Harley L. Halverson for the HP 606A Signal Generator, the HP 355A/B step attenuators were introduced as separate measurement accessories in the 1958 general catalog. They were exceptionally popular because of their wide frequency and attenuation range, and moderate price.

Was this apparently simple product concept of using commercial microswitches up to 1000 MHz (HP 355C/D), a great innovation? The simple answer is that the HP 355 attenuator family stayed as a viable product for FORTY SIX YEARS.

The HP 355C/D/E/F were still for sale in the Agilent Technologies, 2003/2004 general catalog. The original design of Art and Harley would remain constant, and tens of thousands of Model 355 step attenuators were delivered to customers, or integrated in a number of instruments manufactured by Hewlett Packard for a half century. A remarkable product longevity. Special custom versions were available for mounting behind panels of customer test sets.

The animation below shows how the cam-actuated microswitches
switch in a pi-type attenuator pad or bypass it in the HP 355D: