The HP 355 Attenuators

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magVarious Members of the HP 355 Attenuator Family


The HP 355 Series VHF-UHF Step Attenuators


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:


Animation Showing a 355D Attenuator, steping from 0 to 120dB


Inside View of the HP 355 Attenuator


hp355_top hp355_bot

mag1HP 355 Top Cover Removed
Showing Cam Mechanism

mag2HP 355 Bottom Cover Removed
Showing Attenuator Cells and
the commercial (green) microswitches


The HP 355 Wiring



mag3Bottom-inside View of the HP 355D Attenuator
Note the gold colored metallic tabs that are used to test-adjust
the capacitance around the resistors and make a performance spec of 1 GHz


HP 355C/D/E/F Schematic Diagrams, from the Operating Manual



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