Waugh attenuator

Updated July 5, 2011

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New for June 2011! This topic was suggested by Chin-Leong, who was mentored by Ray Waugh. At Chin-Leong's request, we have placed Ray Waugh in the Microwave Hall of Fame!

Ray Waugh's attenuator is a PIN diode network that provides voltage-variable attenuation over a broad bandwidth. By replacing the series resistor in the Pi network with two diodes, three benefits result:

1. First, since the maximum isolation of the network is set by the capacitive reactance of the series diode(s), the use of two diodes in place of one will increase the maximum attenuation or double the upper frequency limit for a given value of attenuation.
2. The twin diodes which occupy the position of the series resistor are physically set up 180° out of phase, resulting in the cancellation of even order distortion products.
3. The resulting attenuator network is symmetrical and the bias network is substantially simplified.

Below is the PIN diode attenuator, before Waugh. Note the excessively-complicated bias scheme, and the asymmetry. You can also learn more from page 152 of Ulrich Rohde's online book, RF/Microwave Circuit Design for Wireless Applications.

Voltage-variable attenuator using PIN diodes prior to Waugh

Here is a schematic of Waugh's improved attenuator:

Waugh's attenuator

You can learn all about this network from Agilent's App-Note AN1048, A Low-Cost Surface Mount PIN Diode Attenuator. You can easily find this through a Google search. The Agilent semiconductor division has since become Avago. AN1048 is where we cribbed the two schematics from, we hope Agilent (or Avago) doesn't mind, but we suspect that Ray Waugh won't!

Waugh's attenuator is also described in:

Waugh, R. W., "A Low Cost Surface Surface Mount PIN Diode PI Attenuator,"
Microwave Journal, vol. 35, no. 5, May 1992, pp 280-284.

Ray's biography is from his paper which appeared in Applied Microwave & Wireless: Raymond W. Waugh is diode applications engineer for the Wireless Semiconductor Division of Hewlett-Packard Company. He is a Senior Member of the IEEE, a member of the Microwave Theory and Techniques Society, and is on the editorial board for IEEE Transactions. He is also a member of SAE. Waugh obtained his BSEE degree from the University of Michigan in 1962 and his MSEE from UCLA in 1968. He has written or co-written approximately 20 papers, and has conducted diode design seminars in Europe, Asia and the United States.

In addition, we have learned that Waugh was anti-patent, so his novel attenuator was adopted quickly. His four-diode attenuator is the most ubiquitous wideband attenuator configuration in the industry.

Waugh's configuration has become the de-facto industry standard for multi-octave, compact, high linearity attenuator. His invention solved the asymmetrical drive requirement of the previous three-diode attenuator and also improved linearity by distortion self-cancelling in the anti-series PIN diode arm.

Waugh served the society in the Editorial Board of MTT Transactions 1999-2000.