A Binary Attenuator

by
Dave Clingerman – W6OAL

Recently I was presented with a surplus three ‘rack unit’ panel that contained three multi value attenuators that were made up of four toggle switches each labeled; I, 2, 4, & 8. I thought it rather strange that an attenuator would be labeled in a binary series. After a little ponderance I realized that the attenuator was a 0 to 15 dB device in one dB steps thusly; 1 we have, 2 we have, then 1 and 2 make 3, 4 we have, 4 and 1 make 5, 4 and 2 make 6. Switches1, 2, and 4 make 7, and we have an 8. Continuing the appropriate addition, we can have one dB attenuation steps all the way to 15 dB in a 50 ohm system. Probably to buy one of these would be rather costly but to make one using 1% resistors (Allen – Bradley) wouldn’t be all that difficult. The rack mount of three of these devices (series-ed) actually would allow me to go from 0 to 45 dB in one dB steps.

Generally as Hams we don’t need this fine an attenuative resolution whereas most of us that do any bench work have pads that are 1, 3, 6 and 10 dB values – and then multiples of 10 or 20 dB to get up to what we want. It occurred to me that if we can acquire 0 to 15 dB in one dB steps why couldn’t we build on the same architecture and create a 0 to 150 dB in 10 dB steps. The series would be 10, 20, 40 and 80 dB. Looking inside the original Allen Electronics attenuator I noticed that all four attenuator sections were PI, the Greek letter p, pads; a series resistor with a shunt resistor on each side of it.

If we were to designate the series resistor “S” and the shunt resistors “P” for parallel, the four required pads would consist of: 10 dB = S96, P71; 20 dB = S248, P61; 40 dB = S2500, P51; 80 dB = S250K, P50 – ohms respectively.

If it is of interest or should one have the interest in building the series sets for the 1, 2, 4 & 8 dB device, they are as follows: 1 dB = S6, P870; 2 dB = S12, P436; 4 dB = S24, P221; 8 dB = S53, P116.

Don – NØYE and I are in concurrence that attenuators of this caliber (home brew or commercial) shouldn’t be used above 1 GHz unless the commercial device is so designated/rated. If you are going to be doing precision measurements above 1 GHz it would behoove you to find a new or surplus device that is rated in the range of which you intend to work.

If folks would like to roll their own 50 ohm, p pads in the range of 3, 6 and 10 dB here are the values: 3 dB = S18, P292; 6 dB = S37, P151; 10 dB = S71, P96. Remember the less lead length you allow the more accurate your pads will be.