Sunday, February 7, 2016

Quick PCB: Pi Attenuator

This tiny PCB can be used to make a useful Pi attenuator for your bench.

Tiny RF Pi attenuators using surface mount resistors.


Attenuators are useful when you need to reduce a signal's strength by a specific amount. Flatness of the attenuator's response is very important to ensure predictable signal levels across the entire bandwidth of operation. Maintaining the impedance match is also important. You can buy high quality attenuators from manufacturers like Mini-Circuits, but sometimes it's more fun or more expedient to make your own. Luckily you can make very low-cost, low-power attenuators that will have sufficient performance for general use in your workshop.

Alan, W2AEW, has an excellent YouTube video on the theory of Pi attenuators and how to design them. Check out his video. I'll also post a link to an online calculator to aid the design process.



Pi Attenuator Schematic

R1 is in parallel with R2, and R4 is in parallel with R5. This allows you to get odd resistor values needed for some attenuator designs by placing common value resistors in parallel. The back of the board has a white silkscreen box for writing in the amount of attenuation as a reminder.

Order PCBs on OSHPark:


Parts List:

R1 through R5: 0603 or 0805 thin-film SMT resistors
SMA Connectors: Edge-Launch SMA for 1.6mm/0.062" board thickness

Using SMT components for these attenuators allows you to achieve best performance at higher frequencies. These boards have a generally flat response up to about 2GHz. Be cautious of the signal levels you put through these and the power ratings of the small SMT resistors.

With some of these boards, SMA connectors, and a book of SMT passives, you should be able to handle any low-power attenuation needs that arise on the bench.

Thanks for reading.

- Dan W.

7 comments:

  1. Why not put 0805 pads on board, 0805 can handle a bit more power than 0603 components...either way awesome build!

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    1. I do have a version with 0805-sized pads. I uploaded it to OSHPark and updated the post.

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  2. Thanks for making the PCBs available at OSHPark. They will come in handy.

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