Building Attenuators to Protect Test Equipment

I have the long term goal of designing and building an amateur radio transceiver. In order to do this, I have realized that it would be beneficial to have both a network analyzer and a spectrum analyzer on hand to test parts of the design.

For a network analyzer, I purchased a NanoVNA H4. The NanoVNA is a budget network analyzer that can be used for antenna analysis, measuring filter responses, measuring amplifier gain, and measuring transmission lines. I paid $100 shipped for my NanoVNA H4 from R&L Electronics. For the spectrum analyzer, I purchased a TinySA. It cost me $57 shipped from AliExpress.

A concern that I have when using these devices is that I may connect them to too strong of a signal and damage them. The NanoVNA has a maximum input of +20 dBm and the TinySA will handle +10 dBm. It does not take much to accidentally generate a too powerful of a signal and to blow up the input stages of either device. These are relatively cheap devices, but I still don't want to destroy them. To protect these devices, I have decided to build a set of attenuators.

Circuit Overview

The circuit consists of one 3 dB attenuator, two 10 dB attenuators, and one 30 dB attenuator. Also included are calibration standards for calibrating my NanoVNA.

How a Pi Pad Attenuator Works

They are basically just resistive dividers. A regular voltage divider could have been used, but the issue with these is that the impedance is different depending on the direction. The nice thing about the pi pad is that the impedance is the same no matter what direction the signal passes through.

Ballpark numbers for resistances were found by using this calculator. Resistors were then substituted with the standard resistor values that I have on hand. The attenuation of the new resistors was found using this calculator.

Build

I used 0603 surface-mount resistors and U.FL connectors to build the design. I milled the pads with a desktop CNC machine. The mill messed up the "OPEN" calibration circuit so I just opted not to build it.

Test Results

Each attenuator was evalulated with the NanoVNA for the 40 meter amateur radio band. A table of the results is seen below. Screenshots from the NanoVNA are also shown. The -30 dB is spot on with expected values. The other two are not as close as I would have liked, but the goal here was to build something that would allow me to not destroy my equipment and not actually to build the perfect attenuator.