Monday, November 10, 2014

VNA fixture

In a previous blog post I mentioned purchasing a batch of 20 MHz crystals from China with the intent of building some crystal filters.  I am facing the need to be able to characterize my new collection of eBay crystals.  I have looked at many of the suggested techniques and out of them all, I like the idea of using a vector network analyzer for the task.

A few years ago, I purchased one of these units from SDR-Kits.

I purchased this at a time when support on Windows platforms for the necessary drivers was quite sub-optimal and as a result this device has not seen much use over the years.  However, I have dug it out, dusted it off and upgraded the firmware and USB drivers, which are now nicely supported on all versions of Windows.

Now, I need a nice fixture that can be used to characterize a batch of Crystals. Leveraging the fine work of Eldon (WA0UWH) I have built a fixture that can be used with this VNA for the task at hand.

The idea behind this is to provide a couple places where components can be connected to the fixture and swept with the VNA to be able to visualize its characteristics over a wide frequency range.

The 8 pin header is provided with three ground attach points, one signal attach point and two pins on each side that connect together.  The idea is to be able to create simple networks of discrete components and be able to sweep them or to just sweep individual components.

The buttons provide the ability to short or terminate the VNA with a 50 ohm load.  As part of the calibration process when the fixture is attached, you want to remove the effects of the fixture itself from the measurements so the VNA is calibrated with a short, through, open and load conditions before attaching components to be measured.

The two resistors are 100 ohm 0805 units that are connected in parallel across the SMA when nearest button is depressed providing a 50 ohm load.  The bottom button shorts the SMA connector when depressed.

The bare copper area at the bottom of the board allows for placing surface mount components across the gap.  These can be held in place with a non-conductive clothes pin or a conductive clip as appropriate.  When measuring surface mount crystals for example, a conductive clip will allow grounding of the crystal case as the back side of the board is an open ground plane while holding the connectors firmly against the top side of the board.

Hopefully this little fixture will help expand the usefulness of the VNA in my projects.  I will share my experiences using this characterizing crystals in a separate post.

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