AD8317 + Arduino RF Power Meter # 2

I built an AD8317 power meter a couple years ago. The AD8317 is specified for operation up to 8 GHz, but it provides useful measurement accuracy over a reduced dynamic range of up to 10 GHz. With suitable software and calibration this module can provide a very useful piece of test gear for the shack.

My own test equipment doesn’t provide any means of accurately measuring power levels above 1.5GHz and unfortunately I’ve not yet been able to attend a MicroWave Round Table event. For those of you that have never attended such an event, one of the many highlights is usually some form of test facility that typically offers frequency / power measurement, spectrum analysis etc. Hopefully we will all soon be in a position where all these excellent events will be able to run again.

My interest in QO-100 and a homemade up-converter meant that the need to do power measurements at 2.4GHz was growing. This seemed a common requirement amongst a small group of fellow QO-100 enthusiasts. As a result of this and the successful completion of another recent project, where five of us constructed a 50MHz Power Reference Source, this new project came about.

This project is based on the PA0RWE Milliwatt Power Meter design and incorporates the W1GHZ Simple RF Power Reference module.

My recent interest in KiCAD resulted in the following PCB:

The PCB has the Arduino Nano, the AD8317 module and the 50MHz Power Reference board attached to the underside. Once again the PCB was designed to have duplicated footprints so that the builder can choose to use SMD or leaded components.

The PCB is a shoehorn fit into a Hammond enclosure. I have to confess this was more by luck than judgement. One of the things I have learnt very quickly is to give careful consideration to the size and type of enclosure you intend to use and where the fixing holes etc will go. Some 3D-printed supports were produced to hold the PCB in place and support the LCD display. Bulkhead N to SMA adapters are used for the RF in and out connections.

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The following photo shows the underside of the completed Power Meter.

Three power meters have been built so far, two are complete and one is awaiting the metal bashing to be done.

Incorporating the 50MHz Power Reference has proved to be very useful for doing basic (albeit at a low frequency) checks of attenuator values, cable continuity / losses.

I have been able to calibrate the power meter up to 23cms using my Rigol SA and for the 13cm band the best I’ve been able to do is to calibrated using the output from my ADF4351 signal generator. Unfortunately I’ve still not been able to calibrate the power meter across its full range using an accurate signal source. The performance of the AD8317 board at 3cms is still very much an unknown.

 

Lessons Learnt:

    • No matter how many times you check your PCB layout the chances of there being some form of a “Gotcha” is always high. On this one I failed to study the data sheet and managed to miss two links needed on the voltage reference chip!
    • Consider the enclosure dimensions and method of mounting the PCB before finalising the PCB dimensions.
    • It can be difficult to drill holes accurately in two of the side walls of the type of Hammond enclosure I used. The internal ribs on the sides (designed to be used to support PCB’s) can cause the drill bit to veer to where there’s minimal thickness. This was a particularly evident when drilling the holes for the N to SMA adapters. Ideally before drilling, the ribs should have been machined off to leave a flat internal surface.
    • Owning or having access to a 3D-Printer is extremely useful. I was able to print and supply the supports and LCD bezel for each of the units.
    • It’s good have a small group build to share ideas and costs in sourcing components etc.

Acknowledgements:

    • PA0RWE for his AD8317 Milliwatt Power Meter design.
    • W1GHZ for his Simple RF Power Reference Module design.
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