Thursday, October 6, 2016

8307 power meter repackage

I was adjusting the gain on several broadband amplifier stages using a noise generator and my AD8307 power meter.  It worked well, but was a little inconvenient without a small test probe.  A couple months ago some one had brought a RF probe from QRP Guys for show and tell at our local QRP club meeting.  I liked the format, but I preferred the direct reading in dBm. that I have with my existing meter.  I guess it was time to see about doing a repackage.
It was very easy to move the components in the layout, and change from an Arduino Nano to a Pro-mini. Switching to the Pro Mini allowed me to stack the display directly over the Arduino.  This reduced the width of the board, and gave me a nice probe like form factor. I etched up a board and assembled the new format meter.  I had one small error in the board, but a simple jumper took care of that. After changing some pin assignments in the sketch, I had everything up and running. Now it was time to see about packaging it.  
I have been playing with the CAD software I use with the 3D printer.  I tried several ways to make a housing with a top and bottom piece, but couldn't get one to print that held together tightly. 
Probe Cross Section
I had made a set of corner pieces for a cabinet that had in a built in PCB holder that had worked well.  So I did a design that was basically a rectangular tube with built in slots to hold the PCB. display board and centering for the display itself. It also kept the 9 volt battery from moving around.  I printed a long tube with this cross section, and checked that everything would slide in easily.  It looked OK, but I wanted it to look more like a probe. With a little cutting and slicing of the design I had a nice looking probe housing with cutouts for the display and on/off switch. Simple pieces closed up the open end and provided a removable cover for the 9V battery compartment.

I kept the SMA connector for the input to the meter.  That allows me to use a cable to check output levels of modules with connectors, or install a probe tip for in-circuit measurements.  One thing I decided to add to the software was  to display the difference between the last peak and current reading.  This allows you to get the stage output level, then go back and take the stage input reading.  The displayed difference should be the stage gain/loss in dB.  I took some readings across several known value  in line attenuators,  and found it very easy to measure stage gain/loss.

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