Tuesday, March 15, 2016
Now that I have the SWR meter nearly finished, I want to get back to working on the spectrum analyzer. I will need to build and align several filters that are higher in frequency than either the SNAJr. or the Chinese SNA cover. Lookng at some posts by Asher Farhan on his blog, he has a simple SNA that he called the Sweeperino.
This uses the same AD8307 power meter I had used before and a SI570 for the signal generator. His version has a 2 line LCD display for frequency and measured power level , and uses a pot for tuning. There is a PC program for doing a display of the sweep, same program is also used for the SPECAN display.
Since I was only interested in displaying the sweep while aligning filters, I did not need the display or tuning pot. I wanted to save the last si570 I have on hand for the spectrum analyzer, so I used a Adafruit si5351 breakout board I had left from another project.
The power requirements for the si5351 board and ad8307 are very low, and could be powered directly from the 5 volt output on the Nano. I used some 100uH. molded RF chokes with additional filter capacitors to feed power to the 5351 and 8307 . I had used this before in the stand alone power meter, and with shielding was able to get a -74 dBm noise floor.
When I built the power meter I found it was fairly difficult to solder the SMD parts without a solder mask, very easy to have shorted traces. After I finished etching a double sided board, I decided to try adding a solder mask.
I found two methods of doing solder mask at home, the simplest appears to be using UV curable paint. I watched a You-Tube video that makes it look like it should be fairly easy.
Other than the paint, the only other thing needed was a UV light source, although you could probably just take it out into the sun to cure. Looking around for UV light sources, I found a small unit that is designed for use with UV curable finger nail polish. This was under $10.00 and can be powered from a USB port or USB power cell. This unit has a push button that turns the light on for 30 seconds at a time, this will make timing the exposure very easy.
I printed up a stencil of where I wanted the mask to leave bare copper on a plastic overhead projector sheet. Then got everything all ready to give it a try. I did a couple of quick tests on some scrap board to get the exposure time down. About 2.5 minutes seems to give good results.
Using a small plastic card as a squeegee and a piece of plastic cut from a zip lock bag, I spread a thin layer of the UV paint on the board and aligned the stencil with the etched pads. I exposed the board to UV light for 2.5 minutes, and then removed the stencil and the piece of plastic. Using a paper towel with some rubbing alcohol, I cleaned off the un-hardened paint. After checking the mask, I exposed the completed board for another couple of minutes to make sure everything was hardened.
The board looked pretty good, with only a small area where the mask came off the ground plane area. Not as pretty as a commercial board, but looks like it will do what I want it to do. Looking at the area where the SMD components for the power meter circuitry will go, everything looked very nice. I think this will make it much easier to solder in the SMD parts using my hot air gun. I used the same process and masked the bottom layer of the board.
Next to build up the board, and get the Arduino software working with this configuration.