I took a break from antenna analyzers to find and build a rig for a friend who is in a nursing home. The requirements are:
I’ve been studying detectors for antenna analyzers and trying to land on one which will be effective. If you’re interested in detectors, read past the fold.
The Fold
This is my first attempt at smd soldering since I left the Marine Corps. I’ve done some component removal for my junk box, but that doesn’t count.
The bridge between pins 13 and 14 doesn’t seem to really be a bridge. When I check under better light with magnification, the bridge doesn’t exist. My ohm meter tells me there isn’t a bridge as well.
My method was the “Apply flux and solder and drag the iron across the pins” method. If you’re careful and the board has good solder mask (which this board seems to have), the results are surprisingly good. Next time, I’ll pay more attention to the alignment of the chip on the solder pads.
What is that chip? It’s an AD8302. I will be using it in my antenna analyzer experiments. I bought 10 of them and 10 dip adapters for a total of $25 on ebay. More about this chip later.
I recently built an analog signal generator covering 1.7mhz to 28mhz in two bands. I wanted to put it in a case so I can easily use it on my bench. I had one project box, and I went to work jamming the sig gen in there. Looking at the results, I probably should have gotten a bigger project box.
Wow, that’s ugly.
The hole in the top of the box is for the rotor of the air variable capacitor to pass by. I didn’t have room in the box for batteries and I haven’t found a secure location for them. It does function, producing a nice sine wave and it will see service on my bench. It will be a constant reminder to think about the mechanical aspects of my projects.
Past the fold are details on my rendition of an ardiuno DDS based signal generator. Included are the code, some schematics (not a full schematic though), and some hints on setting it up.
Signal Generator Details
I’ve built a prototype of my first antenna analyzer. Pictures and some details below.
First Antenna Analyzer Details
One of the features which I’m trying to build into my antenna analyzer project is to measure whether the impedance in the antenna is capacitive or inductive. Judging from other designs, this means I will need two signal sources which are 90 degrees out of phase. The AD9850 data sheet mentions this feature, as do several other ham sites, but I haven’t found a really good description about how to get it done.
The Ugly Details
A google search for DDS buffer amplifier turns up some nice results in the first two hits. These circuits are very similar. My rendition of this circuit replaces the 2n2222 with 2n3904′s since I have scads of them. I also lower the value of the feedback resistor to 470 ohms (R3 in my schematic). This lowers the output voltage a bit, but it also seems to cut back on the harmonics. With 1k feedback I was seeing the second harmonic at 10db down from the original signal. With 470 ohms, the second order harmonic was down between 34 and 40 db over the whole range of tuning.
Schematic and picture to follow
I have a lot of work to do on my arduino DDS signal generator, but it’s good enough to demo. Too bad I’m bad on camera. Once I have this signal generator to the useful stage, I’ll post some details about it.
I spent a fair portion of the weekend testing my new AD9850 DDS modules and attempting to use some bare AVR chips with internal oscillator as cheap arduino. Read past the fold if you’re interested in the progress.
The Fold
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