Category Archives: Test Equipment

Gremlins Lurk in the Junk Heap

Three weeks ago my EchoLink station went deaf. Thanks to the West Central MN Amateur Radio Club’s antenna analyzer, I figured out right away that the problem was in the feedline/antenna system. Ever since then I have been either too busy or too nervous to go on the roof, so it has remained a mystery . . . until this afternoon.

Antonio Mitchell checking SWR on the Edison Fong J-Pole

Antonio, my son, went up on the roof with the antenna analyzer while I watched from the ground. He did a great job taking all the tape and coax seal off the PL-259, disconnecting the coax from the antenna, and hooking up the antenna analyzer to the antenna with a patch cord. “One point two!” he called down to me. There you have it — it was the coax! I realized what I’d done. Here I had some brand-new coax in my field-kit, but I ended up grabbing a different coil of junk coax and got bitten by a Gremlin. I found the new, already-terminated coil of coax and Antonio swapped it for the bad length, carefully wrapping the PL-259 with coax seal and rescue-tape. In short order we had the station back on the air.

All that, and Antonio doesn’t even get to use it yet. Hopefully soon! Antonio has been studying hard for his Technician exam this week. He is eager to take the test.

Thanks, Antonio, for getting the EchoLink back up and running for all of us.

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Filed under Antennas, EchoLink, Elmering, Test Equipment

Minimum-Loss Matching Pad

In my last post I promised to write about the minimum-loss matching pad that I’m using to couple my signal generator to the device I’m testing. The source impedance of the generator is 600 ohms and the output is intended to be terminated in a 600 ohm load, but the device I’m testing is only 228 ohms. The way to match this with the lowest loss is with a transformer, but it is inconvenient and unnecessary to come up with a transformer for every mismatch this piece of test-equipment will face.

Thanks to advice from the ham who is guiding me in this project, I’m using a minimum-loss matching pad, also known as an “L-pad,” to match these two impedances. (I’d tell you who this fine fellow is, but to keep you in suspense about my project I’ll wait until my final write-up. If I name him now, the cat will be out of the bag!) This quick, cheap, and easy match requires only two resistors:

To calculate the value of the resistors and to calculate the loss of the matching pad, use these formulas (A spreadsheet that uses these formulas is available through this webpage.):

In my case R1=472, R2=290, and the loss is -9.25 dB. That loss is pretty significant, but it is acceptable for this application. Remember this is a minimum-loss matching pad, not a no-loss matching pad. Using what resistors I had on hand to come as close as I could to the required values, I soldered this pad on a generic PC board from Radio Shack that I cut in half using my Dremel tool with a cutting wheel:


For more on this topic, I commend to you this webpage on “Impedance and Impedance Matching.”

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Filed under Test Equipment

1Hz-2MHz Function Generator Kit

After building the “Accurate LC Meter Kit” from Electronics-DIY.com, I turned to their “1Hz – 2MHz XR2206 Function Generator Kit”. All parts necessary to complete the kit were included, though not exactly as pictured on their webpage — two of the WIMA capacitors had been replaced with substitutes and there was no IC socket. All components were through-hole; soldering the kit together went quickly and easily.

If you build one of these kits you’ll need to provide your own power source as well as your own pin-connectors (if you choose to use the pins provided). As with the LC Meter, I used a size M coaxial DC power jack to accept a plug from one of the wall-wart power supplies I have around here. I didn’t bother to install a power switch in either unit since I won’t be using them very often; I won’t leave them plugged in between uses.

The fellow at the local Radio Shack gave me some pin-connectors for free, clipping them off of some battery packs that were in a box for recycling, though he only had two-pin connectors. Since one of the pin-sets has three pins, I just soldered a piece of hookup-wire to the third pin. If I had to do it all over again, I wouldn’t bother with these pins — I’d just solder hookup wire right to the PCB. By the way, if you ever try soldering to a pin make sure you clip a heat-sink to the pin before heating it up. The plastic base of those pins melts pretty quickly!

I chose a plastic project box from Radio Shack to house this function generator. Using a Dremel tool with an engraving cutter (at the lowest speed — 5,000 RPM), I put three notches in one side of the box for the potentiometers, a notch on one end for the two switches, and ground down all four stanchions on the floor of the box since otherwise the potentiometers would have extended too high to allow the lid to fit. That Dremel tool sure is handy! A few knobs from Radio Shack finished off the project.

The two outboard switches allow you to select between three waveforms — sine, triangle, and square. I don’t have an oscilloscope so I can’t tell you how the waveforms look, but I can at least tell you that the sine wave sounded pure when I hooked my headphones up to the output with a matching pad. I am pleased to report that the signal generated by this function generator is very stable. Four DIP switches on the PCB allow you to select between four frequency-ranges, and two potentiometers allow you to tune within the selected range. One of these two potentiometers provides coarse tuning, and the other provides fine tuning. The third potentiometer controls the amplitude of the signal generated (note: amplitude decreases as you turn this potentiometer clockwise).

If you build this kit you’ll want to hook it up to a frequency counter. Two pads on the PCB are provided for this purpose. I have a piece of coax hanging out of the back of the box for connection to my own frequency counter — not that you have to use coax, but it was handy for terminating with a BNC connector. (If I were really classy I would have put this coax through its own hole in the project box, but hey, this is a piece of test equipment — I just ran it through the big hole I made for the RCA connector.) When I hooked up my frequency counter I noticed that the published ranges for each DIP switch were just rough approximations, but I was pleased to see that this frequency generator covered the entire published range and more — up to about 2.4 MHz, if I recall correctly.

Here is a slideshow of photographs I took of the completed function generator:

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The source impedance of the generator is 600 ohms and the output is intended to be terminated in a 600-ohm load. In my next post, I hope to discuss the construction of a minimum-loss matching pad to hook it up to a piece of equipment that has a different input impedance.

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Filed under Kit Building, Test Equipment

Accurate LC Meter Kit

Update (6/1/12): Not having heard back from electronics-diy.com, on 5/29/12 I inquired by email about the assembled kit I returned. The next day I received this reply: “We have done a range of tests on your LC Meter kit and have found no problems. Both capacitance and inductance measurements are identical to our kit. We even used brand new microcontroller chip and the readings are the same. Per your request we can either ship the kit back to you, or if you are not fully happy with it offer full refund.” I asked for a full refund, which they promptly granted (minus $5 shipping that I initially saved when placing an order over $99, but now have been charged since my return brings that order under $99).

Update (3/7/12): Yesterday I prompted support@electronics-diy.com for a reply, mentioning the number of pageviews this post has received. I received a prompt and polite response. I learned that I was mistaken in expecting the meter to read capacitors 1 uF or higher, since the published range of the meter is only 0.1pF-900nF. There was no explanation of why I am having problems with inductors that are within the published range of the meter. However, I was quite favorably impressed by an offer to test and fix the kit at no extra charge! I shall take them up on this offer and keep you updated.

Update (2/8/12): I am having trouble with this LC Meter. It gives me the same reading for all capacitors 1 uF or higher, and the same reading for all inductors higher than about 70 mH (this last value is just a guess): 838.8 nF and 83.88 mH, respectively. As you can see the digits are the same. It seems to work for really small capacitors and inductors, but anything bigger and these are the only readings I get. I emailed support@electronics-diy.com on 1/8/12 about this, but as of 2/8/12 I have received no reply. Unless and until I learn the problem is due to some error of my own in constructing this kit, I recommend against purchasing it.

Yesterday evening I finished building the “Special Edition Accurate LC Meter Kit with Blue Backlight LCD”, available from Electronics-DIY.com for $69.95. I have no experience with such devices; a more experienced fellow told me he was impressed by its specifications, so I ordered the kit. Soldering it up was a snap. The main printed circuit-board is all through-hole construction, and the LCD-board that mounts over the top of it requires nothing but a connector.

If you want to build one of these you may want to order this version of the kit instead of the one I purchased: Accurate LC Meter Kit with Green Backlight LCD, for $59.95. My kit’s “Blue Backlight LCD” turned out to be green anyway, and I think the two kits have the same circuit, save an adjustable potentiometer on mine that controls the contrast of the LCD (which I just set to maximum anyway). Certainly the cheap case that comes with the kit I ordered is not worth the extra $10 — to use it you have to carve out a bunch of stuff (to make room for the circuit-boards), including two of the four stanchions that attach to the lid. After going to all that trouble (I used a Dremel tool) you are left with a case that requires adhesive tape to hold down one side of the lid!

The instructions that came with the kit were pretty sketchy, mostly limited to how you need to carve up the case (by the way, the measurements were wrong, so ignore them). The only thing that got me into trouble was the voltage regulator, which gets in the way of the LCD-board (and protrudes too high to seat the lid of the supplied case) if you solder it in the way you normally would (which I did!). By bending the voltage regulator out at angle I managed to get the LCD-board mounted, but the lid still won’t seat properly. Learn from my mistake, and bend the leads of the voltage regulator into a Z so that they lay flat on the board and allow the voltage regulator to sit just off the edge of the board. (Of course, this only matters if you try to use the case provided.)

You’ll need to supply your own power to this unit. There isn’t enough room in the case for a 9V battery, so I purchased a DC socket. You’ll also need to supply your own connectors for testing inductors and capacitors; the photograph on the Electronics-DIY.com website shows them in the case, but they aren’t supplied. I used banana-plug sockets. You’ll also need to supply your own pin-connectors if you use the supplied pins on the circuit-board, and you’ll need your own stand-offs if you want to support the LCD-board (only two of the four screw-holes match up with the lower PCB, but that’s probably good enough).

There is no way to select the units displayed on the screen, e.g. pF vs. nF. But the dearth of selector switches is actually one of the nice things about this unit. There is no need to select a range of capacitances or inductances. The only thing you have to do is plug it in, hit the reset button whenever you want to calibrate it, and stick in a capacitor to get a reading. If you want to test an inductor, you simply press one button to select inductance-mode, then attach your inductor. It just works — and it works with precision.

Here is a slideshow of some snapshots that I took with my cell-phone. They didn’t turn out very well, but they’re good enough to give you an idea of what it looks like. Notice that I used black electrical tape to mask the edges around the LCD. That’s because the opening I made was downright ugly. Next time I’ll try using a cutting wheel on my Dremel tool instead of a grinding tip!

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Filed under Kit Building, Test Equipment