Posts Tagged Radio Control

Spectrum Analyzer IF Amplifier

IF Amplifier cooling down

I’ve decided to hold off posting spectrum analyzer modules until they’re complete.  I’ve been collecting tons of pictures along the way, so hopefully the post for each module will be interesting and visually appealing.  With that in mind, I can’t post about any of the modules with the exception of this one.

For whatever reason, my kit didn’t come with any voltage regulators, and this is the only module besides the mixers from my last post that don’t use any.  The IF Amplifier, as the name implies, amplifies the Intermediate Frequency on the analyzer.  I made a simplified version of the block diagram from Scotty’s site.  I’ve also indicated the modules I’ve finished in green.

Block diagram, green modules are completed as of this post.

As you can probably see, there’s a lot to be done.  Luckily both DDSs, the Log Detector, ADC, Control Board, and one PLO are done once I get some parts to replace the missing ones.  I also need some parts for the Master Oscillator.  Once that (actually quite small) order comes in, I’ll be almost finished with the boards.

Anyway, back to the amplifier.  I was one of the first people on an order of boards back in August ’08 because I agreed to look-over a new revision of the design files.  Of course, by having me look them over essentially guaranteed that there would be errors.  As it happens there was an error on this board.  The engineering change order (ECO) is luckily quite simple, the only problem was that a short section of the border was missing.  Here is a photo of the completed fix:

Fix for the slight error,

You should be able to see that along the border there is some solder wick saturated with solder bridging the gap, and connecting to the capacitor.  Also, on (at least) the 2 resistors (R3 and R4) there are tiny balls of solder.  These happen during the reflow stage when a bit of solder squeezes out from underneath the device.  I haven’t gone over the board picking all these out yet in this photo.  I think it’s important to remove all of these because they may cause shorts.  Also, you may be able to see a slight, shiny, residue around everything.  This is the solder flux that’s included in the solder paste.  I remove this later with some 99% rubbing alcohol.

Completed IF Amplifier

This is the completed amplifier board.  There are actually 2 amplifiers here, mirrored.  In practice, the output of one will be the input of the other.  This serves to roughly double the gain.  Finally, notice on the right side, that many of the parts have been omitted.  On the schematic, Scotty simplified this section.  Technically, he has 2 zero-ohm resistors (basically jumpers) in addition to the capacitor.  I chose to just jump the capacitor across and leave the resistors out.

Anyway, I enjoyed building this one.  I hope you’ve enjoyed reading about it.  I can’t wait to finish the other ones!

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“Scotty’s Spectrum Analyzer”

IMPORTANT!!  — I’ve made a special page for my construction and testing of the spectrum analyzer.  It’s available on the navigation bar below the blog title banner, or here.

A while back I bought a spectrum analyzer kit.  It can display waveforms in the frequency domain from 0 to 1000 Mhz.  I haven’t worked on it for a while, but I’ve decided to start working on it again.  If you’re interested in more information, there is a webpage for the analyzer by the guy who designed it, and there’s a Yahoo group, also. If you’re really-really interested there is a group buy open (until the 28th of march 2010) here.  Anyway, I’m sure it’s of limited use to my usual readers (thanks, friends & family! [oh who am I kidding, most of my family doesn’t read this 😉 ]), but it may be useful to others working on the kit.  I’ll post the “annotated” CAD images for each board here, and add images of the completed boards when I finish them.  Who knows, it may be interesting for someone.  Also, I’ll include my thoughts and observations while building these boards.  Finally, not all the boards that come with the kit include annotated layouts, so I’ll have to dig around and figure out what they do and what to put on them 😉 Read the rest of this entry »

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New video link antenna

Comparing antennas

I’ve finished building the first (and my first) Yagi antenna for the Kadet video downlink.  I mentioned in the first video post that a new, and better, antenna would help greatly.

I was able to find some designs online that fit my needs.  The major constraint that I had to deal with is the impedance issue.  Nearly everything amateur radio related is 50 ohms, and nearly everything video related is 75.  The cheap Yagis paper written by Kent Britain, WA5VBJ, has a 75 ohm 421 Mhz antenna intended for amateur television (ATV).  My transmitter is 434 Mhz, but I figured it would be close enough.  The great thing about these designs is that they can be built using supplies from a standard hardware store.  The elements are made from #10 bare copper wire, and the beam is wood.

Completed antenna

An interesting characteristic of antennas, and RF in general, is that to get a stronger signal you often have to make compromises.  A Yagi works by increasing the directionality to increase the signal.  Unfortunately, because my plane is going to be flying around, I can’t be too directional.  To get better results, without making things worse, I decided to only use the reflector and “driven element”.  By  eliminating the “directors” I hope that I can get the best possible results. (If you’re confused by this “director”, “reflector”, and “driven element” gibberish, the best place to look is the wikipedia article.  But all that is necessary for this discussion is that the reflector is behind the driven element [which connects to the transmitter or receiver] and reflects the signal forward, and the directors go in front and focus the signal into a narrower beam).

Feed line soldered

There were some problems during construction that I should mention to help others wanting to try something like this.  The antenna designs specify that the feed cable should be soldered directly to the driven element.  This should work great on traditional 50 ohm radio cabling, such as LMR or RG-type cables.  These cables have copper braid shield around the circumference.  With the 75 ohm cable used in video, often made as cheaply as possible, a loose aluminum braid is used as the shield.  This is a major problem that I had to deal with.  It took me a while to even understand why the braid wasn’t soldering.  I think I assumed that the braid was made of tin.  After a few hours of searching, I discovered it was aluminum.  Aluminum oxide forms almost immediately and can’t be soldered to, so even sanding the wire doesn’t help.  There are solder pastes and fluxes that help, but I wasn’t interested in waiting for something to be shipped.  My solution, if you want to call it that, was to mechanically attach the braid to some other wire that can be soldered.

In the first image of the post, I’m comparing the new antenna versus the others I used earlier.  When the Yagi was installed, I rotated the antenna 360˚in azimuth to get an idea for how directional the antenna really is.  There wasn’t much of a change in signal quality, so it isn’t very directional.  If the transmitter were further away it may have been more dramatic.  I am motivated to build a few more antennas, maybe with 1 and 2 directors to see which is better.  With that said, I’m pretty satisfied, and I’m hoping for good weather this weekend.

b.t.w: Just to dispel any fears that the shield is shorted to the center conductor, as it appears in the above photo, it was, and I fixed it.  Here is a photo of the feed point as it was when I tested it.  Also, notice that I got my driven elements and directors confused when I wrote on the board 🙂

Another view of the feed point

 

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First video (somewhat) successful!

Flying video

The weather sucked, but I was able to get out the the field this morning. I recorded about 17 minutes of video in total.  In the embedded clip, I edited out the majority of the static.  I’m a little disappointed that the signal quality is so poor.  I found a site that has some cheap Yagi antenna designs in 50 and 75 ohms.  I’ll probably try to build one this week and see how it goes.

More information about the camera is available in these posts, if you haven’t already seen them.

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The Camera module’s new look

Painted to match

Taking advantage of the crummy weather, I decided to paint the camera module to match my plane.  I didn’t just do it for the aesthetics, no really, I swear. 🙂  Actually, the real reason I painted it was to prevent stray light from making annoying reflections on the inside of the window.

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