Posts Tagged video
I’ve finally gotten around to assembling a breakout board for the Skyworks SKY65116 UHF amplifier. It’s really amazing how the state of the art in RF ICs has advanced. They can still be on the expensive side ($6 at digikey), but still relatively cheap when you consider the cost of all the support parts that it takes to build an amplifier from a RF transistor. This particular amplifier has a 50 ohm input and output, and 35dB of gain. It works from 390Mhz to 500Mhz, which means its perfect for the 70cm ham band. The breakout board is stupid simple, copied directly from the evaluation board schematic in the datasheet, but I’ll include schematic and design files anyway.
This is the video transmitter from my first person video experiments. The performance was pretty terrible, even after I tested it using different receive antennas. I’ve even purchased a receive-side amplifier to try, but haven’t done anything with it yet. Anyway, the transmitter had a built-in antenna, so I wasn’t sure how I was going to add an amplifier. I ended up assuming that the output would be roughly compatible with an 50 ohm load. I unsoldered the antenna and installed a bit of thin coax to the antenna port. I scratched off some of the solder mask on either side of the board near the antenna port to make sure I had a solid electrical and mechanical ground connection. The transmitter is pretty crappy, and the prices you can find online are COMPLETELY RIDICULOUS! I wouldn’t pay more than $20 for it. I think that’s about what I paid, it was on clearance.
This image is the testing configuration I used. The camera, power board and transmitter are in the top of the image, and are exactly as I used them for first person video. The added coax can be seen going into the amplifier on the left. Coming out of the amplifier is the cable going to the oscilloscope or spectrum analyzer. The amplifier wasn’t inline all the time, though. I measured the output power from the transmitter at about 25mV into 50 ohms using the oscilloscope. Using Minicircuits’ handy table that comes out to be about .01 mW, or -19 dBm. A measurement from a spectrum analyzer verifies the -19 dBm measurement from the o’scope (see below for image).
I’ve attached a very nice graphic from wikipedia that describes the components of modulated NTSC video. There is something happening here that isn’t obvious, so I’ll explain it. In the spectrum analyzer image, below, you’ll notice that I’ve labeled the luminance and chrominance carriers. The luminance carrier is really the main carrier for the entire signal. It comes from black and white TV era. There are significant DC components in NTSC video, so this carrier is very important. Notice, in the graphic above, that the luma carrier is 1.25 Mhz above the lower edge of the band. This is because NTSC video uses what’s called VSB, or vestigial side band, which means that the lower half of the signal is attenuated. This reduces the spectrum necessary to transmit video. The choice was made to include the carrier and 1 Mhz with of lower sideband while removing the rest. Later, when color TV was added, they needed a way to encode color. This is done by adding another carrier and encoding hue and saturation by modulating the phase and amplitude of this carrier. All this is explained at length, and probably much better, in the wikipedia article on NTSC.
In the spectrum image I’ve included above, it’s clear that the little transmitter uses AM rather than VSB. You can tell because AM modulated signals are always symmetrical with respect to the carrier. If it was VSB, the spectrum on the left side of the carrier would be suppressed. You may notice that the left and right side don’t look 100% alike. This is because it takes time for the analyzer to sweep the band (it does this 30 times a second), and it will be analyzing the spectrum of a different part of the image as it scans.
Well, that was an unexpected tangent! Back to the amplifier… In the above image I have the amplifier in the signal path from the source to the analyzer. It’s disconnected from any power. I’m a little off on the “-60 dBm” text, it’s closer to -64 dBm. I was interested in seeing how much RF would leak through an unpowered amp. It appears that the amp provides a little more than 40 dB of forward isolation between the input and output when it’s unpowered.
Finally, this is the spectrum when the amplifier is powered on. I had to install 40 dB of attenuation on the analyzer to capture this image. The peak of the carrier is almost 5 dB lower than the top line, so it’s about 36 dB stronger than the input. This is inline with expectation, as the amp specifies +35 dB gain. The resulting signal is +15 dBm, which is a modest 32 mW of power. The hope is that through a better antenna and some amplification I can get better performance from the video link.
A word about the legal implications. Ham radio people are notoriously concerned with the rules of everything they do, so I feel obligated to mention them. In the U.S., at least, 434 Mhz is a commonly used ATV (amateur T.V., or “fast scan TV”) frequency. There is some concern due to the proximity to the “satellite only” frequency band of 435 Mhz to 438 Mhz. This means that the carrier is sometimes shifted to 433.92 Mhz, as this transmitter is. Some of the sidebands still end up in the satellite only band, but with much lower power. Because this amplifier only outputs +15 dBm I’m very unlikely to upset anyone with its use, though I should think about adding an overlay with my call sign to the video at this power level. Maybe I’ll have a new 8-bit microcontroller project…
Well, as a (perhaps welcome ;)) deviation from the spectrum analyzer posts, I’ve spent a little time working on repairing a security camera I’ve been hanging onto for a while. I’ve been toying with the idea of installing it near the radio control flying field as someone in the club knows the owner of a nearby business.
The problem with the camera is that it has trouble switching from nighttime mode to daytime mode. At night, it is sensitive to Near Infrared light (such as what is transmitter from your remote control). During the day, however, this sensitivity makes the colors look strange. To cope with this, they have a filter that slides in front of the lens for the daytime. This filter binds, and because of this, the camera is always in nighttime mode. I decided to go ahead and disassemble the camera to try to fix it. My motto: If it’s broke and out of warranty, take it apart! 🙂
You can (kinda) see in this photo the image sensor. This metal mount can move forward and back to accommodate different lenses and adjust their focal length.
Removing the bottom cover reveals the power conversion board.
Removing the top cover reveals the PCMCIA slot. This camera is kinda cool in that it can accommodate either a memory card or a wireless LAN card in this slot. Also, there is an ethernet port on the back. It has an embedded web interface that allows the control of camera functions and viewing live video. Apparently, there is the ability to get some information from the serial port, but I haven’t found any information about it.
Like I mentioned before, the front lens mount can be adjusted front-to-back. The black screw near the top of the frame is used to secure the mount. More to the right of the frame, the spring is used to press the mount against a set of wedges that control the depth setting. The 4-pin connector goes to the lens iris, which is kinda like the aperture of a still camera.
This is the front of the camera disassembled. The image sensor is still attached to the camera frame. The black piece of plastic is the filter module, and the other piece is the metal frame for mounting the lens and adjusting the focal length.
This is the device causing all the trouble. The slight green tint in the left frame is the IR block filter, and the right is clear. The motor is a small gear-head motor attached to a worm gear. The worm gear has very shallow cuts in it. The spring pushes down on a small plastic follower. The whole system is intended to allow the worm gear to continue turning even if the system is jammed. I think this is so that limit switches aren’t necessary. I figured that the problems I’m having are due to excessive friction, which would cause the frame to remain static while the worm gear turns. My first thought was to place a little petroleum jelly on the sliding surfaces. I re-assembled the front of the camera and found that the problem remained. However, I noticed that when the lens mount was all the way against the camera it would stick. I could then solve the problem by keeping the lens not-quite against the camera.
Before re-assembling the camera, I decided to lubricate the adjustment assembly. The black plastic ring on the right includes the adjustment wedges. The middle ring is the precision-machined lens mount with a channel for the plastic ring. The punched metal piece on the left completes the assembly.
Anyway, I put the camera back together. It works great now. I’m a little embarrassed to admit that the problem was simply that the lens can’t be in the closest setting for the filters to work. At least everything went back together without a hitch. 🙂
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.
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).
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 🙂
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.
I’ve just finished building a camera module for my Kadet. When I was building the plane I knew that I was going to try and put a camera and transmitter approximately where the pilot’s head would be in a real plane. Read the rest of this entry »