Posts Tagged Radio Control
It has been a while since I played with the Sky65116 amplifier boards that I built and wrote about. Since getting my own spectrum analyzer, I’m now able to make much better absolute measurements. The DSA-815-TG analyzer is specified as having 1dB of uncertainly across the span, and the SA that I have at work is essentially uncalibrated.
Not only do I now know that the absolute power out of the video transmitter at the fundamental frequency is about 5.5dBm, I can also see the first and second harmonics. I don’t know why I didn’t see them with the other analyzer, but they are disturbingly large. I believe the FCC requirement is that these harmonics should be more than 40dB below the fundamental. By this criterion, the transmitter should not even be sold in the US.
The SKY65116 amplifier has close to 36dB of gain, and a 1dB compression point of 32.5dB. The 1dB compression point is specified as the output power level at which the gain is reduced by 1dB. The easiest way to show this is with the graph from the Sky65116 data sheet, shown below. You can see that as the output power begins to approach 32dB gain drops quickly. The goal is to not force the amplifier to operate in this region. If you do, you’re likely introduce harmonic distortion and other nasty nonlinear effects (i.e. intermodulation products).
So, knowing that the output is +5.5dBm, and we want about +30dBm out of an amplifier with a gain of 36dB, we need to introduce 11.5dB of attenuation (30dBm – 36dB – 5.5dBm = -11.5dB) between the transmitter and the amplifier, at a minimum. It’s easy to make 14dB attenuators with standard value resistors (4×150 ohm and 1×120). I’ve been playing around with QUCS a lot lately, so I’ve provided a model for the attenuator. It’s just about the most boring S-parameter model you’ll ever see… Perfectly flat response at -14dB, but that’s what we’re after.
A while back, I had a bunch of these simple 5-pole filter PCBs made up. I just left them blank until I needed them, and I made two of them into 14dB attenuators using the circuit above. I had two extra poles, so I filled one with a 0 ohm resistor and the other with a 1uF DC-blocking capacitor. The capacitor reduces the low frequency performance, but only less than about 1.5MHz. It’s worthy the trade-off in my mind.
I went ahead and covered one of the attenuators with copper sheet just to make it more of a completed package. I’m sure I’ll need to use it many times in the future. I left the other open so I could unsolder it and make it something else if needed. An interesting thing happened when I installed the cover. The small ripple in the attenuation (around 500 MHz, see below) occurred only after I installed the cover. I assume this is due to parasitic capacitance between the components and the copper covering. It’s still only about 1dB of ripple, so I’m satisfied with it.
Anyway, back to the amplifier. I’ve now got about -8.5dBm going into the amp ( +5.5dBm – 14dB), so with its 36dB of gain, I should expect to see +27.5dBm out of the amp. That’s getting very close to the maximum input power on my SA (+30dBm). It’s always better to be safe with these things, so I used the other 14dB attenuator between the amp and the SA. Now, I should expect to see +13.5dBm on the input. I maxed-out the input attenuation on the analyzer (another 30dB) and gave it a shot. Note that the internal attenuation is calibrated out of what’s shown on the display, and I told the SA about the other 14dB of attenuation, so the power values shown on the display are referencing the amplifier’s output.
In the above image, you can see that we’re getting 27.5dBm out of the amplifier! I love it when a plan comes together! This is the value I calculated, right on the nose. I promise that I didn’t work the math backward! 🙂 Again, it’s so painfully obvious that the transmitter is AM, rather than the VSB signal that it should be.
Now, just for fun, let’s dive back into the video signal coming out of the transmitter. In the image above, I’ve put some markers on the various carriers present in the signal. The luma carrier is in the center at 433.85MHz, which is where we expect it to be. Marker 2 is at 437.45MHz, which is 3.6 (let’s call it 3.57) MHz away, matching exactly where the chroma carrier is supposed to be. There’s no audio carrier, which isn’t a surprise because there’s no audio, though I wouldn’t be surprised to see the carrier. Marker 3 is 19 MHz away; I have no idea what this is or why it’s there. It’s not supposed to be. Same with marker 4. Oh, well… that’s what you get with a shitty transmitter.
Now, what about those pesky harmonics? The Sky65116 is a 390-500 MHz amplifier, so my hope is that the reduced gain by the first harmonic will attenuate the harmonics enough to bring them into compliance. The graph above is the gain v. frequency graph from the data sheet. It’s neither encouraging nor discouraging. It’s difficult to infer what’s going to happen at 800 MHz when the graph stops at 500 MHz. In the image below, it appears that I lucked out. The first harmonic is 42dB down from the fundamental. If I were selling a product, there’s no way I would send this out for compliance testing. It would just be too risky, I’m not that confident in my measurements. By my math, it would cost less than $2 in parts (single unit quantities) to make a decent low pass filter. That’s the right thing to do. When I modeled it (in QUCS, again), I calculated that the harmonics would be within compliance even without the rolloff of the amplifier. With the rolloff, the harmonics would be well below the noise floor.
I’ve had a ton of fun redoing this experiment with my new spectrum analyzer. I’m going to write lots more about the analyzer in the future, and I’m really looking forward to it. Coming soon is an exploration of the skyworks low noise amplifiers. Between these two products, I expect to have a solid video link over 1000 feet or so.
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…
It has been a while since I finished the transceiver modules, and I how now used them in an actual application. But, before I talk about that, I’d like to show some pictures of the process I used to put them into enclosures. I had some of these cast aluminum enclosures lying around, so I thought I’d use them. They’re a little on the heavy side, as the completed weight is around half a pound, but it’s well within the carrying capacity of my Kadet.
Before diving into the process of cutting the holes, I want to show some images of the transceiver board with the RFI fence installation process.
For use with the spectrum analyzer project, I found some sheet copper at the craft store. It was sold at a local crafts store, and I think it was for etching. I chose the thickest one they had. So far, the best way I’ve found to cut it is using an exact and straight edge. I tried scissors, and it didn’t really work.
Once I had a strip of copper cut, I cut openings for the power and control traces and soldered it onto the PCB. I also soldered it onto the SMA connector. Once all that was finished, I soldered on a lid. Lots of solder flux helps here.
Once the board was prepared, both by soldering on a fence and replacing the pin header with a right-angle one, I began to prepare the enclosure. I was intending to drill a hole for the SMA connector, then cut a hole for the digital connection. The SMA connector hole was trivial to make, though the connector I soldered onto the board was a little short. I ended up having to use an O-Ring from the hardware store (look in the plumbing section) to hold it in. When an antenna or cable is screwed on the O-Ring compresses, having the nice side-effect of sealing it.
For the digital connection, I decided on using a DB-9 connector. I figured it was a prolific connector, so I should have lots of connectors laying around. That didn’t turn out to be as helpful as I had hoped, but I’ll get into that later. To create nice holes for the DB-9 connectors I decided that I could use an old PCI bracket as a template. I lined the bracket up against the side of the enclosure and traced it. On the black box, I traced it using a knife, and on the grey box I used sharpie.
Once the outline was traced onto the box, I drilled holes for the retention screws. Then, I drilled out as much as I could of the trapezoid shaped interior. I most used the drill press, then the dremel with a router/cutter bit. I made sure to leave a margin inside the perimeter to remove with the files. I had avoided purchasing a set of jewelers files for a while, I think I assumed that they were expensive. They’re not, you should get a set.
In the photo above, you can see the finished penetration for the DB-9. I beveled the inside edge to make room for the fillet on the connector that I had. The black box got a male DB-9, and the grey box got a female one. The holes need to be about the same size, as the male shroud always has to fit over the female connector body.
In addition to the RF and digital connectors, I needed a way to securely mount the internal circuit boards. The way I chose to do this was first to drill holes in the bottom of the box, then “countersink” some screws into it. I have countersink in quotes because I don’t have a countersink bit, so I used a larger drill bit. You can see the results of this in the headline picture of this post. Though I think it looks pretty good, I still decided to buy a drill & tap for 4-40 screws after building the black box. For the grey box, I used the tap and screwed directly into the box. This requires slightly less hardware and looks pretty good, I think.
For whatever reason, the board I built for the black box using some veroboard-style construction didn’t work the same as the breadboard. Because I was under time constraint (I was planning on flying one of the transceivers over the weekend. I decided to put it back on the breadboard and use it as the base station. This version uses an FTDI cable to connect to my computer.
For the grey box, I used an extra ATMega48 breakout board I had. This one worked just fine in the enclosure, so I flew it. I also built a power regulation/distribution board, seen on the right of the photo. This concludes this article. Now that I’ve got at least one flight worthy transceiver I can test them in flight. That’ll be detailed on a future post.
I maiden’d my new plane, the Mud Bug, last weekend! I was a ton of fun. Now, I’ll tell you all about building it, and converting it to use brushless motors.
All the parts in the kit are laser cut, and fit perfectly. The design of the plane is such that the shape of the wing is created almost exclusively by bending the wing’s top skin into shape. There are only a few ribs, and no bottom skin at all. It’s remarkably light.
The kit calls for a stick-mount geared, brushed motor. These are getting pretty out-dated, and aren’t very efficient. I really like working with brushless motors, so I had to devise a way to mount it. I chose the E-flight park 250 motor because it was pretty inexpensive and only a few dollars and grams more than the park 180. It’s possible to mount it inside of a carbon fiber tube (using glue), but you need to use their tube, and it wasn’t in stock. I just decided to use the cross-mount adapter and make a plywood plate that mounts on the stick. Where the stick mounts, I added another, smaller, piece of ply to help support it. It ended up working perfectly.
I cut a pair of sheets of 1/64″ ply to use as scab plates on the balsa firewall and fuselage parts. I had read online that those pieces of the original kit were somewhat weak.
This is the scab plate on the back of the fuselage portion of the motor mounting. These are only there to spread out the loads transmitted from the stick to these parts.
The motor mounting process went beautifully. At first everything fit so tightly that I didn’t need any glue. Half way through the first flight it had vibrated enough to polish the wood parts that they could slip. I just put a few drops of thin CA, and it was fine.
The flights went great. That park 250 has waaaaay more power than you need with a 7×6 APC Slo-fly prop (which is the only prop I’ve tried). The plane flies easily at 1/4 throttle. At full throttle it gets very small very fast. When flying slowly, it’s also very agile. It’s possible to complete an entire circuit in 1/3 the length of the runway.
Even though it sports GIANT tires, it’s still easy to nose-over during landing in even short grass. Given that it has almost no mass, no damage was sustained.
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. 🙂