|Oct 09, 2001, 06:58 PM|
In-flight video power converter
Ok, deep breath, here we go
Let me state here and now that as far as electronics are concerned I know enough to be dangerous and a significant hazard to any electronic device within my grasping range. *watches magic smoke rising to the ceiling from last oops*
Recently I constructed a DC>DC converter based on the National Semiconductor LM2587S-ADJ to power my airborne video equipment from the flight pack.
The on-board video system consists of:
Trango Systems VTR Eagle 2400 liberated from its 11.1 oz steel case. (I'm an over-kill specialist)
Supercircuits PC60XSA B&W camera w/audio (no longer available to the general public but thatís off-topic)
DC>DC converter 5-40V in configured for 12V 1A max out.
Initial testing with all the bells, whistles, servos, Oooga horn, (just kidding) and flight motors running I get no interference either way, video is clean and all R/C components are behaving themselves like good little devices.
Here's where I get in trouble and my limitations become evident. The camera also has a switching converter on it, (KIA7805PI). I am getting a high frequency hum that is getting transmitted on the audio channel of the transmitter, (and possibly not doing any good to the rest of the video transmitter). My gut instinct is the camera voltage converter is creating this noise. I am sure it is the camera having tried powering it separately with no noise. Disabling the audio section of the camera does not help.
Besides going back to school, anyone have any ideas?
Thanks in advance
|Oct 09, 2001, 11:40 PM|
I built the circuit shown on the data sheet for that same chip for my project. I use an X10 cam, requires 12V out. Built exactly as shown, I get no noise. Did you use the specified components, particularly the caps and inductor? If so, I wonder if you designed your own circuit board? I did, and it's pretty compact.. I wonder if you did, if there is noise coming from the board? If you didn't use a board, but instead used standard stripboard, you may get a lot of noise from that, as there is a lot of capacitance between the strips, and other problems that I'm too dumb to understand.
Anyhow, point is, I built the same circuit as in the data sheet, and it's quiet on the camera side. My receiver didn't like it that much, so I ended up using a seperate pack to power it for now, until I can be bothered to hook up my 'scope and check if I can drop that interference with a filter.
|Oct 10, 2001, 03:16 AM|
It sounds like the swithching frequency of the converter might be in the audio range.
What about wrapping the converter in plastic (to prevent shorting) then wrapping it again in foil? That might shield the noisy emissions from the converter.
|Oct 10, 2001, 04:50 PM|
I built this converter from a Surface Mount power protyping kit manufactured by Pioneer-Standard. This converter is flawless and has had absolutely NO ill effects on the R/C devices with no sheilding. It is only the voltage converter on the camera board itself that is creating a problem.
BTW, these are great people to deal with and next day delivery is SOP for these folks.
You are indeed correct about the camera on-board converter switching frequency or one of it's harmonic products being in the audio range. The noise changes frequency whenever supply voltage changes. The manufacurers of this camera did a decent job of filtering switching noise on the audio/video output side but apparently had no concerns about switching noise being inserted on the power supply side.
With the camera powerd by a seperate source the is zero noise.
If the camera power is "in-circuit", either using the V-out side of the converter or direct connect to the flight battery I get "noise".
There is a negligable amount of this noise getting through the audio/video section of the camera. I was only able to see it when I hooked the audio output of the camera directly to the audio-in on my PC and used Winscope with the gain cranked way up. This is not a problem. To bad I don't have a real o-scope.
BTW, if you are wondering why I am insisting on using the audio and need a clean signal, I will be using it to transmit GPS data to a ground station.
Guess I get to study up on power filters.
|Oct 11, 2001, 04:54 AM|
Yeah, a good audio band-reject filter would probably clear up the signal before the GPS NMEA data in modulated on it.
The other way that might work is to encode the GPS data into the horizontal frames during the vertical refresh blanking. This is the way the TV's closed captioning is done and there are lots of closed captioning decoders that you could use to decode the data.
Good luck with the project. It sounds fun! In flight video with coordinated realtime GPS data.
|Oct 11, 2001, 07:55 AM|
I already have my eye on a system that uses the audio channel for the NMEA data. Ground station software processes it for a video overlay with 7 programmable data display feilds. No flight instruments needed, (but provided anyway as a back-up).
It had better be fun. I started this project 5 years ago, (not just the video system but a semi-autonomous UAV system).
Nothing new there. However none of the systems I have encountered has used Virtual Reality as a piloting aid. To date I have been trying to accomplish a NASA level program on a dime-store budget, (my very shallow pockets).
That wouldn't make me ambitious or anything, would it?
I have had a major set-back and loss of considerable money thanks to the actions of an equipment manufacturer that will remain un-named for the present. They are not anyone that E-Zoners would deal with I can assure you. Live and learn.
The aims and potential of my project caught the attention of some Entomology/Agriculture Researchers. If everything goes well we should have our proposal before the National Science Foundation by summer 2002. There are additional organizations that might help with funds but I will leave that part for my academic partner to deal with.
Keep 'em flyin'
|Oct 11, 2001, 05:15 PM|
I find it very unlikely that the converter has a design frequency in the audio range. What's more likely is that the converter has a loop oscillation in the audio range for some loading conditions. I would go back over the build and check for an improper component, probably a capacitor. As you know, caps have tricky markings. If you are accidentialy off by x10 in a part (easy to do) the circuit may be unstable. This applies to any capacitor in the circuit. You might try adding additional output capacitance (say--tripple or more of what's already there) and observe any difference.
|Oct 12, 2001, 08:42 AM|
We are discussing 2 different voltage regulators. The one I built is not causing the problem. The one built onto the CAMERA itself is the culpit. The problem is following the camera.
The camera was designed to run on 8-15Vdc so it is pretty flexible. It has it's own dedicated regulator/converter. I can power the camera directly from the 8-cell flight battery with no adapter needed. I prefer not to tho, I get snow in the image from the speed controllers.
For clarity I will refer to the one I constructed by it's file title, design#3.
No matter where the camera power is placed in the circuit, either directly connected to the flight batt. or connected to the 12V-out side of the design#3 converter, I get noise. Interestingly if the camera power tap is "up-stream" of design#3 and the speed controllers are on the "noise" changes in frequency with throttle setting, (i.e. voltage changes). So Quacker, I think you may have nailed it for me. It is possible that the camera converter is defective. I am going to try your suggestion on the camera converter.
Depending on your end use this setup may be the ticket or may be major overkill. The Trango Eagle is intended for commercial applications and is very expensive. The version I have has been superceded by the Eagle PLUS. The major difference between the two, the EaglePLUS also has a data pass through mode capable of transmitting RS232 at data rates from 1200-9600 baud with provisions for transmitting RS422/485. I am getting that one next. Wish it had been available when I made the original purchase. Retail ranges from $975-1000 for the TX/RX pair. You may be able to find someone who still has the original EAGLE version in stock at a discount since it has been replaced with a new revision.
With the supplied "rubber-ducky" antennas LOS range varies from 3000-6000ft. With my 14db gain antenna this could stretch to 2-4 miles. With movement involved I am staying within 1/3 published maximum figures for image quality. Practical range w/14db gain antenna, 1 1/2 miles.
Video is 460 lines/30fps. Image quality is only limited by your input. There are also two programmable alarm channels, (very useful for our application) in addition to audio.
Current system, (camera, converter, transmitter) AUW with original "rubber ducky" transmit antenna is 7.5 oz. Replacing the antenna and heavy SMA connector I can loose another 2.5oz. I have been following the exploits of yb2normal's foray into that arena with intense interest.
The transmitter manufacturers page is:
Hope I have covered everything. Thanks much.
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