Jump to content
Sign in to follow this  
dilbert

EMI problem with video overlay telemetry

Recommended Posts

I'm having a serious problem with my TL100 overlay board when I use it with my blackwidow 200mW system. The TL100 works fine by itself. I set it up with a video source and captured its images every 5 minutes for 24 hours. The altitude tracked the pressure altitude changes very well and airspeed indicator was always zero. Now, when I connect the blackwidow TX system, the altitude and airspeed reading go all over the place. I tried builing a metal enclosure for the board and sealed it well, but didn't eliminate the problem. I power the TX and TL100 from a separate batteries. I'm not sure where to go now. My plane is also electric with a brushless motor also powered by its own battery. Am I the only one with this problem?

Share this post


Link to post
Share on other sites

I've noticed that the TL100 unfortunately is quite sensitive to the RF energy radiated by the TX. When I use the 100mW TX I need to put the TX and board as far as I can from each other, and then place the TX antenna in the orientation that would affect the least.

And when the TX is a 500mW it's a real pain.

Share this post


Link to post
Share on other sites

So I'm not the only one with the problem. I don't have much of a choice for TX placement. I use foam planes (GWS e-starter) with the TX on the tail and the TL100 on the main cabin. I was surprised that a metal box failed to shield the RF. I didn't connect the box to the board ground. EMI/EMC is still black magic to me (and I'm an EE). I wonder if anyoune else have any tricks to minimize the problem that makes altitude/airspeed readings unusable.

Share this post


Link to post
Share on other sites
I was surprised that a metal box failed to shield the RF.
That only helps to reduce radiated EMI/RFI. The box must be grounded to your circuit.

There are other noise paths besides the radiated stuff. It also can travel through conducted paths. What have you done to protect all the wire leads going to the board? Hint: Toroids, feedthrough caps, and discrete EMI filtering, used in a combination that solves the problem.

Share this post


Link to post
Share on other sites

Nothing yet. I did another experiment. Hooked up the TL100 with a separate video source and separate battery playing back to a monitor. The readouts were rock solid. Next I powered up the blackwidow by itself and moved closer to the TL100 still in its metal case, and sure enough, the airspeed and altitude start jumping all over the place. So, the interference must be coming in through the external leads. I guess I'll have to experiment with ferrite beads. Do you think I should connect the case to the TL100? It appears to have separate ground planes, so a connection point is not obvious.

Share this post


Link to post
Share on other sites
Do you think I should connect the case to the TL100? It appears to have separate ground planes, so a connection point is not obvious.

You should connect it to the lowest impedance location in the TL power supply. I suggest a heavy wire to the TL's Vreg IC gnd.

Share this post


Link to post
Share on other sites

I'll try that. Turns out the TL has only one ground plane (on top of the board); the back has the power plane. I also added a ferrite bead to the power leads and it helped a lot. Now I have to do the tach and volt sensors.

Share this post


Link to post
Share on other sites
Turns out the TL has only one ground plane (on top of the board);

A solid ground plane would be the ideal lowest impedance point. So, that is what you should ground the metal box to (assuming you even need the metal box).

Share this post


Link to post
Share on other sites

I think we're getting somewhere now. I've grounded the board in two spots to the box (by the video ports and by the voltage regulator. I temporarily placed the tach and voltage sense leads inside the box and tried again. Almost no interference. Only a minor disturbance on the variometer display. Now I have to deal with the tach and voltage sense leads.

post-5-1137986210_thumb.jpg

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this  

×