fangin.com RC, photos & tech

Today I flew the PVC quad on another shakedown flight (after fiddling with the configuration via the GCS). I was flying over a fairly large oval, not very high but on the far side from where I was sitting when all motors lost power. I believe failsafe on my Futaba kicked in. You can hear in the video that the motors stop and then just before hitting the ground, they briefly start again. I walk over to the quad from the other side of the oval, perform a quick inspection to make sure nothings broken or fallen off and without any other changes or resets, power up the OP CC3D by holding yaw right and away I go.

 

 

 

 

I've used this Futaba Tx and Rx pair in nitro fixed wing models at much further range and never had an issue. The main difference is now I have a 150mW 1.3GHz video transmitter (http://www.readymaderc.com/store/index.php?main_page=product_info&cPath=11_30_38&products_id=1018) about 20cm away from my 7 channel 2.4GHz receiver. I've read about guys using low pass filters on the vTx so I thought it might be worth checking the interaction between the 1.3Ghz vTx and the 2.4GHz Rx. Fortunately a guy from work that came out to watch carries a spectrum analyser around in his car so we set about looking at how "dirty" is the 1.3GHz vTx.

 

The screenshots of the specan show a 2GHz wide band centred at 2GHz (so it's showing from 1 - 3 GHz). The first screenshot is using the "Max Hold" function on the specan which is effectively a cumulative trace that shows the max detected levels across the band and holds it. This screenshot shows my 2.4GHz Tx as well as the in building wifi and a few other small peaks. At this point, my video transmitter is powered down.

 

 

The second screenshot on the same scale (1 to 3GHz) after the vTx has been powered up for a few seconds. You can see a peak at 1280MHz (vTx frequency) but disturbingly, across the whole band are is a whole lot of noise coming from the vTx. The second harmonic at 2560MHz appears to be right where we see the Futaba Tx.

 

 

The effect of this noisy little transmitter means it's desensitising my 2.4GHz receiver. It's like someone talking really loud (1.3GHzvTx) standing right next to you (2.4GHz Rx) when you're trying to listen to instructions from someone talking in a normal voice on the other side of the room (the 2.4GHz Tx). 

 

To address the problem a 1.3GHz low pass filter can be fitted in-line on the vTx between the output and the antenna. The low pass filter allows the video signal to be transmitted down in the 1.3GHz band but anything above roughly that frequency is heavily attenuated. The noise coming from the vTx is no longer shouting in the ear of the 2.4GHz receiver so I can hear the instructions from the transmitter much easier and at a further distance. I've ordered this low pass filter (http://www.foxtechfpv.com/1400mhz-low-pass-filter-p-439.html) from foxtechfpv to hopefully solve the problem.

 

I forgot to mention why 1.3Ghz and 2.4GHz interfere with each other - harmonics. Check it out on wikipedia http://en.wikipedia.org/wiki/Harmonic

 

What I found most useful today was being able to see the problem right there on the spectrum analyser. Being able to see the video transmitter peak at 1280MHz and then all the  smaller peaks across the rest of the band including the 2.4GHz area right where my Futaba Tx is.

 

What will be interesting to look at next is after I fit the low pass filter, how does it look on the spectrum analyser. I expect it should look much cleaner. Something else I also want to look at on the specan is the 1 watt 1.3GHz vTx  fitted to my DJI F450. I've flown that at much further range without issue. Is the vTx a much better one where it transmits only on the frequency its supposed to? I'll hook it up and find out.

 

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Without the "Max Hold" function on the specan, this is how it looks.;

 

First with the 1.3GHz vTx off

 

Then with the vTx on