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900 Mhz Video Downlink Interfering with 2.4 Ghz Uplink

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LAN_403

Execration

Sat Jul 28 2007, 11:03PM

Registered Member #929 Joined: Sat Jul 28 2007, 06:14PM
Location:
Posts: 3

As part of my current job, I am in charge of putting together a R/C aircraft capable of being video-piloted 4-5km (3-4 miles) via line-of-site. It will carry scientific instruments over sites that are normally hard to get to, such as the terrain surrounding the peak of a volcano.

To accomplish this goal, a 2.4 Ghz XtremeLink uplink was chosen due to its advertised 5 mile range. This is the only off-the-shelf system that could potentially supply the range I need. So far weâ€™ve tested it to 4.5 km on the ground (after 4.5 km line-of-site was lost), beside a highway where there are lots of obstacles and potential sources of interference such as power lines etc.

The downlink chosen was a 900 Mhz 2-Watt video transmitter. The base station is using an 8 dBi patch antenna, which should be more than enough to cover a 5 km range. Here is where my problem starts.

When I went to range-check my 2.4 Ghz radio with the 900Mhz transmitter turned on, I was unable to get more than 50 feet away at full power before I lost total control. This is MAJOR interference. I was puzzled, especially considering the separation of the two frequencies. A normal range check consists of unscrewing the 2.4 Ghz antenna on the Tx, and holding a button which reduces the output power to 10%. You are then supposed to be able to get at LEAST 50 feet (150 reported by some) from the aircraft before transmission is cut off. I couldnâ€™t even make it 50 feet with antenna on and full power, let alone 5 km! I need help in troubleshooting this interference problem.

The first thing I did was try separating all the wires and components from each other, outside the aircraft in a test area. I noticed that if any servo wires came too close to the 900 Mhz antenna, the servos would lock up to one side and wouldnâ€™t respond until at least 5 inches of separation was given. Once separated, they worked flawlessly. After separating the 900 Mhz transmitter and its power supply from the 2.4Ghz receiver and its power supply, with no wires crossing each other, the range was significantly increased. I could now successfully perform a range check, and probably fly successfully as long as I wasnâ€™t flying first-person. However, the range check indicated that my control crapped out sooner with the 2 Watt video Tx on that without. In other words, the 900 Mhz is still causing interference, just not as bad. If I am going to fly 4-5km however, this interference cannot be ignored, especially if a range reduction is noticed on the ground. My goal here is to have both of these components operating with each other such that no impact can be detected if the 900Mhz Tx is on or off.

I will now detail my exact setup for further clarification. For my uplink, I have the 2.4 Ghz receiver with its own dedicated battery. The elevator and rudder outputs are fed into a Co-Pilot stabilization system. This device takes readings from an attached IR sensor and adds control inputs to the rudder and elevator to keep the aircraft completely level during flight. Servo extensions are used, but they can be eliminated if need be.
The transmitter is running off the main battery pack. A DC-DC Converter is used to regulate the 14.8V 4-cell lithium batteries to 12V. Long power lines are used to reach the Tx, which is mounted just past the trailing edge of the wing. The colour KX131 camera is being supplied a regulated 5V power supply from the unused BEC of the speed controller, and thus is also running off the main batteries. A coaxial cable is taking its output and feeding it into the 2 watt Tx via an RCA cable.

I originally had a lot of wires passing each other, and I am blaming that configuration for my initial, horrible range. After the components were separated, and kept as â€˜modulesâ€™ (Tx and Rx separated), the range greatly increased, however itâ€™s still not as good as when the 2 watt 900 Mhx Tx is turned off. Is there anything that can be done to stop this interference? My boss suggests it isnâ€™t the 2.4Ghz receiver antenna picking up the 900 Mhz signal, since it works, just only up to a certain range. He says perhaps the pre-amp or diode arrays in the receiver are being saturated by the 900 Mhz RF. Perhaps a set of in-line filters or some shielding could prevent this? Please see the attached images for further clarification of my setup. Any help is greatly appreciated, thanks.

24receiverpg0

[Edit: Image size]

...

Sun Jul 29 2007, 01:15AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

Besides physically moving the antenna apart, there isn't much you can do without a degree in RF engineering... You can buy filters that will block 900mhz but pass 2.5ghz, but building one yourself will probably just end up further reducing the range due to the losses in a poor filter.

Execration

Sun Jul 29 2007, 02:37AM

Registered Member #929 Joined: Sat Jul 28 2007, 06:14PM
Location:
Posts: 3

... wrote ...

there isn't much you can do without a degree in RF engineering...

I am psoting in the hopes that an RF Engineer could offer some advice. I work with people who have plenty of RF experience, and from their experience, RF is a very tricky subject. I figured the two frequencies would be far enough apart to avoid any interference, but apparently the 2 Watts of 900Mhz RF output is getting into the 2.4 ghz receiver anyways. Where should the filters be placed? At the base of the antenna?

...

Sun Jul 29 2007, 05:16AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

The filter would be spliced inline with the antenna somewhere...
We use stuff like this, but they are quite expensive...

Something like this might work, but that has a 10db return loss...

WaveRider

Sun Jul 29 2007, 09:35AM

Registered Member #29 Joined: Fri Feb 03 2006, 09:00AM
Location: Hasselt, Belgium
Posts: 500

If you have access to a spectrum analyser, check the level of 3rd harmonic output (at 2700MHz) from your 900MHz DL transmitter. My bet is that this is bleeding through into your UL receiver, saturating the front end thereby reducing sensitivity. (The front-end band-select filter on the 2400MHz Rx is probably not selective enough to stop this.)

A 2700 MHz band-stop filter on your receiver may clear this up...

Let us know how it goes. You are right, RF can be tricky stuff!!

Steve Conner

Sun Jul 29 2007, 10:03AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Here are some things you can try in roughly increasing order of desperation:

Low-pass filter on the 900MHz transmitter (to reduce harmonic output like Waverider said)

Cavity bandpass filter on the 2.4GHz uplink receiver (highly selective, passes 2400, stops 2700 and everything else)

3rd harmonic filter on the 900MHz transmitter using coaxial stubs or whatever (in case the low-pass filter wasn't enough)

Enclosing the 2.4GHz receiver in a shielded box with EMI filtering of all servo and power wires etc. that penetrate the box (in case the 900MHz or 2700MHz interference is getting in by a route other than the antenna. The cavity filter you added previously stopped it getting in the antenna port, so now there is theoretically nowhere it can get in...)

Doing likewise to the 900MHz TX if it wasn't shielded already (in case harmonics are getting out elsewhere than the antenna port)

Relocating the 2.4GHz and 900MHz antennas as far apart as possible: for instance one in the nose and one in the tail: or on opposite wingtips

Alcohol abuse

Resignation

A new career in landscape gardening

Dalus

Sun Jul 29 2007, 10:23AM

Registered Member #639 Joined: Wed Apr 11 2007, 09:09PM
Location: The Netherlands, Herkenbosch
Posts: 512

Do you know if your reciever is shielded, the 900 Mhz rf could be directly bleeding into the circuits of your 2.4 Ghz reciever. Maybe you should check this first by shielding your reciever with a piece of aluminium foil wraped arround it.

WaveRider

Sun Jul 29 2007, 02:03PM

Registered Member #29 Joined: Fri Feb 03 2006, 09:00AM
Location: Hasselt, Belgium
Posts: 500

If you work with people with RF experience, you should check the 3rd order spurious emissions of the 900 MHz transmitter using a spectrum analyser. Your RF colleagues will know how to do this. If they are more than 60-70dB down from the carrier, they will not likely be the culprit. In this case, 900MHz signals are likely leaking into your Rx LNA or mixer possibly causing harmonic mixing. Rx shielding and decoupling of power supplies/low-freq wiring will likely put a stop to this.. All this depends on the radio architecture, though...

If harmonics from the transmitter are indeed the problem, a 2700 MHz notch filter on the Tx output as Steve says, is probably better as using one on the input of the Rx will degrade its noise figure (and sensitivity). Shielding everything is a must. Many transceivers in this frequency range use ceramic coax filters or LTCC filters which sometimes pass significant power at the third harmonic..

Cheers!

Dave Marshall

Sun Jul 29 2007, 10:06PM

Registered Member #16 Joined: Thu Feb 02 2006, 02:22PM
Location: New Wilmington, PA
Posts: 554

I'd be interested to know what local oscillator frequency each of the transmitters is using. I've encountered some poorly constructed UHF/microwave equipment that was phenominally noisy well below the intended TX frequency, because of poorly constructed/filtered local oscillators and frequency multipliers. Its possible if they're on LO freqs that are very close, there is some serious cross talk going on between the circuits. If this is the case, shielding should help alot.

Dave

Carbon_Rod

Mon Jul 30 2007, 06:27AM

Registered Member #65 Joined: Thu Feb 09 2006, 06:43AM
Location:
Posts: 1155

The curse of noise. =]

Some ideas:
a.) Check if your controls work at best case range with everything off.

b.) Fly the antennas as far from each other as possible (shielded until each wingtip perhaps.)

c.) Put RF chokes, inline 5ohm carbon resister, and high speed 33uF tantalum capacitors on the Power supplies to each.

d.) tuned RF filter on each antenna RX. (Dave is correct about cheap units that cross talk.)

e.) Consider half-duplex antenna switching with a sync chip locked on to the video's frame rate (~30fps to ~15fps). (Limited range, but at the other end it could key a 1/3 period clear broadcast window.)

f.) Switch to Digital camera and some packet system with error handling.

Cheers,

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