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pseddon

Oracle Diversity and Airwave 625 receivers

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I have an Oracle diversity system with two Airwave 625 receivers and goof-patch antennas driven by a PAL signal.

One receiver seems to be quite different to the other as the picture is over contrasted and partially distorts or pulls as the signal strength varies. Sometimes the picture slips to the left. I have swapped the antennas and that makes no difference.

Is there any hope/adjustments for the receiver or should I change it?

Regards Peter

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If the video quality changes with video content, then it is usually a video tilt (poor low frequency response) issue. So the first thing to check is the coupling cap on the video output. Perhaps the cap is bad (use a low ESR 470uF Electrolytic or Tantalum) or perhaps is installed backwards.

To check to see if it is backwards, operate the video link while you measure the voltage across the cap. The meter will show if the polarity is reversed. If it is, then toss the cap and correctly install a new one.

If that checks out fine then you might try playing with the variable cap on the PLL circuit. You can get to it by popping off the module's top metal lid. You need to use a non-metallic wand to tweak the "pot." I normally don't recommend touching this adjustment, but you have nothing to loose.

Beyond that, try a new AWM625Rx module.

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Thank you Mr RC-Cam,

Here is a bit more info:

1. Changed the video coupling capacitor - no change

2. Changed the receiver - no change

3. Changed the antenna - no change

4. Fed receiver from separate 5v supply - no change

The video wavform has some frame synchronous (50Hz mains?) interference on it that is causing the line synch pulses to disappear and the video to be pulled into where the synch pulses should be. The frame synch pulse is also distorted and is nearly zero at its start and slowly gets bigger through the frame synch period.

I can now only suspect the wiring/layout or maybe the video cap, which is from the same batch as the original.

regards Peter

ps what does the pll pot do - set the centre frequency for the pll that determines the frequency of the output (2.4Ghz)?

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2. Changed the receiver - no change

Just to confirm, you installed a new receiver and it has the same problem? If so, how does the bad receiver board differ from your working one?

If you unplug the two receivers (bad one and working one), and swap them on Oracle, does the problem follow the bad receiver?

What are you using for a video transmitter? It needs to be an Airwave so that everything matches up.

The frame sync pulse is also distorted and is nearly zero at its start and slowly gets bigger through the frame sync period.

That is a description of very bad Video Tilt. Video Tilt is a low frequency performance problem, usually due to the video coupling cap but can also be caused by the RF modules. Are you sure the video coupling cap is the right value and installed the correct way?

hat does the pll pot do

The PLL sets the operating frequency. If it is not perfectly correct, the video may have distortion. But since you now have two modules that do the same thing, this isn't the problem.

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Just to confirm, you installed a new receiver and it has the same problem? If so, how does the bad receiver board differ from your working one?

If you unplug the two receivers (bad one and working one), and swap them on Oracle, does the problem follow the bad receiver?

What are you using for a video transmitter? It needs to be an Airwave so that everything matches up.

That is a description of very bad Video Tilt. Video Tilt is a low frequency performance problem, usually due to the video coupling cap but can also be caused by the RF modules. Are you sure the video coupling cap is the right value and installed the correct way?

The PLL sets the operating frequency. If it is not perfectly correct, the video may have distortion. But since you now have two modules that do the same thing, this isn't the problem.

Both receivers seem to exhibit the same problem.

I changed the layout and components on the first bad receiver and no change. I haven't yet had a fiddle with the pot - I will report back if I get any joy.

On the second receiver I built a test rig and tried it on a different video coupling capaitor again. I have noticed that the video output when driving a 75R load is twice the amplitude video signal (about 2.5v peak from the top of the frame sync pulse) compared to the one good receiver. I tried adjusting the pot on the second bad receiver and could improve things but the amplitude was still very high and things seemed to be unstable with the problem appearing and disappearing.

I have now sent off for yet another receiver!

Thanks for all the help.

regards Peter

Edited by pseddon

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Well all is now working and this is why:

I tried using an airwave transmitter and that gave different results again but seemed to show I had one very faulty receiver and one that had some high frequerncy interference at the top of the picture.

A new Airwave receiver has enabled me to get two good receivers working with the Oracle diversity system but the new one has a higher video output signal than the other good one.

The transmitter that I had been using to check everything and now seems to be faulty is one of those small back of the camera types with 10mW output. I will check the channel settings but I wonder if 'size matters' and the smaller the worse the quality of the signal.

thanks for all the help, regards Peter

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Glad to hear you are on your way to solving the problem.

The issue with mixing brands of A/V equipment is because there is often a difference in frequency allowances, video pre-emphasis techniques, and audio sub-carrier operation. For those reasons I encourage folks to stick with the same brand of wireless Tx/Rx A/V equipment that is designed to work together. Otherwise, goofy things may happen.

Plus, those little Tx's stuck on the back of CMOS cameras are usually very poor choices. There's only so much you can do with $1 worth of components. :)

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