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HomeAircraftHelicopterRadio - Servo - Gyro - Gov - Batt › Interaction between channels 60 and 88
01-13-2003 09:21 AM  15 years agoPost 1
smudge

rrApprentice

Wiltshire, UK

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We had a scary time yesterday. I switched on my radio, (channel 88, PCM), and noticed that the servos were moving in time with my freind's flying helicopter. He was using channel 60, PCM.

He had nearly complete control of my helicopter. This was a channel 60 pcm transmitter, controlling a channel 88 pcm reciever.

The same happened with both of my helis, so it is unlikely that both receivers had a fault. The recievers are both dual conversion, FP149 and FP148.

Has anyone else seen this? It is very worrying, and shows the requirement for a good frequency monitor.

Bye for now,

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01-13-2003 11:13 AM  15 years agoPost 2
GT6

rrApprentice

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Hi Smudge

I think this effect lies with the spurious responses of the 455kHz second IF filters in the Rx. Somewhere I have the data sheets for some of the commonly used ones but I can't find them just at the moment. However I do recall seeing some pretty big 'holes' in the stop bands where the rejection was only -40dB or so. The separation between channel 60 and channel 88 is 280 KHz. The signal from your friend's Tx would be going into your second IF filter at (455-280) = 175KHz. If the IF filter has a spurious response in its stop band at 175KHz then it could make it through to the demodulator of your Rx. I'm a little surprised that the demodulator would demodulate something so far off the centre frequency but...

Since the main use of the first IF is to eliminate the image frequency for the second IF (910KHz or 91 channels away) I guess it may not have done much to weed out this one.

Did you have solid control of your model as soon as you turned your Tx on?
Regards
GT6

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01-13-2003 12:07 PM  15 years agoPost 3
donlynn

rrKey Veteran

New Zealand

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R30 135 tanks

Gidday
My concern would be will you pick it up on the scanner b4 turn on???!!!!
regards Don

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01-13-2003 03:08 PM  15 years agoPost 4
GT6

rrApprentice

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Hi Don

Very good point - if the effect is what I think then you won't pick it up with a scanner without also knowing where the spurious responses of the Rx are so you know what channel spacings to avoid. It would make running the pegboard a nightmare - you would need a peg for the actual frequency you were using and one for each 'sprog'
GT6

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01-13-2003 08:49 PM  15 years agoPost 5
donlynn

rrKey Veteran

New Zealand

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R30 135 tanks

Gidday
So is a option to have a spare battery, identical rx,xtal and servo on your heli's channel in your pocket to try before turning tx and rx on ?

Regards Don

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