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PCM fail safe

Old 07-08-2002, 12:29 AM
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lvmdl
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Default PCM fail safe

I´want to know if the FM radio can activate the fail safe system
in a PCM radio when both are turned on in the same frecuency.
Thanks Marcelo.-
Old 07-08-2002, 12:58 AM
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INJUN_EAR
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Default PCM fail safe

Old 07-08-2002, 02:17 AM
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mglavin
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Default PCM fail safe

It is possible for an FM/PPM system to swamp a FM/PCM RX on the same frequency. It largely depends on the strongest signal being transmitted.

This is strictly a dB (signal strength from the RX's point of view)
issue. A RX will accept the strongest signal that it recognizes. When FM/PCM is the communication medium, it makes it actually easier for a RX to lock in and act appropriately (as far as the RX is concerned) on that strongest (dB-wise) signal.

That said I think it is somewhat harder for this to occur than you'd imagine, but it can happen. PCM RX's enjoy approximately a 20db SNR advantage above other types of modulation... If I take a message, encode it as FM/PCM and FM/PPM, transmit both via FM, as does our TX's and slowly raise the noise level, the FM/PPM signal will drop out and become garbled at up to approximately 20 dB before FM/PCM. As the noise goes up, FM/PPM becomes unusable, FM/PCM keeps flying until such time the noise gets much higher.

PCM pulses don't have to be perfect; they may change amplitude, width or both. The D/A decoder is only looking for the presence of pulses, either they exist or they do not... If a PCM system is experiencing servo dither, it is due to a servo loop problem it is not associated with an RF link.

When were using FM/PPM which is analog, the pulse positions can be shifted slightly by RF noise between the TX and RX. Depending on the severity of the RF noise/noise's being picked up by the RX dither or glitching may occur and be realized via the servos movement and or lack there of control.

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