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Multiple FM transmitters and a single reciever???

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enggashar

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Is it possible to have multiple(upto 4) FM transmitters of the same frequency and recieve the transmission of all of them on a FM single reciever?
Please help me out in this regard.
Also let me know if there is any alternate solution.
Thanks
Ashar Ahmed
 

throwaway18

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If the transmitters are active at different times so only one is transmitting at any time then yes.
 

biff44

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The RF portion of an FM receiver is almost always a limiting amplifier, so the largest received signal gets through, and lower powered jamming signals get clipped. If the jammers are similar in power to the main signal, you get garbage out. If the jammers are lower in power to the main signal, they are ignored.
 

    enggashar

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enggashar

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Thanks Mr. Biff44, but can i assume from your theory that if i put 3 transmitters and 1 reciever, and only the one tx who transmit data will be picked up by the rx. the other two tx although powered up nut not transmitting will not destroy the data of the one who is transmitting?
 

throwaway18

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If a transmitter putting out an unmodulated carrier is significantly stronger at the receiver then a weaker signal carrying data then the receiver will just produce silence.
 

artem

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throwaway18 said:
If a transmitter putting out an unmodulated carrier is significantly stronger at the receiver then a weaker signal carrying data then the receiver will just produce silence.
That is true for limiting amp - discriminator based detectors. If there is no overlapping of frequency spectrum of each transceiver obfuscating transmitted data , in theory it could be possible to detect even if they are transmitting signal at same time and on same carrier frequency. But i think FM is not advantageous for such usage.
 

throwaway18

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in theory it could be possible to detect even if they are transmitting signal at same time and on same carrier frequency.
Yes, in theory anything is possible. It's an interesting idea but I don't think it would be worthwhile in practise. It would need a crystal filter tuned to the data. If the transmitter is near the receiver the LNA or the IF amplifers before the filter may clip.

Even with well engineered transmitters the phase noise of the carriers is to be 40dB down 3kHz from the carrier.

To get two transmitters with the same carrier frequency to not overlap they would have to have DC coupled modulators.

In my experiance a few watts from a transmitter a mile away can be enough to desensatise a receiver on an adjacent channel, 25KHz channel spacing.
 

biff44

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Well, he said "the same frequency". You would need to modulate the 3 carriers with some sort of orthogonal modulation, of which FM is not one.
 

artem

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You did not understand me correctly - english is not my native language .

It is nothing to do with adjacent channel if carriers are same , i am not intending frequency division approach related to carrier but to modulating frequency which comes to VCO of FM transceiver, though blocking receiver by few watts on mile means receiver is not good.

Modulating RF frequency with sine will give us multiple of harmonics grouped around carrier .
If each transceiver will be modulated with different frequency sine (modulating frequency) for same carrier frequency, the sum of spectrum will be nonoverlapped spectrum of sum of all signals - allowing us unabmoguosly detect modulating sines as they will be separated . The condition for choosing of modulating sine frequencies - sines and their significant for reception harmonics (let say up to 2nd) should not be equal one to each others. Hypothetic receiver (of course TX's and rx must be crystal controlled) can separate these harmonics - that could be for example multiple pll's working on IF operating in strict narrow frequency ranges so no lock on neighbour frequency will be happened.

So orthogonality is in modulating frequency not carrier .
 

throwaway18

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The frequency component of an FM signal carrying a sine wave will have phase noise limiting how close together the compoents can be.

There will be intermodulation between the frequency components in the receiver. Intermod products 30 or 40dB down will destroy weaker data signals.

The PLL's will take a long time to lock and unlock restricting the data rate.

If you modulate by switching between an unmodulated carrier and an FM signal carrying a sine wave then I think you would generate frequency components on top of other transmissions when switching between states so you have to limit the bit rate.

I'd have to do a lot of maths to consider this in detail but I think this would work much worse than using frequency division.
 

artem

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throwaway18, as you could see above i already mentioned - "in theory" and "not advantageous" .

If this approach could be fesible enough - FM had already to be in place as prime modulation for CDMA .

But it is not and i do not claim opposite.

Regards
 

fasaghandis

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In the theory,if they radiate sam program at the same time and at the same frequency like the single frequency network it is possible. But its coordination in practice is impossible.
 

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