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Here’s a question I once asked an FM engineer: what if you frequency modulated and amplitude modulated the same carrier? Would it be possible to transmit 2 different audio programs? The engineer said he didn’t know.
IF it worked, it would provide an analog method for what’s being done now with digital systems: sending out multiple program streams from one station.
FM sub-carriers are not part of the discussion, as they reside within the FM envelope.
Total posts : 45366.
Carl,The simple answer is yes, it can be done.
Here’s the not so simple answer. If one looks at the spectrum of AM and FM signals on either a spectrum analyser or in a textbook one sees that sidebands are produced on both sides of the carrier frequency. To follow this it is important to note that this view is in the frequency domain where an oscilloscope presentation is in the time domain. The time domain pattern results from the instantaneous vector summation of all the sidebands and the carrier.
One will see in the frequency domain that the AM and FM sideband signals occupy the same spectrum space (though the FM sidebands extend beyond those of AM) so applying AM and FM at the same time will produce composite sidebands which will sum as distorted AM and distorted FM. On another board it was reported that a particular Ramsey AM25 was simultaneously producing FM and the poster stated that he could hear the distortion in the AM received audio. This can well be the case since an AM receiver will interpret the FM sideband contributions as AM which do not accurately represent the modulating audio.
If one takes a sine wave audio signal and applies it to an AM transmitter and observes the output on a spectrum analyser it will be seen that the amplitude of the carrier does not change. On AM, the amplitude of the carrier is constant!
The same test on FM will reveal that the amplitude of the FM carrier changes! In fact at certain ratios of deviation to modulating frequency the carrier disappears entirely. The reason it appears just the opposite on an oscilloscope is that in the time domain we are looking at the composite signal which is the sum of the carrier and all the sidebands and, for example, on FM this sum is constant giving the result that the FM amplitude does not change.
So I suppose I have over answered your question. As always, things are not that simple since modulation signals could be encoded by a combination of phase and amplitude (phase changes are FM) modulation and this is done with digital systems (QAM) and was done with analog stereo AM. But a receiver designed to receive and separate the signals would be needed. In fact, these letters I type will be sent to you using a digital signal which is a combination of AM and FM over the cable leaving my house.
Neil
Total posts : 45366Thanks to radio8z I don’t have to think about AM on FM anymore. Now let’s see, what’ll I think about next…
Total posts : 45366Good question, Carl, and I think you got a pretty good answer by the first follow-up reply.
Simultaneous AM and FM (with the same program material) was the basis for “Noise Free Radio,” a system proposed by George Yazell, a oldtime radio engineer, to mitigate impulse and other noise on the AM band. George demonstrated his system in the ’80s, I think it was, even taking it to the Nat’l Assn. Broadcasters (NAB) convention one year. His demo, which was cobbled-up from a variety of consumer and ham gear, was really quite impressive. He used one of those ‘plasma globes’ as a source of noise that obliterated the AM component of the broadcast, but the FM part rode right through it. George was invited the next year to actually deliver a paper to the NAB. An engineer from Harris Corp. stood up and essentially called him a nut, that the “…mathematical analysis of the system just doesn’t work.” George simply smiled, turned on his demo and said “Voila,” or something to the effect. The Harris guy had no comeback for that and sat back down. George was able to get FCC approval for short-term tests on a Florida AM station, but nothing ever came of Noise Free Radio. I still have a .pdf of George’s paper, which I can e-mail upon request. Just one of those ideas, shrouded in doubt and mystery, that might have amounted to something if pursued. Certainly simpler than HD Radio!
Total posts : 45366Thank you Electrojim for providing such a conclusive answer to my FM On AM question. I would enjoy seeing the .pdf on George’s work, thanks for that.
carl atsignhere kdxradio dothere com [edited by moderator so spam crawler will have a harder time finding it]I think NFR might be a preferred way of upgrading AM rather than so-called “HD.”
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