All of you masters of electronics that are reading this, don't laugh, but I've never been able to find this out in any of my books. Is it possible to use amplitude modulation where frequency modulation is used, or vica versa? Or is that impossible? As you move higher up the electromagnetic spectrum, do rf modulation types change?

 

It makes no difference, it's just that FM is generally better than AM (for most purposes). But FM requires more bandwidth, so it's generally only used at higher frequencies - the higher you go, the more bandwidth you have, so FM gets more popular the higher you go.

But you can use either at any frequency - have you seen a lead guitarist bending the strings while playing? - that's frequency modulation at audio frequencies!.

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A couple of extra bits of information.

The receiver is likely to know the difference. There are some that will handle either but as a rule in AM mode it won't make sense of FM and vice versa.

Without going into a lot of detail ('cause my knowledge is limited) FM is often used to describe true FM as well as PM, Phase Modulation. I don't think the receiver can notice the difference however the method used to generate them is different.

An AM signal is actually two sidebands and a carrier. It occupies a bandwidth thats approximately 2 times the highest audio frequency. The sidebands are the result of the carrier being mixed with the audio. The carrier frequency doesn't change.

An FM signal deviates from the center frequency by an amount that is limited by the standards for the situation. Some services use less deviation (Narrow Band FM) and some, like broadcast use more deviation taking up more space (WBFM). In older ham FM transmitters you would notice that someone's deviation was too low because they sounded quieter even with a full strength signal. I assume that listening to an NBFM signal with a reciever set for WBFM might sound similar.

It seems as though AM and it's cousin, SSB (Single SideBand)are used for long haul communications. Bandwidth certainly has something to do with it but it isn't just the space it takes up. It seems that the more spread in a signal the more distortion that results when it makes hops and skip around the world.

There's more to it but I thought this might be a little help.

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stevez

 

Obviously, an FM receiver won't demodulate an AM signal, an FM receiver is designed to remove any AM variations in the signal. However, you can demodulate FM on an AM receiver, to a certain extent, it's called 'slope detection' - you tune the AM receiver slightly off tune, this gives it a frequency to amplitude characteristic, which will resolve FM.

It's a standard radio ham technique :lol:

But really, you need a receiver that matches the transmission you are trying to receive.

Just as you say, the result is exactly the same, most radio ham FM transmitters actually use PM rather than FM.

The big advantage of SSB is that it goes a lot further, simply because you have much more power available - you mentioned an AM signal has a carrier, and upper and lower sidebands - all the information is actually contained in either one of the sidebands. So by throwing away the carrier and one of the sidebands, you can concentrate all the power in the one sideband left - increasing the available power a great deal, probably between 3 and 4 times the AM power.

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nigel that was very helpful info. can u recommend some good books on this topic.

 

I would suggest looking for books about passing the RAE (Radio Amateurs Exam), it's about 30 years since I took mine :lol:

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Hey, thanx a LOT guys, u just solved a year long riddle for me. I just found this forum yesterday.

 

Nigel, at the risk of getting into one of those drawn-out discussions with you :roll: , I have a couple of questions/comments. I need to preface them by saying that I have no practical experience with transmitting and receiving FM/PM signals (except for listening to radio).

Frequency is the derivative of phase, or, alternately, phase is the integral of frequency. It seems to me that an FM demodulator receiving a phase-modulated signal would have a response that rises at 6dB/octave, and would therefore have terrible low frequency response. A PM demod receiving an FM signal would roll off at 6dB/octave, yielding lousy high frequency response. Maybe this doesn't matter in the audio range that ham radio covers (although even a decade would have a 20dB change)? Or am I missing something?

 

RonH - there is a page or so in the 94 Radio Amateur's Handbook that might start to address your concerns. I suspect that similar text exists in other editions. I think I understand it but not enough to explain it.

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stevez

 

Jeez, Steve, I haven't been to a library in years. Do you suppose that volume is posted somewhere on the Internet? :wink:

 

I know of no way to access this or similar references via the net. Wouldn't that be nice if we could do that and do it for free! Somehow I think "free" is the problem.

Send me a message and I'll work on getting you a copy of the pages.

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stevez

 

I think you are - amateur radio is only concerned with voice response, and you only use narrow-band FM for bandwidth reasons - but the speech quality is excellent.

As far as I'm aware, the phase modulation employed produces the same output as 'proper' frequency modulation - it's usually done at about 12MHz, you then multiply that by 12 to reach the 144MHz amateur band. The audio amplifier and modulator are normally filtered, presumably to restrict it to a speech bandwidth - but it could correct for your frequency rrsponse worries as well.

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I was just thinkin' the other day, is it possible to use both FM and AM at the same time? One would be able to transfer twice as much information if this were possible. Or is it just impractical?

 

I don't see any problem with transmitting it - an FM modulator works in the very low power stages of the transmitter, an AM modulator usually works on the output stage. So you could easily apply both types of modulation - but as to been any use or not, I've no idea!.

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They've actually been doing AM plus FM for a long time.

DSB - Double SideBad w/supressed carrier is almost AM. An FM signal plus DSB signal is what is transitted as FM stereo. What we think of as real FM occupies the first 15 kHz of the bandwidth then the DSB part is centered around a frequency of 38 kHz above the base frequency. The DSB portion contains the extra information that allows separation of the left and right channel. If all you have is an FM mono receiver it only needs a bandwidth of about 15 kHz. An FM stereo reciever bandwidth has to be quite wide - over 50 kHz, to handle all the information. As part of the transmitted information there is a 19 kHz pilot signal that the receiver locks on to. In the past (maybe currently too) they added even more stuff, way above the DSB stuff - called SCA and with the right radio you can hear that.

I have some friends who work for communications companies - they make commercial, military and gov't systems. They've learned to pack a ton of information into a signal - it boggles my mind.

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