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what is the diffrence bewteen AM and FM

10/04/2008 8:35 AM

what is the diffrence bewteen AM and FM

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Pathfinder Tags: "AM and FM modulation"
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#1

Re: what is the diffrence bewteen AM and FM

10/04/2008 11:07 AM

http://en.wikipedia.org/wiki/FM

AM=Amplitude modulation where the height of the wave form changes.

FM=Frequency modulation where the hieght remains constant, but the hertz or frequency changes.

This was too simple of an answer--so you should read up on it! Good stuff.

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#2
In reply to #1

Re: what is the diffrence bewteen AM and FM

10/04/2008 3:13 PM

Boy, a picture is worth a thousand words. A GA to you!

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#3
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Re: what is the diffrence bewteen AM and FM

10/04/2008 3:32 PM

Thanks--I pulled the gif off the same link in the post incase the link doesn't get used.

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#4
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Re: what is the diffrence bewteen AM and FM

10/04/2008 11:22 PM

GA from me too! (in this case, GA stands for Great Answer) I wish I had had that image/movie back when I was teaching electronics!

Kudos to whoever it was that created it...

Dick

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#5
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Re: what is the diffrence bewteen AM and FM

10/04/2008 11:41 PM

I am adding it to my permanent collection of gifs.----And, Thanks dk.

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#7
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Re: what is the diffrence bewteen AM and FM

10/05/2008 10:00 PM

Great visual, but here is the rest of the story. The top waveform is the information that is to be broadcast. It is audio frequency - what we can hear. The higher frequencies shown in the curves labeled AM and FM represent the carrier modulated by that audio tone- that which can be efficiently broadcast from a reasonably sized antenna. For AM, this is 530 kHz - 1710 kHZ in the USA, and the same in EU plus 150 - 280 kHz as well. For FM, the band is 88 - 108 MHz. The AM waveform represents the multiplication of the carrier wave by the audio waveform. Your AM radio multiplies the incoming broadcast signal by a frequency that is 455 kHz off the broadcast frequency, then filters, amplifies and detects the amplitude modulation on the resulting 455 kHz waveform. With the FM waveform, the frequency deviation from the carrier frequency is proportional to the audio signal amplitude. When the audio signal amplitude is zero, the broadcast signal is precisely the carrier frequency. When the audio signal is at max amplitude, the frequency deviation from the carrier is at maximum. Typically about 15 kHz for the FM broadcast band. SInce FM is broadcast in stereo, the deviation of two channels up to 15 kHz from the carrier results in a total of 30 kHz total spectrum occupancy at max audio amplitude. The rate at which the frequency deviation changes gives the audio frequency. If the broadcast signal is changing at 1000 cycles per second, you will hear a 1 kHz audio tone from your radio speaker.

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#8
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Re: what is the difference between AM and FM

10/05/2008 10:32 PM

Okay............I am going to have to re-read this when there is total quiet here. I will give you a GA and hope that your information is as rock solid as it sounds--I will have questions for you, possibly.

Are you a H.A.M. or work in radio?

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#9
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Re: what is the difference between AM and FM

10/05/2008 11:15 PM

I am an electrical engineer who specializes in radio frequency interference (rfi), more commonly called electromagnetic interference (EMI) these days. Clearly, to understand and combat rfi, you have to have a working understanding of radios, both in terms of broadcasting and receiving.

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#10
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Re: what is the diffrence bewteen AM and FM

10/05/2008 11:21 PM

When was it that the AM band was extended to 1710kHz? I seem to remember 540-16xx, and I definitely remember being exasperated by the local road conditions being broadcast on 1710, when in my previous car ('87 model - used up to '04) the radio wouldn't tune that high.

I had never thought of AM modulation as multiplication (or it could well be that I knew that many years ago and forgot), but it does make sense, as long as the audio signal is considered to vary from 0 to 1.

Now the AM receiver is another story. The receiver ADDs (not multiplies, unless I am seriously mistaken) the incoming signal to the local oscillator signal, which is tuned to a frequency 455kHz higher than the incoming signal, so the difference signal is always 455kHz, which can be efficiently amplified by the IF amplifiers (tuned to amplify only 455kHz) in a superheterodyne receiver, and the result detected by a diode to produce the audio output.

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#13
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Re: what is the diffrence bewteen AM and FM

10/05/2008 11:51 PM

I do not recall the year when the AM band frequency range was extended, but like you I recall owning radios that didn't cover the extended range. In fact, I own several such radios. I like old radios and collect them. My wife would say I don't collect as much as simply don't throw old stuff away, and she's right. But that is neither here nor there. I think the discussion about adding or multiplying signals on the transmitter side is looking at the same phenomenon in different ways. Consider the following trigonometric identity from high school days: sin (a+b) = sin a cos b + cos a sin b. You are correct in your description about the receiver mixer function. I am uncomfortable with the simple term addition, because the mixer is a highly nonlinear device and the mixing of the received rf signal and the local oscillator causes all manner of mixer products (the accepted terminology and the reason for my perhaps injudicious use of the word multiplication). The intermediate frequency filter is there to select the desired mixer product out of the many created. I think we are saying the same thing in a different way.

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#6

Re: what is the diffrence bewteen AM and FM

10/05/2008 6:26 AM

If you were from the 60's, AM is American Music and FM is Foriegn Music.

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#11

Re: what is the diffrence bewteen AM and FM

10/05/2008 11:40 PM

AM- Analog Modulation.

FM- Frequency Modulation.

Regards

Earnest.

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#12

Re: what is the diffrence bewteen AM and FM

10/05/2008 11:45 PM

It is all about broadcasting. AM: Aplitude Modulation; FM:- Frequency Modulation.

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#14

Re: what is the diffrence bewteen AM and FM

10/06/2008 3:26 AM

I may as well throw my two cents in. If you take an audio signal of a maximum peak to peak voltage of A... Offset it by a dc voltage of A/2... and multiply it by the carrier frequency, you will get an amplitude modulated signal at 100% modulation.

Interesting point is that if you multiply A without the DC offset, you get double sideband suppressed carrier... and if you can eliminate either of the sidebands, you can get single sideband. Anyhow, your carrier signal goes away.

Bill

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#15
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Re: what is the diffrence bewteen AM and FM

10/06/2008 7:43 AM

OK, now you have me curious... What kind of circuit is used to multiply analog signals?

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#16
In reply to #15

Re: what is the diffrence bewteen AM and FM

10/07/2008 9:55 AM

Something called a balanced modulator... which is a diode bridge network of sorts. It is not a perfect multiplier, but it is a close approximation. It is also used to beat a recieved RF signal against a local oscillator to get an IF signal in superhetrodyne radio receivers... also known as a mixer.

In hidden transmitter hunting, when in close proximity to the transmitter, we will use one to move the hidden transmitters frequency off by 1 MHz or so such that our receiver is not looking at the same frequency as the transmitter. We also attenuate the signal at the same time by varying the level of our 1 MHz local oscillator going into the balanced modulator.

Wiki could probably give you a better description than I.

Sincerely

Bill

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#17
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Re: what is the diffrence bewteen AM and FM

10/07/2008 10:11 AM

Just as a quicky... frequency translation is a multiplication in the time domain, but is an addition (or subtraction) in the frequency domain.

Bill

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