advantages of FM over AM

[syeda amna]

why it is said that AM consumes more power than AM? why modern communication systems prefer FM not AM?

 

[ckshivaram]

FM is more clear in transmission than AM.Its wave length is short whereas the frequency is high and vise verse for AM. Here the frequency is modulated and in AM amplitude is modulated. Natural or human activity like traffic etc doesn't effect the FM transmission whereas AM transmission gets effected.
1. Lesser distortion. Frequency modulated wave is less susceptible to intereferences from buildings, traffic etc which provides improved signal to noise ratio (about 25dB) w.r.t. To man made interference.
2. Waves at higher frequencies can carry more data than the waves at low frequency.
3. Smaller geographical interference between neighboring stations.
4. Less radiated power.
5. Well defined service areas for given transmitter power.

 

[betwixt]

The wavelength has nothing to do with it. I use AM and FM at frequencies as low as 100Hz and as high as 12GHz, both use AM and FM equally well.
AM and FM can also be equally efficient. In some circumstances the AM modulator may consume power which drops the overall system efficiency but FM runs full power even when there is no modulation so there are plus and minus points for both methods.

The primary reasons why FM is used are:
1. It is less prone to some kinds of interference, bear in mind that almost all natural and man-made interference is seen as amplitude changes.
2. It does not require linear amplification stages.
3. It is easier to synthesize frequency shifts than amplitude shifts making digital modulation simpler.
4. It allows simpler circuits to be used for frequency tracking (AFC) at the receiver.

There are other reasons. If there is a disadvantage with FM, it is the need to encode digital signals before transmission so the receiver can correctly find the center point between '0' and '1' bits.

Brian.

 

[syeda amna]

thanks for your replies.
I have read FM is used in mobile radio systems rather than AM. is it correct? why?

 

[ckshivaram]

FM, offers some distinct advantages. It is able to provide near interference free reception, and it was for this reason that it was adopted for the VHF sound broadcasts. These transmissions could offer high fidelity audio, and for this reason, frequency modulation is far more popular than the older transmissions on the long, medium and short wave bands.

FM, In addition to its widespread use for high quality audio broadcasts, FM is also sued for a variety of two way radio communication systems. Whether for fixed or mobile radio communication systems, or for use in portable applications, FM is widely used at VHF and above.

 

[yemyat]

The most important advantage of FM over AM is the superior noise characteristics. You r probably aware that static noise is rarely heard on FM. The addition of noise is to a received signal causes a change in its amplitude. Since the amplitude changes in AM contain the intelligence, any attempt to get rid of the noise adversely affects the received signal. However in FM, the intelligence is not carried by amplitude changes but instead by frequency changes.

---------- Post added at 13:44 ---------- Previous post was at 13:30 ----------

I forgot ur first question and sorry about that. Another advantage that FM has over Am other than superior noise performance is the fact that low-level modulation can be used with subsequent highly efficient class C power amplifiers. Since the FM waveform does not vary in amplitude, the intelligence is not lost by class C power amplification as output amplitude due to the LC tank ckt flywheel effect. Thus, there is no need for high-power audio amplifiers in an FM transmitter and, more important, all the power amplification takes place at about 90% efficiency (class C), as compared to a maximum of about 70% for linear power amplifiers.

 

[saif haider]

the most important point to be noted for AM is that antenna design changes, especially the length as frequency is inversely proportional to wavelength and when we use frequencies of such short wave length. we have to design antenna of long wavelength that why we see that AM and FM carrier frequency reception is dependent on antenna length and orientation. also when we attach the antenna of long length. impedance is generated because of factor like standing wave ratio which increases power consumption.

 

[betwixt]

I must be a novice, I've only held a broadcasting license for 40 years, can someone explain that last post to me in simple terms?

Brian.

 

[yemyat]

I think he wanted to tell about AM needs larger antenna size than FM. When one uses large antenna, it will take more power consumption.

 

[keith1200rs]

It is simply a case that some people don't know the difference between modulation and frequency. Using FM instead of AM doesn't reduce the antenna size at all. Using VHF instead of MW does.

Keith

 

[saif haider]

frequency=(c/λ)​

If I try to listen to a typical radio broadcast on an FM frequency of 100 MHz (100,000,000 Hz), the waves carrying my program are about 3m (10ft) long. So the ideal antenna is about 1.5m (4ft) or so long. A shorter antenna will still pick up signals, but a longer one will be more effective. That's why you often have to pull out your antenna on a radio: folded in, it's not long enough to resonate (electrically oscillate) with the radio waves you're trying to capture.

Now for AM, the waves are 1000 times greater, so how come you don't need an antenna that's 3km (1.8 miles) long to pick them up? Well you do, you just don't know it's there! The AM antenna inside a transistor radio is a huge length of thin wire wrapped around a ferrite (iron-based magnetic) core, which greatly boosts the incoming signals, and that means it can be much smaller and more compact but still pick up the signals you need.

So far so good, but what about cellphones? How come they need only short and stubby antennas. Cellphones use radio waves too, also travelling at the speed of light, and with a typical frequency of 800 MHz (roughly ten times greater than FM radio). That means their wavelength is about 10 times shorter than FM radio, so they need an antenna roughly one.

 

[keith1200rs]

You are confusing frequency e.g. 100MHz with modulation method e.g. FM. You can transmit AM at 100MHz. It is not the modulation method which determines the frequency or the antenna length.

Keith

 

[saif haider]

OK it means that mobile sets could also receive AM by using the headset. GUDY lets go for AM no more FM.

 

[mystic07]

1. effect of noise in digital communication can be eliminated
2. We can compress/reduce the signal we want to transmit
3. digital signals can be encrypted so that unauthorized person cannot access it.
4. with digital techniques, extremely low error rates producing high signal fidelity is possible through error detection and correction
5. digital circuits are more reliable and can be produced at a lower cost than anlog circuits
6. combining of signals using time division multiplexing is simpler than combining of analog signals using frequency division multiplexing

 

[keith1200rs]

OK it means that mobile sets could also receive AM by using the headset. GUDY lets go for AM no more FM.

There is no reason why AM could not be used at any frequency. AM used to be more common at all frequency bands such as VHF. Now FM is used even on short wave such as the UK CB radio.

Keith

 

[DXNewcastle]

Its depressing to read from people posting on this electronics forum that they believe there is some necessary connection between radio frequency and the method of modulating a carrier.

I wonder if the second of the OP's questions was also founded in that mistake?

why it is said that AM consumes more power than AM?

by which we can assume (s)he meant "why it is said that AM consumes more power than FM?"
Presumably, if the OP was comparing AM & FM commercial broadcasting, then the lower frequency AM transmissions would be aiming to reach much wider audiences, with coverages of 100s or even 1000s of kilometer radii. This can usually be contrasted with a radius of 10s of kilometers for higher frequency FM transmissions. But of course the greater range of the AM transmissions is purely attributable to the lower frequencies and remains independent of modulation method.

 

[betwixt]

I wholly agree. It seems to be a commonly believed myth that FM means VHF and above and AM means long, medium and short waves. Frequency and modulation method are not related, at least beyond the Nyquist limit. It's interesting that even at frequencies around 1.2MHz in the MW band there are public broadcasts using COFDM to carry digital radio station broadcasts, and very good quality they are too! Equally, there are lots of AM signals around 1.8GHz in my area. Antenna size and polarization have absolutely nothing whatsoever to do with the kind of modulation.

Brian.

 

[saif haider]

i think, i talked about the carrier frequency which your r modulating (making changes in form of amplitude, frequency and phase).

 

[DXNewcastle]

Yes, saif haider, you did say so. But you actually said "AM reception is more dependent on antenna length and orientation" which is completely untrue. There is NO link between modulation method and antenna length, just as there is NO link link between modulation method and frequency.

These misconceptions don't help people on here looking for help (Yes, I know these misconceptions are consistent with some common practices in commercial broadcasting, but there is absolutely NO link between Modulation and Frequency; and absolutely NO link between Modulation and antenna length and orientation).

 

[saif haider]

i take my word back. "AM carrier frequency is more dependent on antenna length and orientation".