yes, i've just learned that too. the sidebands are actually needed to reconstruct the original audio accurately. They extend infinitely in both directions, but in the real world you're limited by your available bandwidth. So, FCC "allotted bandwidth" includes not only your deviation range, but also the sidebands! That's a new insight for me. Therefor, the receiver must be demodulating the whole complex waveform containing the sidebands, not just the sinewave at transmission freq. Wow.

"mechanism which allows you to "slope tune" an AM receiver to listen to FM signals."

what!? ok, i'm going to have to research that.

http://www.radio-electronics.com/in...reception/fm-slope-detector-discriminator.php

"AM Noise is not a great problem with PLL detectors,but with Ratio Detectors & the like which convert FM to AM so as to detect it,this can be a major source of received noise."

ok, thx, i'm thinking PLL.

"The loss of information still affects all types of detectors,though,appearing as distortion."

thx, very helpful..

http://www.rdlnet.com/downloads/articles/am_noise_article.pdf

"2x Deviation is a fair "rough guide" to deviation for narrowband FM,but is not the whole story in the wideband case."

2x what? the highest freq in your source audio?

commercial broadcast FM is considered wideband, right?

I did say "2x deviation",which for 5kHz deviation would be "roughly"10kHz occupied bandwidth at the radiated frequency.

I made this point because it is often quoted by folk who have spent their working lives with narrowband Comms equipment.
It is just a rough guide,& isn't strictly correct,even for narrow band FM.(NBFM)
Yes,Broadcast FM is wideband (WBFM)

This article goes into detail about how to calculate deviation, and talks about sidebands. I don't understand the math.
http://www.silabs.com/Marcom Documents/Resources/FMTutorial.pdf

The Maths aren't all that hard to learn,so I recommend you do so.
The downside is they aren't hard to forget,either,till you get to the point where I am now---I look at it ,& say:-"Yeah,that makes sense",but I would struggle to work through it.
Just for now,try "reading around" the Maths,as there is still a lot of good stuff in the text.

"The dynamic range of any radio system is the ratio between the maximum modulating signal level & the minimum modulating signal level which it can reproduce at the receiver end of the system."

Yes. Ie, difference between the loudest sound and quietest reproducible sound.

"Analysis of FM modulation is not something which you can do on the basis of a few paragraphs in this forum. You need to do your own research"

I'm continuing to read everything i can get my hands on, but there's a lot i don't understand. I come to forums to get help where i don't understand the research.

"I haven't done any serious study in this field for about 30 years,so you have pretty much "wrung me dry".

i highly doubt that, you're a great resource.

thx]

If you can get your hands on an ARRL Handbook ( your local Library may have one),they have some good stuff.

In my (2001) edition.in Chapter 12" Modulation Sources (What and How We Communicate)",
There is a section on FM,which includes the useful & interesting Bessel Function graph.
Read it in conjunction with the section "Mixers & Angle Modulation" in Chapter 15 "Mixers,Modulators & Demodulators"

Of course,the Chapters may be different in newer issues,in which case,you will have to search through the book to find them.
The Chapter Index at the front of the book is good--the main index at the back is quite hard to navigate.

 

Is bandwidth determined by the absolute deviation occupied (eg "±75 kHz" or "±3 kHz")?

Or, is bandwidth determined by deviation RELATIVE to the frequency of the carrier?

In other words, does a ±75 kHz deviation on a 90 Mhz carrier contain the same amount of information as a ±75 kHz deviation on a 900 Mhz carrier?

thx!

According to FCC rules (if that matters) it is ABSOLUTE. If referreed to the center frequency, this is normally defined as "percentage bandwidth." While percentage bandwidth is an important parameter for actual r.f. circuit design, it is "invisible" to the actual data or other information.

 

According to FCC rules (if that matters) it is ABSOLUTE

That's interesting, can you be a bit more specific? You mean they dole out commercial FM BC band in fixed channel widths?

If referreed to the center frequency, this is normally defined as "percentage bandwidth".

Isn't percentage a relative value?

While percentage bandwidth is an important parameter for actual r.f. circuit design, it is "invisible" to the actual data or other information.

Invisible? You mean bandwidth has no affect on the amount of information that can be transmitted?

 

That's interesting, can you be a bit more specific? You mean they dole out commercial FM BC band in fixed channel widths?

Yes,that is one of the primary functions of the FCC where Eric lives,ACMA where I live,& any number of such acronyms in other countries.

Isn't percentage a relative value?

Yes,& it is important in some aspects of Broadcasting.
For instance,an AM Broadcast Station I used to work at went out to 10kHz audio bandwidth.
This meant its occupied bandwidth on the Medium wave AM band was 20kHz.
At its carrier frequency of 720kHz,its "percentage bandwidth" was 2.78%.

At 100MHz,the same "percentage bandwidth" would correspond to an occupied bandwidth of 2.78MHz!

The percentage bandwidth of a FM Broadcast station with an occupied bandwidth of around 200KHz is
about 0.2%.
From this you can see that the relationship between occupied bandwidth & centre frequency is important,& can be expressed as "percentage bandwidth".

Invisible? You mean bandwidth has no affect on the amount of information that can be transmitted?

We have touched on this earlier,now,with a better appreciation of the term "percentage bandwidth",can you see what Eric is telling you?

Eric is a very erudite gentleman,& his posts tell you something if you read them closely,& don't just leap in with another question.

 

That's interesting, can you be a bit more specific? You mean they dole out commercial FM BC band in fixed channel widths?



Isn't percentage a relative value?



Invisible? You mean bandwidth has no affect on the amount of information that can be transmitted?

Yes, ALL commercial FCC allocations have specific channel widths....this applies to AM broadcasting, FM broadcasting, and 2 way radio. The bandwidth CERTAINLY limits the amount of information you can transmit, the percentage bandwidth does not

 

My question was not about the role of the FCC, i understand the function of the FCC. My question was whether all allocated channels have the SAME width. Sorry if I wasn't clear.

I understand percentage bandwidth is the ratio of absolute bandwidth to carrier center-freq. But I don't understand your point. Are you saying we need to employ percentage bandwidth to maintain the same spectral efficiency at various carrier center-frequencies? Or are you saying agencies like the FCC employ percentage bandwidth when allocating channels? Or...?

I appreciate everyone's erudition. But I still don't understand the comment about invisibility. Sorry, I'm sure the meaning is quite plain!

 

My question was not about the role of the FCC, i understand the function of the FCC. My question was whether all allocated channels have the SAME width. Sorry if I wasn't clear.

I understand percentage bandwidth is the ratio of absolute bandwidth to carrier center-freq. But I don't understand your point. Are you saying we need to employ percentage bandwidth to maintain the same spectral efficiency at various carrier center-frequencies? Or are you saying agencies like the FCC employ percentage bandwidth when allocating channels? Or...?

I appreciate everyone's erudition. But I still don't understand the comment about invisibility. Sorry, I'm sure the meaning is quite plain!

Percentage Bandwidth is important to Engineers when they have to design RF Amplifiers,Antennas,& suchlike.

For instance,in Australia's old Analog TV system,Channel 2 had a Vision Carrier frequency of around
64 MHz,& Channel 28 had a vision carrier freq of around 528 MHz.

The channel width (Occupied Bandwidth*) specified in the TV standards was 7 MHz,so in the CH2 case,the
Percentage Bandwidth is around 10.9%,whereas for Ch 28 ,it is about 1.3%.
It is evident,that the challenges to an Engineer of making equipment with a flat frequency response over
the required channel width* are greater in the lower Channel case.

The Ch2 Station carried exactly the same amount of information as the CH28 Station.
The spectral efficiency is thus the same in each case.

Percentage Bandwidth plays no part in allocation of Channels in a particular Radio Service.

The FCC,ACMA,& others licence you to operate your FM (or other) station on a given centre frequency.
This is normally the only unique thing about your Station.

They also specify many things about the signal you may radiate.
In this context,Maximum Frequency Deviation & Maximum Occupied Bandwidth are specified .
These are exactly the same specifications for all FM Broadcast Stations.
It is then up to the Station Engineer/s to design a Station which meets the Standards.

When you sit at home & check that your nice new FM Station is still on the air,you listen with an FM receiver which is the same as the one in other homes.

This will probably be some form of Superheterodyne.
In such a Receiver,the Received Frequency is "down -converted" to an IF frequency with a centre frequency of (commonly) 10,7MHz,where it is amplified,passed thru a filter with a bandwidth (also specified in the Standards) similar to the specification for Occupied Bandwidth of your Transmitter.
It then goes off to be detected.

If you now retune to another Station--the IF doesn't know the different between this & if you had just tuned off your Station then back on.
The Occupied Bandwidth (in kHz) at the IF Frequency remains the same as that of your Station----it doesn't care about Percentage Bandwidth at the original Frequency radiated from any Station in the FM Band----hence invisibility!