Radio question

[Pengineer]

Just seen a MCQ regarding sidebands in FM transmissions, can't remember exactly but something like
a) does not exist (or not used)
b) same as for AM
c) same as for AM but.........


Any clues please
Or any other radio comm/nav MCQ's (and answers)

[Bus429]

Pen,

AM - increased depth of modulation increases sideband power and thus total transmitted power.

FM - total transmitted power always constant but increased depth of modulation requires increase in bandwith. The differnce is the bandwith required to transmit a relatively undistorted signal. Increased depth of modulation means increased deviation and therefore an increased modulation index. More distant sidebands require significant amplitudes (of modulation).


Does this help? Not from memory!!! I have a text book to hand!

:) :confused:

[ 12 August 2001: Message edited by: Bus429 ]

[Clever Bloke]

Also an FM signal has a theoretical infinite number of sidebands.

[Pengineer]

As you can see, I'm preparing to sit the radio exam. It seems to be very theoretical compared with the others I've done with lots of things to try and remember, freqs, bandwidhts, channels, formulae etc etc.
When i was doing electrics there was lots of little memory aids for left and right hand rules, Ivy Watts, CiViL for ac circuits etc etc, does anyone know any clever little radio related ones? My memory is getting worse nowadays and every little helps.
Thanks

[Bus429]

Pen,

I'm a thicko at the best of times. When I was doing my radio course, I was told to just accept some theory rather than try to understand it!! One book I read stated that it is not certain that radio waves actually behave as stated.

I will try to get you the ISBN of the book I used for the theory (written by an Australian and an American).


;) :D

[Pengineer]

Next question:
In a SSB transmitter a 10Mhz carrier is AM by a 10khz signal, the o/p is...
A)A range of tones from 10-10.01MHz
B)10.01khz
c)9990khz

And why?

Also with the advent of 8.33khz channel spacing is the number of channels available now 760 x 3?

[Bus429]

Oops - did not RTFQ!!!

Pengineer - try this book:
Electronic Communication Systems by Kennedy and Davis. Published by McGraw-Hill, ISBN 0-07-112672-4 ;)

[ 14 August 2001: Message edited by: Bus429 ]

[ 14 August 2001: Message edited by: Bus429 ]

[GoodToGo!]

Pen, errrr, I think 'A' in answer to your last question. By the way I think you mean 10.01Mhz for the answer in 'B' (not Khz)

Cheers!

:cool:

[Bus429]

Original frequency + (fc+fm) + (fc-fm) I think! :D

[Don D Cake]

Re the SSB question.

When you mix signals (and that's all you do when you AM modulate), say Fc and Fm you end up with Fc, Fm, Fc + Fm, Fc - Fm and Fm - Fc.

So in a double side band transmitter with a carrier of 10MHz (Fc) and a modulation frequency of 10kHz (Fm) you would transmit the carrier frequency (Fc) and two side bands (Fc + Fm and Fc - Fm). The modulation frequency (Fm) would be filtered out and Fm - Fc is a negative value so it can't exist. That is you would transmit three discreet signals, 9.99MHz, 10MHz and 10.01MHz.

It is inefficient to transmit all this stuff so you can get away with transmitting one side band only and use a regenerative receiver with a local oscillator. This type of transmission is called Single Side Band Suppressed Carrier.

This is where your question is vague, you have to make some assumptions. You have to assume that the carrier is suppressed and that the side band transmitted is the upper one. In this case the answer would be 10.01MHz. If the lower side band is used the answer would be 9.99MHz.

[Don D Cake]

Oh and re your FM question....

I would guess c).

If you AM modulate 10MHz with 10kHz you get two side bands 10kHz either side of the carrier (as mentioned above). If you FM modulate the same signals you get an infinte (in theory) number of side bands spaced 10kHz apart either side of the carrier.

So it's the same as AM but....(they go on for ever)

And I've just done it using an RF signal generator and a spectrum analyser to prove it to myself.