Ruben Undheim said:
Hello!
The reason for that I asked was that I have studied some
schematics of the superheterodyne receiver, and they have
all been with either two IF's(10,7 and 455), or only one
455KHz IF. Therefore I couldn't understand how it was
possible to receive 200KHz bandwidth radio, when I couldn't find
a 455KHz filter with more than 35KHz bandwidth. My problem was
that I thought that it was not possible to get sound out of a
10,7MHz IF without the 455KHz stage,
since I had not seen any examples for it.
Hi.
There is no problem with retrieving the sound from an FM signal with such a
design, dual conversion is used for other reasons.
A low IF is good for selectivity and demodulation reasons, but it leaves the
image frequency closer to the wanted one. For example, with a 455KHz IF, the
receiver will also pick up a signal 910KHz (twice the IF) away from the
wanted signal - just as if it were on the same frequency.
A dual conversion uses a high first IF, allowing the front end circuit to
attenuate the image at 21.4MHz away (for a 10.7MHz IF), and a low second IF
allowing selectivity. The first IF filter will have a bandwidth far below
910KHz - attenuating the 2nd IF image frequency before it gets to that
stage.
For broadcast stations, using wide band FM, an IF of 10.7MHz is reasonable.
A lower IF may not allow for the 200KHz bandwidth. As it already places the
image 21.4MHz away, another conversion stage before it is not required.
For communications radios, using speech, narrow band FM is used. It is very
possible to get such signals within a 10KHz channel spacing, with receivers
having 60dB or greater attenuation on that adjacent channel. This requires a
low IF, so the dual conversion is useful to get the best of both high and
low IF systems.
I know I've said it before, but this is a great book for radio topics:
Radio Handbook, 23rd Edition.
William I Orr, W6 SAI.
Chapter 6 covers receiver fundamentals, and includes topics such as image
response, selectivity and dynamic range.
Beg or borrow a copy if you can, although it is always worth having a copy
about. I got my copy new, but I usually go down my library and local charity
shops looking for cheap books for sale.