but Mr. Ron is that we can we use RF tuned amplifier instead of RF amplifier so that we don't need a bandpass filter. Because by using this, we will be able to tune our desired stations at the RF stage itself. Anyway we are going to attenuate all other frequencies in the band other(than the required station) at some point before demodulation.So why maintaining undesired frequencies till the IF stage.Can u get what i am trying to say? and also don't know the exact purpose of the IF amplifier which is after the mixer section. Can u/anybody clear my doubts?
Perhaps I was not clear before. Yes, if you are receiving a relatively narrow bandwidth signal (and broadcast FM is indeed relatively narrow), then a tuned RF amplifier is the right way to go so that a separate filter is not necessary.
You are incorrect when you say that you will be able to tune the RF stations at the RF stage, and this goes to the heart of why we have an IF at all and why we use a superheterodyne configuration. The problem is that it is not possible to make LC (inductor-capacitor) filters narrow enough at 100MHz to allow only one FM station to pass. In addition, it is very very difficult to make a narrow filter that tunes evenly across the entire band. The station bandwidth is only about 180 Khz. This is a percentage bandwidth of 0.18% and such a narrow filter cannot be practically made using simple coils and capacitors. It is possible to make a narrow filter with other technologies such as crystals or SAWs, but such filters are made at fixed frequencies and are not tunable over a range of 88 to 108 Mhz.
This is an old old problem that was faced by early receiver inventors many years ago. They discovered that the most effective kind of receiver is the superheterodyne structure. In this kind of receiver, a special filter is manufactured with the exact bandwidth of one channel (eg 180 Khz for FM) but at some specific fixed frequency, usually a lower one than the band you are receiving. One common frequency used over the years has been 10.7 MHz and the channel filter that I mentioned is often made out of ceramic materials for this frequency. To make this filter useful, we use a "frequency converter" to change the frequency of the signal from what we receive on the antenna (eg 99 MHz) to the frequency of the channel filter. The frequency converter is designed to be tunable over the range 88 to 108 MHz and at any frequency it is tuned to it converts that frequency to 10.7 Mhz.
This idea has the additional advantage that it is much easier to make high gain amplifiers that remain stable at a lower frequency than at a higher frequency, and at a fixed frequency these amplifiers can be optimized very carefully without having to worry about frequency variation. It is also much easier to make a good demodulator at one fixed frequency than over a variable range of frequencies. So it is common to put most of the amplification, all of the channel filtering, and the demodulator at a lower, fixed, frequency and use a frequency converter in front of this. We call this fixed frequency section the "Intermediate Frequency" or IF section of the radio.
So, the short answer to your question is that we have an IF section so that we can practically have good selectivity. Without an IF filter, you will not get good selectivity.
Your in good hands over here. Hey RadioRon, let's add a few important factors to this concept.
