Why is intermediate frequency required

Introduction: The term IF refers to Intermediate frequency which lies between baseband frequency and carrier frequency on frequency spectrum. Down converter. RF PA Power Amplifier is used to provide amplification to the signal before transmission into the air. In RF receiver, received frequency spectrum which is at higher frequency is converted to spectrum centered at intermediate frequency which is at lower frequency. There are three different types of RF receivers.

Here RF mixers are not used. I referred this question, but its answer didn't explain about the need for IF conversion. If you are only interested in receiving a signal from one station you may not need to have or use an intermediate frequency.

You can build your receiver to tune in to just that frequency - the tuning needs to be sharp - you need to reject all possible other sources that may pollute the signal you want. This is done by a bunch of band pass filters that together, have a passband that is wide enough to cope with the signal you wish to receive but not so wide that it lets others in.

Now say you wanted to tune in to 2 stations - you'd have to re-align all this filtering to coincide with a new station. Historically radios were simple and moving a bunch of tuned band pass filters to a new centre frequency would be hard. It was a lot easier to have a bunch of fixed band-pass filters that did the majority of all the unwanted channel rection rather than trying to align them as you tuned the dial. Thus super-heterodyne receivers were conceived.

The incoming broad range of many radio stations were "mixed" with an oscillator that can be simply tuned with a dial - this produced sum and difference frequencies and usually the difference frequency became the new "wanted" frequency. Don't hang me on this - it could equally be at Maybe someone can modify my answer or advise me on this.

It's a small matter though because the point is that once you were able to manipulate the incoming signal's carrier frequency you can feed the result through a tightly tuned fixed set of band-pass filters before you demodulate. There is a sensible reason - if the oscillator were exactly tuned to pick up 88MHz i. Of course if someone started transmitting just outside the FM band you may pick this up but I believe that legislation prevents this.

Following recent activity in this question I remembered that there is another valid reason for using an intermediate frequency. Consider that the signal from an antenna might be in the order of 1 uV RMS and then consider that you'll probably want the radio circuit to amplify this to something like 1V RMS forgive the hand waving at the demodulator. Well, that's a gain of 1 million or dB and, no matter how hard you might try, having a circuit board with a gain of dB is a recipe for feedback disaster i.

What an IF gets you is a break in the signal chain which prevents oscillation. So, you might have 60 dB of RF gain then convert to your IF and have 60 dB of IF gain - the signal at the end of the chain is no longer frequency compatible with what happens at the antenna and therefore, there is no theramin effect!

Some radios might have two intermediate frequencies - for just this reason alone you can reduce the RF gain to 40 dB and each IF stage can have a gain of 40 dB and NO theramin.

IF makes the receiver both more economical and higher quality. RF parts are trickier to make and use, and the circuitry more beset with problems of stray capacitance, inductance, noise, ground loops and interference. More so the higher the frequency. But we must have an RF front end because the signal at the antenna connection is just too weak to do anything with but amplify it. Necessary but expensive, designers want to minimize the amount of RF circuitry. OTOH, we want good selectivity. Transmissions are allotted bandwidth, and multiple transmitters are under pressure to be squeezed together next to one another in frequency.

We want a flat passband for the desired frequency, and complete blockage of frequencies outside that. Perfection is impossible but tradeoffs can be made for a "good enough" filter. This takes advanced filter design, not just a plain LC tuned circuit. While this could be done in RF, in theory, in practice it'll be tricky and expensive, and hard to make stable against temperature changes and aging. We can make better filters meeting complex response requirements at lower frequencies, e.

The lower the frequency, the easier it is to design a decent approximation to a rectangle response function filter. Turns out that making the down-converter - the local oscillator and mixer - is relatively easy and economical.

Overall the system is most economical with minimal RF front end amplifiers, a down converter, and a beefy well-designed IF section doing all the fancy filtering. I find it interesting that this design strategy has held up over decades for many different systems utilizing wildly different technologies. Old vacuum tube radios looking like wooden furniture in the ss, transistor radios in the s, tiny cell phones and bluetooth devices today, giant radio astronomy telescopes, spacecraft telemetry, and more.

Basically it's to allow the demodulation circuit to be made very sensitive with a narrow bandwidth. If the demodulation circuit had to be wideband say, able to work for any frequency from MHz for FM , keeping a flat response across the entire frequency range would be difficult. Instead, the tuner is wideband and then beat heterodyned to a single intermediate frequency and sent to a very optimized demodulation circuit.

Early radios used Tune RF stages to amplify weak radio signals to the point an AM "detector" could convert them back to audio. These TRF radios would have anywhere from one stage to as many as 12 stages.

The more stages, the better the reception for weak signals and the better the image rejection rejection of nearby frequencies. This worked well when there were only a few radio stations but did not work well when more stations started crowding the airwaves. A TRF radio uses a tuned circuit whose Q for each stage is set to allow all of the frequencies for the audio bandwidth being used to pass through and a little amplification to boost the signal to usable levels. This had a few drawbacks as others have pointed out and a few they missed.

If the stages were too high in gain they might start oscillating and the radio stops working. Even with ganged variable capacitors, getting all the stages to stay on frequency was hard so provisions were made at some stages or all stages for "trimming" the signal. This is why pictures you see of early radio sets had so many knobs. Quite a few were for the "trimmer" variable capacitors and others were tube bias adjustments to set the gain to prevent feedback.