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What Is an IF, or "intermediate Frequency"?

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❶This is called filtering. Transmissions are allotted bandwidth, and multiple transmitters are under pressure to be squeezed together next to one another in frequency.

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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.

The main lesson points are: 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. When several stages of filters are used, they can all be set to a fixed frequency, which makes them easier to build and to tune. Lower frequency transistors generally have higher gains so fewer stages are required.

It's easier to make sharply selective filters at lower fixed frequencies. There may be several such stages of intermediate frequency in a superheterodyne receiver; two or three stages are called double alternatively, dual or triple conversion , respectively.

Intermediate frequencies are used for three general reasons. Active devices such as transistors cannot deliver much amplification gain. So a high frequency signal is converted to a lower IF for more convenient processing. For example, in satellite dishes , the microwave downlink signal received by the dish is converted to a much lower IF at the dish, to allow a relatively inexpensive coaxial cable to carry the signal to the receiver inside the building.

Bringing the signal in at the original microwave frequency would require an expensive waveguide. A second reason, in receivers that can be tuned to different frequencies, is to convert the various different frequencies of the stations to a common frequency for processing.

It is difficult to build multistage amplifiers , filters , and detectors that can have all stages track in tuning different frequencies, but it is comparatively easy to build tunable oscillators. Superheterodyne receivers tune in different frequencies by adjusting the frequency of the local oscillator on the input stage, and all processing after that is done at the same fixed frequency, the IF.

Without using an IF, all the complicated filters and detectors in a radio or television would have to be tuned in unison each time the frequency was changed, as was necessary in the early tuned radio frequency receivers. A more important advantage is that it gives the receiver a constant bandwidth over its tuning range. The bandwidth of a filter is proportional to its center frequency.

In receivers like the TRF in which the filtering is done at the incoming RF frequency, as the receiver is tuned to higher frequencies its bandwidth increases.

The main reason for using an intermediate frequency is to improve frequency selectivity. This is called filtering. Some examples are, picking up a radio station among several that are close in frequency, or extracting the chrominance subcarrier from a TV signal. With all known filtering techniques the filter's bandwidth increases proportionately with the frequency. So a narrower bandwidth and more selectivity can be achieved by converting the signal to a lower IF and performing the filtering at that frequency.

FM and television broadcasting with their narrow channel widths, as well as more modern telecommunications services such as cell phones and cable television , would be impossible without using frequency conversion. In special purpose receivers other frequencies can be used. A dual-conversion receiver may have two intermediate frequencies, a higher one to improve image rejection and a second, lower one, for desired selectivity. A first intermediate frequency may even be higher than the input signal, so that all undesired responses can be easily filtered out by a fixed-tuned RF stage.

In a digital receiver, the analog to digital converter ADC operates at low sampling rates, so input RF must be mixed down to IF to be processed. Intermediate frequency tends to be lower frequency range compared to the transmitted RF frequency.

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The intermediate frequency is created by mixing the carrier signal with a local oscillator signal in a process called heterodyning, resulting in a .

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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.

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RF vs IF This page describes difference between RF(Radio Frequency) and IF(Intermediate Frequency). It also explains how one frequency is . An IF, or intermediate frequency, is a transitional radio frequency situated between two other frequencies. It is often created by mixing two frequencies together via a signal enhancement process called heterodyning.

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RF = Radio Frequency (LO + IF) IF = Intermediate Frequency (LO - RF) Relation Between RF and IF: See the figure below. When the IF is up-converted (The frequency of IF is increased) through an UC (Up-converter) by adding it to the Local Oscillato. Other articles where Intermediate frequency is discussed: superheterodyne reception: This different frequency, called the intermediate frequency (IF), is beyond the audible range (hence the original term, supersonic heterodyne reception); it can be amplified with higher gain and selectivity than can the initial higher frequency.