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.
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.
This, as you can imagine, would make tuning in a radio station quite a production and when the "old man of the house" was going to listen to the radio it was a big event. It was known before the turn of the 19th century that if two oscillators were near each other that they would "beat" against each other and produce a new signal as in the case of two flutes tuned to the same pitch. This was exploited in several interesting ways at the beginning of the 20th century. The first use was in a baseband CW detector that converted a radio signal to audible sound much more cleanly than the barrater and other convoluted detector devices.
The Theremin uses heterodyning of two oscillators where one has it's tuning capacitance supplied by a small plate or wire and the users hand. Major Armstrong in the US and a few others in Europe realized during WWI that this could be exploited to make a receiver that had only a few very high gain stages and much simpler tuning filters.
The mixer stage would take the incoming RF, heterodyne it against the local oscillator and due to the nonlinear behavior of the mixer stage produce both a sum and a difference frequency. Usually it was the difference frequency that was lower than the RF or oscillator that was used. At 1MHz, the LO is set for 1. Instead of many tuned stages whose gain was tailored to prevent oscillation as their input and output frequencies was all the same, one or two higher gain stages for the RF could be followed by one or more carefully designed stages all operating at a different fixed frequency that did not need to be adjusted.
From a many sectioned tuning capacitor that were very expensive and difficult to produce you need only two or three sections that become a much smaller expense. This was also easier to tune as the selectivity of having the IF at KHz meant no radio stations at that frequency would exist since the broadcast band is KHz to KHz.
Dharmaputhiran 2 6 It isn't an answer, but do note that some receivers use multiple IF stages at different frequencies. This answer is focussing on radio receivers such as AM and FM. Andy aka k 9 The IF is slightly bigger than half the frequency range it covers, and this is to avoid generating images inside the band.
The name of the musical instrument you refer to is 'theremin'. EJP thank you and yes, the IF has to be bigger than half the range - silly me! 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. However, the choices for the IF are most dependent on the available components such as mixer, filters, amplifiers and others that can operate at lower frequency. There are other factors involved in deciding the IF frequency, because lower IF is susceptible to noise and higher IF can cause clock jitters. Modern satellite television receivers use several intermediate frequencies.
The downlink signal is received by a satellite dish. One of the two blocks is selected by a control signal from the set top box inside, which switches on one of the local oscillators.
This IF is carried into the building to the television receiver on a coaxial cable.
The intermediate frequency is created by mixing the carrier signal with a local oscillator signal in a process called heterodyning, resulting in a .
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.
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.
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.