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My question is about the following paragraph:

enter image description here

The text implies that the heterodyning process is basically done to obtain a fixed frequency before amplification takes place. Here I don't understand why. Doesn't the incoming AM signals to the trf receiver already have fixed frequency as well? AM has fixed frequency with varying amplitude.

What am I getting wrong here? I've read some related question but couldn't find an answer.

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What's with the annoying highlighting in the text you show? – Olin Lathrop Jan 16 at 14:51
Only tangentially related, but note that once you modulate the amplitude of a fixed frequency signal it no longer has a really fixed frequency. If memory does not fail me, it(s Fourier transform) now has a frequency band of width equal to de modulating signal frequency band, around a "carrier" (fixed) frequency. What the process does it to lower the carrier while keeping the band shape. – Rolazaro Azeveires Jan 16 at 18:42
Worth noting here that the original business model for radio stations was to sell 'sealed sets' that were only tuned to that station. A TRF was entirely adequate for that ... but superhets existed anyway, and were easily jail broken to tune to any station in the band. – EJP Jan 16 at 19:59
@EJP : where was that the original business model, and what period? Never heard of it in the UK – Brian Drummond Jan 16 at 21:36
@BrianDrummond Hmm, certainly in Australia: see also here. My grandfather was involved at both ends of this business, as a director of 3AR and a furniture retailer including radios, which he would have bought in as chassis and had fitted into custom cabinets. – EJP Jan 16 at 23:26
up vote 1 down vote accepted

Let me try to simplify the answer for you. An incoming RF signal has to be amplified a number of times, maybe 3, 4 or even 5, to get something loud enough to be "heard". In a TRF receiver, each stage of amplification has to be tuned (to resonance) for the incoming signal, so that only the desired frequency passes through. That requires each stage to have "operator controlled" variable capacitors, coils, transformers, etc., that need constant adjustment. Others have explained the difficulties in getting all those "variable" circuits to perform as desired. (All those components add significant cost, too.) In the superhet, all that tuning is done in basicly 2 circuits, RF amp and local oscillator. Converted to an IF (intermediate frequency) in the mixer, the IF stages of amplification are all tuned to the same frequency, one time, where they stay. Take commercial AM radio. It uses an IF freq of 455Khz. You can get a bunch of IF stages tuned to 455Khz with a constant bandpass, amplification, filtering etc. To get all those stages to perform at 500Khz and 1600Khz with the same results is not so easy, as the electronic components respond differently at different frequencies. I'm an electronic technician, not an engineer, but I hope I've help clear the waters, and not muddied them more.

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very clear explained the big picture – user16307 Jan 18 at 15:15

Yeah the incoming RF frequency is fixed, but every station has a different fixed frequency. That makes tuning the sort of filter you need for decent reception really tedious! Especially since early radios often had a separate tuning knob for each tuned circuit.

Brilliant new innovation : the Osram Music Magnet Four has THREE tuned circuits, with all the tuning capacitors controlled by the same knob! In about 1932... But it's still not enough, and component variations mean the three circuits don't stay in tune right across the band.

So by 1934 it was normal to build one really good filter, with lots of tuned circuits, tune it ONCE, and leave it... And convert all your favourite stations to that frequency with a mixer stage...

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i dont get why one receiver can not tune all bands of AM with the same RF receiver just by adjusting resonance by a tuning knob. why was that hard to achieve? – user16307 Jan 16 at 15:04
Because you need to adjust 4 or more resonances simultaneously with limited accuracy components that don't track each other accurately. 3 was on the edge of being practical, the triple tuned TRF circuit linked above had a much shorter commercial lifespan than Windows Vista or your last cellphone... – Brian Drummond Jan 16 at 15:32
...that bad? :-) – Rolazaro Azeveires Jan 16 at 18:44
@Brian Drummond sorry to take your time. but lets say we have a trf receiver. why isnt one tuner enough to pass narrow a desired rf frequency? i mean we can tune the receiver circuit (LCR circuit) by a knob and we can only pass the desired rf signal. and then demodulate the carrier and amplify this signal. what is that 4 or more resonance thing? – user16307 Jan 16 at 18:55
@LvW : Yes, most TRF sets used positive feedback (aka reaction, or regeneration). It also improved selectivity to some extent, the basic idea is still seen as a "Q multiplier". Part of tuning them was to advance the reaction control to get enough gain, then back it off if oscillation started. – Brian Drummond Jan 16 at 21:28

... 'at which the tuned amplifying circuits can operate with maximum stability, selectivity and sensitivity'.

That rather woolly phrase is correct, but lacks the emphasis to direct you to what is happening.

The input frequency is converted to a fixed frequency, and then filtered. Further amplification and filtering takes place at this frequency.

The input signal does not enter the radio by itself, it has potentially dozens of other, often stronger, signals at nearby frequencies. These must be filtered out before too much amplification takes place, otherwise they will overload the amplifiers and contaminate the wanted signal.

It is difficult enough to build a good filter, even more difficult to build a good filter that can be tuned over the whole range of frequencies that the radio is to receive. Better to just build one good filtered amplifier at a lower intermediate frequency, and convert the wanted input frequency down to that.

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do u mean this: in trf, first the rf signal is tuned and then amplified. but because the good tuning is so hard all other unwanted rf signals will go into radio as well. but in superhet case the local oscillator tunes the freq. very well to a fixed one and amplifier design is made very spesific to this fixed freq. did i understand right? – user16307 Jan 16 at 14:59
@user16307 In essence, that's it. In practice, both types will have some variable frequency tuning after the antenna, to reduce the range of unwanted signals a bit. But that's where the TRF stops, where as the superhet can finish the job properly at a fixed frequency. – Neil_UK Jan 16 at 16:12

Basically, it all boils down to the fact that it is much easier to build a tunable oscillator than a tunable filter. To tune a filter, you need to adjust the values of several different components very precisely at the same time. This is a gigantic PITA and it doesn't really work well across a wide frequency band. The advantage of a super het design is that all of the filters are fixed, and the only thing that is adjustable is the local oscillator. This vastly simplifies the tuning process.

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Never mind that you can build a fixed-frequency filter with parameters that far outclass anything tunable, using quartz crystals or SAW resonators... – ThreePhaseEel Jan 16 at 22:03

In the old days the first triode valves got a bit dodgy towards the top of the AM broadcast band .This was probably due to parasitic capacitance .There was only LC tuned circuits for filtering .This meant that choosing a frequency lower than the carrier frequency made it easier to get the gain and the selectivity needed.Hence the term"Intermediate Frequency "or IF was coined .Having this frequency fixed made it easy to get constant selectivity across the AM BC band .TRF schemes tended to lose selectivity towards the top of the band .The AM broadcast band is unique in that its frequency dynamic range is about 3:1 where most other bands have a frequency dynamic range of 1.1:1 or less .Nowdays you can have an IF thats above the incoming carrier frequency because there are lots of other types of filters to choose from and because high frequencies have got much easier .Remember that the total gain needed for a credible reciever is around 1000000 so 1 microvolt into the antenna terminal will give 1 Volt in the speaker .The use of an intermediate frequency allows this gain to be better distributed making self oscillation less likely .

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