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Variable capacitors are used in tuning many circuits of radios. I read the theory of basic tuning and resonance frequency. What I understand by changing the capacitor we change the resonance frequency of the radio tuner and select a particular frequency. My question is:

1) What is selected here? Carrier frequency or signal e.g audio frequency? They have totally different frequency bands. If carrier wave is selected, why the modulated lower freq. audio also pass through the circuit. Isn't it contradicting to the theory?

2) If one tunes a receiver circuit to a frequency how can a band of frequencies like audio signals pass through. I mean tuning seems to let pass only 1 particular frequency not a band.

I'm afraid I lost the route at points when trying to understand and confused.

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Tuning a radio to a particular station using a tuning capacitor or any other method selects the carrier frequency.

The frequency-selective circuits in a radio are not "perfect" - they allow a band of frequencies to pass, not just a single discrete frequency, so the carrier and the sidebands generated by the modulating signal (audio) can apss through the RF and IF stages of the receiver.

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  • \$\begingroup\$ "The frequency-selective circuits in a radio are not "perfect" Do you mean that the imperfection lets us to receive the audio signal and otherwise it would be cancelled out and we would have only 1 single sinusoid passing the tuner circuit? \$\endgroup\$ – user16307 Apr 6 '14 at 0:05
  • \$\begingroup\$ Yes - we cannot build a filter circuit that will pass only a single frequency. Any real filter or frequency-selecitve circuit will pass a narrow band of frequencies. \$\endgroup\$ – Peter Bennett Apr 6 '14 at 1:20
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Good question, if a little late!

This was a big debate in the early 1930s, when high performance tuned circuits started becoming practical (either through triple-ganged tuning or superhet) and the "short wave" (now medium wave or AM) band started getting crowded.

People discovered that tuned circuits could be "too good" selecting the carrier and the sideband components closest to it, allowing only the LF part of the audio spectrum to be reproduced. The result was a bass-heavy, dull sound accused of being "drummy".

Some attempts to fix this included the "Stenode" which claimed to maintain tight tuning "so that a signal only 1 kHz away from the carrier could be eliminated" without impairing audio fidelity.

By 1934 this was debunked as a fraud; (sorry I can't find online references to that!)though some radio makers still called a treble boost circuit to flatten the response a "stenode" circuit.

By that stage (1934) "sideband theory" was widely accepted, and as it was easiest to detune a filter to set the correct filter bandwidth once and leave it there, the superhet "supersonic heterodyne" radio took over from the simpler "straight" sets.

Naturally you had to compromise between high fidelity (this was before AM transmitters were tightly regulated as to how much spectrum they could broadcast!) and rejecting interference, and no one compromise would work for all situations.

So here is a serious hi-fi AM tuner, from 1947, the Leak VS where you can adjust the bandwidth of the IF filter to get the best sound from any station you tune. I heard some lovely Asian traditional music on the 19 metre band (about 15MHz) - turned out to be Radio Hanoi!

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With CW telegraphy, you have a single frequency signal. Your tuned circuit basically either passes it or it doesn't, and you tune to pass it.

With AM (amplitude modulation), you have three closely spaced signals, the upper sideband, the carrier wave, and the lower sideband. The selectivity of the tuned circuit is such that, when the circuit is tuned to the carrier wave frequency, it will also pass both sidebands.

An FM signal has a much more complex spectrum, but it is symmetric about the carrier frequency, and the principle is the same. When the tuned circuit is tuned to the nominal carrier frequency, the selectivity of the circuit is such that it passes the whole spectrum.

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