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Why is it required in radio receiver to tune the LC circuit to the the resonant frequency of the station in order to receive the station ? How this tuning filters the desired signal from the other signals. Can anyone explain this to me please ?

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    \$\begingroup\$ Google "bandpass filter" \$\endgroup\$ – EM Fields Jun 1 '16 at 11:41
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    \$\begingroup\$ Which LC circuit? There are many. The local oscillator does not tune to the station's frequency, but is offset by the intermediate frequency (IF) and the narrow filtering is done at the IF stages . Search under 'superheterodyne' \$\endgroup\$ – Chu Jun 1 '16 at 12:41
  • \$\begingroup\$ @EMFields - this is not really about bandpass filters. Those are generally used when the goal is not to tune, but rather to pass a range of possibilities, or in more advanced cases to create a fixed-frequency IF filter which is flat across the desired modulation bandwidth. The question is specifically about LC circuits that need to be tuned for the specific station of interest. \$\endgroup\$ – Chris Stratton Jun 1 '16 at 13:29
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    \$\begingroup\$ @Chu: Since the OP was asking about tuning to the resonant frequency of the station, and cited using an LC to do it, it's apparent - at least to me - that he's talking about about the arrangement used on crystal and TRF radio sets, which is also the same arrangement used on the front end of most AM and single frequency radios. The only time I've ever come across a notch in a radio is to kill the image. But that was a long time ago. \$\endgroup\$ – EM Fields Jun 1 '16 at 13:47
  • \$\begingroup\$ @ChrisStratton: Since the OP wrote "Why is it required in radio receiver to tune the LC circuit to the the resonant frequency of the station in order to receive the station ?" it seems obvious that that's what he was talking about. Unless, of course, his lack of control of the language led him to ask something entirely different from what he thought he was asking. \$\endgroup\$ – EM Fields Jun 1 '16 at 13:51
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The L-C filter used to tune to particular radio stations has high Q. Put another way, it passes only a narrow notch of frequencies. When the notch-pass filter is tuned to a particular frequency, it will pass that frequency but heavily attenuate others. Radio stations are spaced far enough apart in frequency so that the signals for other than the desired station are attenuated to below the "don't care" level.

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  • \$\begingroup\$ A band-pass filter is a device that passes frequencies within a certain range and rejects (attenuates) frequencies outside that range. A notch filter, on the other hand, is a band-stop or band-rejection filter with a narrow stopband (high Q ) and is a filter that passes most frequencies unaltered, but attenuates those in a specific range to very low levels. It is the opposite of the band-pass filter. Learn more here and here \$\endgroup\$ – EM Fields Jun 1 '16 at 12:39
  • \$\begingroup\$ @EMFields - a sharp LC filter can be either a peak or a notch, depending on the placement of the components relative to one another. The reference here is not to a bandpass filter. \$\endgroup\$ – Chris Stratton Jun 1 '16 at 13:23
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    \$\begingroup\$ @EMFi: I thought it was quite clear how I was using "notch filter" by saying "it passes only a narrow notch of frequencies". Notheless, I changed "notch filter" later to "notch-pass filter" so that even those deliberately trying to get confused can't misinterpret what I say. \$\endgroup\$ – Olin Lathrop Jun 1 '16 at 17:06
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    \$\begingroup\$ @EMFi: It's not really a bandpass filter because it doesn't have two separate rolloff points, one between the pass band and the low side, and one between the passband and the high side. "Notch-pass filter" describes the characteristics better. \$\endgroup\$ – Olin Lathrop Jun 2 '16 at 10:39
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    \$\begingroup\$ Utter nonsense. A bandpass filter is characterized by its bandwidth and its rolloff after the frequency of the -3db amplitude points on both the low side and the high side skirts. Have a look here \$\endgroup\$ – EM Fields Jun 2 '16 at 11:36
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Tuned LC circuits actually have several roles in a typical receiver.

One is as a preselector - basically what Olin described, where the filter is tuned to pass one desired station's frequency and attenuate all the others. However, this is not typically as sharp or precise as desired to select only a single station from the band.

Modern receivers are heterodyne based - they effectively "go out and grab" the signal of interest, and move it to a fixed "intermediate frequency" where a fixed frequency filter can more specifically pass it and block adjacent channels. Often the IF filter uses other mechanisms than an LC resonant circuit, but it is typically surrounded by some LC circuits, possibly in the form of transformers with adjustable ferrite caps. The IF is typically also at a lower frequency than the signal of interest, and the bandwidth of a filter tends to scale with its frequency, so a filter built for a lower frequency is more easily made sharp.

To "move" a signal it is necessary to use something called a mixer which can subtract (or add) the frequency of a Local Oscillator with the input frequency. So a tunable oscillator is built with an output that differs from the desired receive frequency by the intermediate frequency. A simple way of building a local oscillator is to build an oscillator tuned by a variable LC circuit, typically a fixed inductor and a plate capacitor connected to the tuning knob. If a preselector is used, it is often tuned by an additional variable capacitor stacked on the same shaft.

A broadcast AM receiver typically converts to an IF of 455 KHz, filters there and demodulates there. In contrast a broadcast FM receiver will typically convert to a first IF of 10.7 MHz, filter there, then convert with a fixed second LO to a second IF of 455 KHz for demodulation. One type of FM demodulator requires making a second copy of the signal with a phase shift, which can be accomplished by an LC circuit tuned to do that to a 455 KHz signal.

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    \$\begingroup\$ All broadcast FM receivers I've seen are single conversion, running the demodulator at 10.7 MHz. If they have a 455kHz IF strip, that's for the AM section. Achieving the required phase linearity over 200+ kHz at 455kHz centre frequency would be ... tricky. \$\endgroup\$ – Brian Drummond Jun 1 '16 at 23:08
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    \$\begingroup\$ 'Dual conversion' (first to 10.7MHz and then to 455kHz) is used for narrow band FM - in such things as analog cell phones and pagers, VHF transceivers and radio control receivers. \$\endgroup\$ – Bruce Abbott Jun 2 '16 at 1:41
  • \$\begingroup\$ Can you provide a datasheet or a link to a datasheet or even a link to any manufacturer who produces a double-conversion superheterodyne or subheterodyne FM broadcast radio receiver? \$\endgroup\$ – EM Fields Jun 2 '16 at 19:31

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