Why is a parallel LC circuit used to filter out the right frequency from the antenna? Shouldn't it be in series?
We have a maximum impedance at resonant frequency if we connect in parallel, so why isn't the signal being blocked?
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1\$\begingroup\$ Imagine you get a current from the antenna. The parallel RC has maximum impedance at your wanted frequency, giving maximum voltage to the diode. \$\endgroup\$– user16324Commented Mar 15, 2021 at 21:26
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3 Answers
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The selected signal is being blocked - from going to ground, that is.
The LC circuit shorts frequencies outside of the resonance to ground. The resonant frequency is rejected - it doesn't go to ground.
That leaves just the selected frequency available to feed the demodulator.
Since it is a resonant circuit, the selected frequency is higher than if it were just fed straight to the demodulator.
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The impedance of a parallel LC circuit (i.e. a "tank") is at a maximum at its resonant frequency. That means that at resonance the maximum amount of signal will be rectified by the diode.
Other frequencies which are away from resonance are shunted to ground.
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Here's the series-tuned version of the crystal radio receiver circuit.
With the parallel-tuned circuit, the diode-clipped, parallel-resonance voltage is output to the phones. High-impedance diodes and phones would be required to match the high impedance of the parallel-resonant circuit.
With the series-tuned circuit, the diode-clipped, series-resonance current is output to the phones. Low-impedance diodes and phones would be required to match the low impedance of the series-resonant circuit.
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1\$\begingroup\$ That's good. A few words explaining why it is done this way would be useful. \$\endgroup\$– Russell McMahon ♦Commented Aug 17, 2021 at 6:54
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