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?
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.
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.