radio - What role does the capacitor play in an AM demodulating circuit?

The goal of this question. Demodulate the AM signal.

(Please assume a diode demodulating AM radio of the most imaginable simplicity.)

I am assuming an alternating current sine wave is coming into a wire from the antenna? In order for an envelope to coexist with the sine wave the amplitude is varying I assume?

If the antenna is the correct length then resonance of some fundamental frequency of the EM wave allows the signal to increase in strength in the antenna before it enters the diode? An assumption on my part.

The signal is then converted to DC but since the strength of the signal determines the fluctuations in the "envelope" the voice information is in "duplicate" so the bottom half is cut out of the rectifier , no? From this point I am lost.

The sketch shows a capacitor in parallel. I know what a capacitor does in principle. Is it attempting to maintain the voltage as the current changes since capacitors don't like to have their voltage changed and resist it, no? Please complete my picture to demodulate the AM signal.

Answer

We need to start at the beginning.

MODULATION

At the source the music signal (represented by a sine wave) is combined (used to modulate) a high frequency carrier signal at the transmitter.

• The music signal contains low frequencies (as humans hear from about 20hz to 20Khz)


• The carrier signal is much higher frequency (say 100 KHz).

note: the carrier signal is a fixed frequency as each radio station operates at a different frequency.

TRANSMISSION

This modulated signal is now transmitted a long way over the air.

RECEPTION

Somewhere far away, say 20 km, we now need a way of recovering that original music signal from the 'received signal' we receive at the antenna.

To do this we are going to use a circuit called an envelope detector.

The envelope detector is first going to convert the AC signal into a DC signal.

RECTIFICATION

Imagine the received signal (the modulated signal) is a 'messy' sine wave that varies 10V peak to peak (it goes from -5V to +5V).

We are going to ignore the negative half of this 'messy' sine wave.

A diode only allows current to flow one way, and hence has the effect of producing a constant voltage

• the -5V to 5V signal has an average voltage of 0V


• the 0V to 5V signal has an average voltage of 2.5V

DEMODULATION

Now imagine that this 'messy' sine wave is not a 'true sine wave' but is actually made up by a more complicated waveform that contains the original message signal (at low frequencies) and the 'messy' bits at high frequencies.

The envelope detector is now going to remove the high frequencies from the low frequencies. This is done with a lowpass filter (resistor and capacitor). A lowpass filter lets low frequencies pass but stops high frequencies from passing.

Now we have recovered the original signal!

note:

Sometimes a picture can really help understanding:

I have tried to give you the bigger picture and have skipped some details here.