AM demodulation and DC offset

[Edward Sutton]

Hello all.

I’m new here so apologies if this is a dumb question for the more advanced members but I’m struggling to find my answers with google.

I am trying to understand the concepts of AM radio broadcast and receiving an audio signal. I feel I understand the demodulation/detection circuit using a diode followed by a capacitor to take one side of the signal and then filtering out the RF carrier. The problem I have is it leaves you with a positive or negative offset in that the full waveform is above or below the zero line. What is added or changed in the circuit to move the wave form so the zero line runs through the middle of the waveform and can be properly reproduced by a speaker. Am in missing something very simple.

Thank you for your time in advance.

 

[Audioguru]

You simply use a series coupling capacitor to pass the audio but block the DC. This cannot feed a speaker, it feeds the input of an audio power amplifier that feeds power to the speaker.

 

[Edward Sutton]

Cool, I I know it would need amplification before getting to a speaker, I was just being simplistic to illustrate what I want to use the signal for. Is this “series coupling capacitor” the same cap used to filter to rf carrier? If so then the act of blocking dc move the zero line so there is positive and negative information. Sorry to labour the point but I just want to be sure I understand before I move on to FM.

 

[duke37]

The RF bypass capacitor is in parallel with the signal to pass the RF to ground.
The DC blocking capacitor is in series with the signal line.

 

[Audioguru]

While you are talking about an AM detector diode and the DC it produces, did you know that any half-decent AM radio uses the DC to control AGC? AGC is automatic-gain-control so that a weak distant station's audio is almost the same level as a strong local station.

 

[Edward Sutton]

Thank you all, I think with a little more hard staring at the drawings it’ll click. Cheers

 

[Edward Sutton]

Hi again. So with a lot more staring I think I understand. The only confusion I have left is a matter of series vs parallel. In a simple am detector circuit the diode is depicted with the capacitor at a right angle to it. A resistor is in place after the cap in parallel. I find it hard to tell if this mean the cap is parallel or series with the diode. The capacitor must be series with the diode to block the DC and parallel with the resistor to form the low pass that removes the carrier correct? Thanks again, I’m sure this is small fry for you all on here.

 

[Audioguru]

You are correct, the resistor and capacitor are in parallel and are in series with the detector diode. Yes, the capacitor filters out the carrier frequency. No, the capacitor does not block the DC from the diode, the DC and audio frequencies pass on to the next stage.

 

[Edward Sutton]

Ok but doesn’t a capacitor in series only pass AC and block DC? Why does the DC pass through?

 

[duke37]

Ok but doesn’t a capacitor in series only pass AC and block DC? Why does the DC pass through?

1. Yes
2. It does not.

Post a copy of the circuit you are talking about with the components labelled.

 

[Audioguru]

The capacitor goes to ground, it passes high carrier frequencies to ground, and it blocks the DC from shorting to ground.. The DC and audio from the diode passes on to the next stage.
Look at this:

 

[Edward Sutton]

Ok. So so in this diagram I’ve uploaded C10 is filtering off the DC and letting the AC signal through to the amplifier stage?

 

[duke37]

You have the right idea but the time constant RC of your circuit is 1ms.
The Mullard manual of transistor circuits uses 0.01μF and 5k giving a time constant of 50μs.if my sums are right.

I think the resistor is far too high. A semiconductor diode would be better driving a lower resistance.

 

[Audioguru]

C7 will charge when the input signal goes positive and it will not discharge so after a part of a second the AM detector will not work. If C& is used to block DC from the signal source then the anode of the diode needs a resistor to ground to discharge C7 each cycle.

 

[Edward Sutton]

Thank chaps. Yes I know the values are wrong but construct is the right idea. Thanks for your help.