AM Radio problem

Hi, the attachment contains a diagram of the AM radio circuit we built for a lab

I have a question about Figure 2 Section 3. From what I understand, audiable information is encoded as amplitude of a low frequency signal. This signal "rides" on the broadcasted high frequency signal. Section 3 is a low-pass filter that filters out the high frequency.

Here is the question: for the 0.1uFD capacitor, what will be the optimum resistor value? Why?

My text book has example of low pass filter, but it's a RC circuit in series. I don't know how a parallel RC circuit will behave

Is section 3 and sectoin 4 related? I know that section 4 is just an AC coupler and an inverting amplifier.

Thanks for any suggestion
(PS: I asked this question in another forum but this place seems to be more suitable for simple question)

phoenixy said:

Hi, the attachment contains a diagram of the AM radio circuit we built for a lab

I have a question about Figure 2 Section 3. From what I understand, audiable information is encoded as amplitude of a low frequency signal. This signal "rides" on the broadcasted high frequency signal. Sectopm 3 is a low-pass filter that filters out the high frequency.

Here is the question: for the 0.1uFD capacitor, what will be the optimum resistor value? Why?

My text book has example of low pass filter, but it's a RC circuit in series. I don't know how a parallel RC circuit will behave

Is section 3 and sectoin 4 related? I know that section 4 is just an AC coupler and an inverting amplifier.

Thanks for any suggestion
(PS: I asked this question in another forum but this place seems to be more suitable for simple question)

Click to expand...

I may not be doing you any favors by redesigning this for you, but below is my best shot without building it (although I did simulate it).
What you have is basically a crystal set with an audio amplifier. Your basic problem is the inverting amplifier. Diode detectors need to work into a high impedance to be efficient. Your load is 100 ohms. Do a Google search for "crystal set design". You will be amazed at the amount of work that has gone into them as a hobby.
So, regarding your questions, your detector is working into much too low a resistance. If the signal amplitude were high, like a couple of volts peak-to-peak, you could think of the diode and capacitor as a peak detector, but with low amplitude signals, the diode becomes more like a very nonlinear resistor. I confess that I settled on 100pF empirically.
You will get much more signal amplitude from the circuit below than from your original design.

yeah well, that didn't really help me for the lab write up. Nevertheless, your effort is much appreciated!

I'm afraid I have insufficient knowledge to understand your design. I'm only 3 months into electronic. Although I know what the diode does in my circuit, I barely know its internal operation. I havn't even touch transistor yet!

Anyways, do you happen to know any good electronic website for me to start up?

Hi phoenixy,
It ain't much but you should check out:
**broken link removed**
It has some stuff worth looking at.

captainkirksdog said:

Hi phoenixy,
It ain't much but you should check out:
**broken link removed**
It has some stuff worth looking at.

Click to expand...

thanks!