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AM Radio problem

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phoenixy

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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)
 

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Roff

Well-Known Member
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)
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.
 

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phoenixy

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

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?
 
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