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AM radio - quite Confused

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Franklin

New Member
I'm quite new to Electronics, and have been working on understanding an AM radio circuit. I think I've gotten most of it, but I'm stuck at a few points.

The schematic can be seen at http://support.radioshack.com/support_games/doc60/60245.htm, or, for a better picture, at https://www.electro-tech-online.com/custompdfs/2003/11/64800.pdf.

Here are my questions:

A) Block (1) - I recognize the tuned circuit, but its not in any configuration that I'm familiar with. Normally, I've seen the signal come in, and a tuned circuit bypass to ground, and then signal out. Here, I don't see any signal in (where does the antenna connect to?) or signal out - only to ground. How is this working?


B) Why is an inductor used to couple to Q1?

C) There are several capacitors which bypass to ground along the way - from right to left, a 0.1uF, a 0.01uF, another 0.01uF, and a 0.05uF. Are these all used to detect the AM - that is, to let the HF drop to ground while the audio carries through? Why do you need so many?

D) Block (4) - How can you couple with only a 0.022uF capacitor - won't this block all but the lowest frequencies?

I'd like to thank anyone who can help me, by answering any of these questions!
 

Optikon

New Member
Franklin said:
I'm quite new to Electronics, and have been working on understanding an AM radio circuit. I think I've gotten most of it, but I'm stuck at a few points.

The schematic can be seen at http://support.radioshack.com/support_games/doc60/60245.htm, or, for a better picture, at https://www.electro-tech-online.com/custompdfs/2003/11/64800-1.pdf.

Here are my questions:

A) Block (1) - I recognize the tuned circuit, but its not in any configuration that I'm familiar with. Normally, I've seen the signal come in, and a tuned circuit bypass to ground, and then signal out. Here, I don't see any signal in (where does the antenna connect to?) or signal out - only to ground. How is this working?


B) Why is an inductor used to couple to Q1?

C) There are several capacitors which bypass to ground along the way - from right to left, a 0.1uF, a 0.01uF, another 0.01uF, and a 0.05uF. Are these all used to detect the AM - that is, to let the HF drop to ground while the audio carries through? Why do you need so many?

D) Block (4) - How can you couple with only a 0.022uF capacitor - won't this block all but the lowest frequencies?

I'd like to thank anyone who can help me, by answering any of these questions!

I'll take a stab at a few of these (someone please correct me if I am wrong) BUT, I think:

A) The antenna is from terminal 4 & 5, the inductor shown. Earlier in the explanation it shows this. This in conjunction with the variable cap, tunes this receiving part of the circuit.

B) I think what you are looking at is part of the antenna, both the inductors there in the front end, are antenna.

C) The decoupling around the output amplifier IC makes sense to me because, you need to bypass the input of the IC and you should bypass the output as well because, the IC has limited powe supply rejection. Think of the power supply inputs to the IC as input signals as well. If there is noise on these lines from the battery, the high frequency part of it will get coupled directly to the output so a bypass helps there also.

D) This part of the circuit is volume control? Think of that capacitor and variable resistor as a variable attenuator. The circuit passes lowe freqeuncy well but it still passes higher frequencies, only they are much more attenuated which is exacly what you want since you are controlling volume here (magnitude of the signal going into the output amplifier).

I have no experience with radio circuits but that is my take on it..
I hope someone more knowlegeable than I, could comment.
 

Franklin

New Member
I have no experience with radio circuits but that is my take on it..
I hope someone more knowlegeable than I, could comment.

Optikon - I'm not sure if your answers are accurate.

Could someone with knowledge of AM Radio circuits please respond?
 

crust

Member
Franklin said:
D) Block (4) - How can you couple with only a 0.022uF capacitor - won't this block all but the lowest frequencies?

That capacitor in conjunction with resistor VR to ground form a high pass filter with time constant 1/RC. I believe this is used to prevent DC from entering the amplifier while still passing the audio signal.
 

Franklin

New Member
Does anyone have any answers to any of my other questions?

Also, what is the point of the 47uF and 100uF strung across the battery?
 
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