AM Radio circuit

[Franklin]

I've 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/2003/11/64800-2.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!

 

[Franklin]

I'd really appreciate it if someone could help me out..

 

[motion]

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?

The AM band is at the low end of the radio spectrum and would require a very long wire antenna. A ferrite core bar antenna is used instead and is magnetically coupled to the tuned circuit.

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

It is actually the secondary of a transformer coupled to the tuned circuit. Using a transformer matches the low impedance of the base terminal of Q1 to the tuned circuit. Without this, the tuned circuit will be loaded and selectivity will suffer.

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?

Yes, the circuit removes the carrier from the detected AM signal. Notice that capacitors are placed on each stage of amplification. Without the capacitors, any carrier signal getting past the filter will be amplified along the audio and will overload succeeding stages if there is no filter in them.

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

The cut-off frequency will depend on the the value of the 0.022uF capacitor as well as resistor VR. The value of VR must be high enough so that the 0.022uF will not block the lower frequencies.

 

[Franklin]

motion - thanks for the great info!

I'm still not clear on one the initial LC, and its coupling. I followed what you send about the first L serving as an antenna, and the second L as actually being the secondary. I assume that the 0.05uF is to drop the RF to ground. But...

A) How is the tuned circuit working? I don't understand how the L and C in parallel is working here. From what I understand, an L and C in parallel will have near infinite impendance at the resonant frequency. To make a filter with it, you put it as a bypass to ground, between signal in and out. All but the desired frequencies go to ground. That doesn't seem to be what is going on here - it would look like here, the LC can conduct everything but the resonant - ie, desired - frequency. At the resonant freq, it's impendance should be near infinite. How exactly is the signal going through?

Or maybe it's like this:
At nonresonant frequencies, the signal shorts through the parallel LC, and doesn't enter the circuit. At the resonant frequency, it can't go through the C and short, so it needs to go through the circuit... does this make any sense???

B) Could you please explain - step-b-step (I'm a beginner) the way the signal goes from the secondary to Q1? It seems quite confusing, with one end of the secondary going right to the base, and the other end going through a diode and then a resistor...

thanks!

oh, one more question:
are the 47uF and 100uF capacitors strung across the battery terminals just to absorb sudden extra draws of current from the IC's? (if they were to eliminate battery noise, wouldn't one suffice?)

 

[motion]

The current through the tuned LC is maximum at resonance. The current induced at the secondary is proportional to this. The LC will still work even if one side were not tied to the ground.

 

[k7elp60]

The 100uF capacitor is for as stated earlier. The 47uF along with 1K between terminals 30 and 33 is decoupling for the first two stages, Q1 and
Q2. This 1K resistor is going to drop some voltage depending upon the
current draw of Q1, and Q2.
Both Q1 and Q2 are biased into conduction with the 330k resistor that is
connected from the collector to the base.

The diode between terminals 17 and 18 is a detector. The coil between terminals 6 and 7 has the modulated carrier of the tuned circuit induced into the coil by magnetic coupling. The .05uF capacitor removes the carrier from the rectified modulated carrier. The audio
variations across this coil change the bias of Q1 and are amplified by
Q1.
The 47k, 10k and the .05uF in the base circuit of Q2 appears to be a crude
automatic volume control.