Tuning Circuit for AM Radio

[Glenn900]

I am confused by the design of an AM radio tuning circuit--a circuit that we will build and test in an upcoming lab. (See the attached image for the schematic given to us in the lab description.)

According to the schematic, aren't all of the tuning components in parallel? If they are in parallel, then won't the speaker see the same maximum voltage output as that across the inductor (L2), regardless of the voltage input frequency? I am confused as to how this circuit can employ resonance to tune the frequency output sent to the speaker.

I am also confused about the "low pass filter." The parallel configuration of resistor and capacitor shouldn't attenuate voltage output based on frequency, since they are parallel with the voltage output. Adding a resistor in series with the diode, however, would seem to make a simple low-pass filter.

Could anyone offer an explanation as to 1) how this circuit uses resonance to tune to a frequency, and 2) how the low-pass filter works?

(This is not a homework or problem set question. I can walk into lab and receive credit for building the circuit, but I'd like to actually be able to understand how it works!)

 

[jbsteele]

The tuning circuit works by employing resonance to tune to a certain frequency. The inductor (L2) and capacitor (C2) form a tuned LC circuit that resonates at a certain frequency. When the input frequency matches the resonant frequency of the LC circuit, the voltage output is amplified. This amplified voltage is then sent to the speaker. The low-pass filter works by attenuating high frequency voltages that are outside of the range of the resonant frequency of the LC circuit. The diode in series with the resistor acts as a voltage limiter, allowing only lower frequency voltages to pass through. This prevents the signal from becoming distorted due to high frequency noise.