Initially I wanted to build a detector radio. With the lack of a very good super long antenna and high impedance headphones I decided to feed the microphone input with the weak signal. The soundcard can directly deliver the amplified signal to the speakers and recording and visualization is inclusive.
You only need a high frequency capable diode to demodulate classic AM radio waves into audible signals. Now you can hear the strongest channels parallel. The volume is fading with atmospheric fluctuations and signal strengths.
The inductor suppresses the mains hum and other low frequency noise.
You might wonder why I didn’t add a tuning capacitor. It can be seen that a low input impedance is damping the LC oscillator thus preventing a narrow band selection and amplitude rising. So it wouldn’t help the reception that much in this circuit.
Higher input impedance can be achieved with a transistor circuit, the emitter follower. A stack of tree BC558 is sufficient for a selective LC oscillator. Now you can listen to the cannel you like and not to the strongest.
With an Audion the damping ration can be lowered, even below zero causing a permanent oscillation which enables cw reception like Morse, data or time signals.
At the point below oscillation occurs the signal is boosted by about 10dB. The oscillator is now the narrowest. It jumps to nearby channels which makes the reception more stable.
With your PC the spectrum of the audio input helps to scan for stations. Whether you listen to long and medium wave or shortwave radio can be determined by the frequency spacing between stations which is 9kHz for LW & MW and 10kHz for SW. Furthermore digital and analog broadcasting can be identified:
The input bandwidth is broader than the audible range and therefore enables a spectrum up to 100 kHz. The last picture shows an excerpt of the audio spectrum of the recording below.
Can you guess which instrument made the dot pattern? (wait 5’00”)
