I build this circuit and it works. I wanted to look at the output waveform by my soundcard oscilloscope software. I would expect something like the following picture: enter image description here

But the output waveform was this:

enter image description here

The input of the circuit was a song played from my computer. The output of the circuit (the antenna) was connected to the input of the virtual oscilloscope. The other input of the virtual oscilloscope was connected to ground. enter image description here

My questions are:

1- Why is the output waveform different from standard (or normal) FM waveform?

2- Why is the frequency 100.1 Hz only? I received the signal at 88 Mhz.

Thank you very much,

  • 1
    \$\begingroup\$ If you want to debug this circuit, don't use a music file. Generate a tone on a known frequency with a tone generator app \$\endgroup\$
    – Voltage Spike
    Jan 28, 2016 at 20:46
  • 3
    \$\begingroup\$ What is a "virtual scope"? A software that reads from the soundcard audio input and displays it? The bandwidth (max. frequency) of the soundcard is max. ca. 20kHz. You can't expect to see a signal in the MHz range. \$\endgroup\$
    – Curd
    Jan 28, 2016 at 20:47
  • 4
    \$\begingroup\$ Your sound card oscilloscope can't pick up anything faster than 24kHz or 96kHz \$\endgroup\$
    – Voltage Spike
    Jan 28, 2016 at 20:48
  • 2
    \$\begingroup\$ The 100Hz signal could be a rectified line frequency hum (2*50Hz = 100Hz). \$\endgroup\$
    – Curd
    Jan 28, 2016 at 20:49
  • 3
    \$\begingroup\$ On a good sound card with 24 kHz sample rate, there will be an anti-alias filter damping signal components above 12 kHz into oblivion. But all sound cards I know of that still have some relevance do at least 44kHz. Still way too low to receive 88Hz, though. \$\endgroup\$ Jan 28, 2016 at 22:49

1 Answer 1


Why someone hasn't answered this beats me....

Your sound card input is good for 20 kHz and maybe 80 kHz on a good sound card sampling at 196 kHz. 88 MHz is well over a thousand times too high in frequency.

Look at the time base dial on the scope picture - it goes down to 1 ms and, in 1 milli second 88,000 cycles of FM carrier would have occured.


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