# How to explain a frequency pic at 1.5 kHz?

I have a pic in the VLF band at 1.5 kHz. What would be the origin of this peak?

The graph was obtained by running display_qt in Linux from a laptop.

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display_qt is a GNU Radio example program. It does not display any actually existing electromagnetic signal but internally generates a 1500 Hz sinusoid.
// Source will be sine wave in noise