A small interactive model of LoRa chirp spread spectrum. The upper view is a time-frequency trace like a spectrogram. The lower view slows the oscillating signal down enough for human eyes, so you can see the waveform itself tighten as instantaneous frequency rises.
1 · TIME–FREQUENCY TRACEwhat a spectrogram-like view shows
+62.5 kHz
CENTER ± 62.5 kHz
TIME →
2 · SIGNAL WAVEFORMconceptually slowed down — not literal 915 MHz scale
AMPLITUDE
TIME →
Why the second pane looks different: the diagonal line above is not the physical shape of the radio wave. It is a graph of frequency versus time. The actual RF signal oscillates continuously; during an upchirp those oscillations become progressively closer together as frequency rises, then the frequency wraps and the pattern opens up again.
TEXT→BITS→SYMBOL→CYCLIC CHIRP→RF
The chirp keeps its sweep law. The symbol changes its cyclic starting position. In the upper pane that moves the wrap point; in the lower pane it changes where the slow-to-fast oscillation pattern begins.
SEND SOMETHING THROUGH THE TOY TRANSMITTER
Message → symbol parade
This deliberately simplified mapper turns each character code into a value inside the selected SF symbol space. It is for seeing successive chirp shifts, not reproducing LoRa's real whitening/FEC/interleaving pipeline.
RECEIVER GAME
Can you hear like the radio?
Hide the answer, inspect the chirp, and use the symbol slider to match the mystery symbol.
What to notice: LoRa uses chirp spread spectrum: frequency sweeps across the configured channel bandwidth while information is represented through cyclic shifts of that chirp. At SF7 there are 27 = 128 symbol values; at SF12 there are 4096. With 125 kHz bandwidth, symbol duration grows from about 1.024 ms at SF7 to 32.768 ms at SF12. The longer symbol time trades data rate and airtime for greater processing gain and sensitivity.