Math Bridge: LoRa Chirp Offset and FFT Bins

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Math BridgeLoRaWANChirp bins

How do ten bits become one chirp starting frequency?

Follow this chapter's SF10 symbol from time and sweep slope to its dechirped FFT bin.

Eddie, the electronics guideEddie guides
The one targetMap a symbol number to its starting-frequency offset.
The chapter caseSF10, 125 kHz bandwidth, and symbol 256.
What it buys youA physical reading of the chapter's dechirp and FFT figure.

A field team faces an unresolved physical question: How do ten bits become one chirp starting frequency? They must answer it before changing spreading factor on the real device. Predict the direction first.

See the relationship before changing it

The figure reads from left to right. The blue card is spreading factor. The middle card applies this page's relationship. The green card is possible states. Walk the arrows once: set the input, apply the rule, then read the result with its unit.

The retained audit below checks several chapter fixtures. This added model holds every other chapter fixture fixed, so the numeric fixture does not switch without explanation.

Spreading factor changes possible states An input card leads through the page relationship to the possible states result. SET INPUT ONE CONTROL APPLY RULE predict calculate check units READ RESULT
Walk the arrows. Dechirping cancels the shared ramp, so the remaining tone's bin number preserves the symbol's cyclic start offset.

Derive the baseline in four named moves

  1. 1

    Name the input. The chapter baseline for spreading factor is 256.

  2. 2

    Name the relationship. states = 2^10 = 1024 Tsym = 1024/125000 = 8.192 ms Δfbin = 125000/1024 = 122.0703125 Hz offset(256) = 256 x Δfbin = 31.25 kHz

  3. 3

    Substitute the chapter fixture. Set spreading factor to 256. The page ledger gives possible states as 1024.

  4. 4

    Read the result. Keep the stated output unit beside the value. Use it only inside the technical boundary on this page.

Predict, then change spreading factor

Try Predict the direction of possible states. Move one control, calculate, then check your prediction.

256
Chapter baseline
Possible states

Observe Dechirping cancels the shared ramp, so the remaining tone's bin number preserves the symbol's cyclic start offset. Reset the control to 256 and compare possible states.

Explain Only spreading factor moves here. The other chapter fixtures remain fixed.

Check yourself

What should you do before trusting a moved-control result?
Answer: Predict its direction, apply the shown relationship, keep the units, and reset to the worked baseline.
What does this small model leave out?
Answer: Only spreading factor moves. Field effects named in the page's technical boundary stay fixed.

Honesty note. This teaching model changes only spreading factor. It replays the chapter fixture; it is not a field prediction.

1. Start with the physical story

Each LoRa symbol is a chirp across the channel. At SF10 there are 1,024 possible symbol values. The value chooses a cyclic starting-frequency offset; the receiver removes the common sweep and reads the remaining steady tone as an FFT bin.

Eddie: Every runner follows the same circular track. The symbol number says where that runner starts.

2. Name every algebra move

1

Count statesRaise two to SF10 to get 1,024 values.

2

Find symbol timeDivide the state count by 125,000 Hz.

3

Divide the channelSplit 125 kHz into 1,024 equal FFT bins.

4

Place the symbolMultiply bin spacing by the symbol number.

3. Reproduce the chapter case

states = 2^10 = 1024
Tsym = 1024/125000 = 8.192 ms
Δfbin = 125000/1024 = 122.0703125 Hz
offset(256) = 256 × Δfbin = 31.25 kHz

The chirp slope is 125 kHz divided by 8.192 ms, or 15.26 MHz/s. Symbol 256 begins one quarter of the way across the channel.

4. Try one real input

TryMove the SF10 symbol number and watch its bin and starting-frequency offset move together.

Spreading factor
Possible states
Symbol value
Symbol time
Chirp slope
FFT-bin spacing
Offset
Offset
Channel position

ObserveSymbol 256 lands at 31.25 kHz, exactly 25% across the channel; symbol 512 lands halfway across.

ExplainDechirping cancels the shared ramp, so the remaining tone's bin number preserves the symbol's cyclic start offset.

Technical boundaries.

This is an ideal symbol map, not a complete LoRa receiver.

Timing
Clock error, frequency offset, and sampling windows affect real bin estimates.
Waveform
Wrapping, coding, whitening, headers, and packet structure sit outside this one-symbol screen.
Detection
Noise, interference, synchronization, and implementation shape the error rate.

Correct, not complete: use the measured state named above before release.

5. Use the result in the modulation review

Read the chapter figure as a reversible chain: bits choose a symbol, the symbol chooses a cyclic offset, dechirping leaves a tone, and the FFT returns the bin.

6. Record the evidence state

Record spreading factor, bandwidth, symbol value, sample rate, frequency-error allowance, dechirp reference, FFT size, detected bin, packet settings, SNR, and error results.

7. Check yourself

Why are there 1,024 bins at SF10?
Answer: Ten bits select 2^10, or 1,024, possible symbol values.
Why is symbol 256 one quarter across the band?
Answer: 256/1,024 is 0.25, so its offset is 25% of 125 kHz.
Does this ledger predict packet error rate?
Answer: No. It maps the ideal symbol; synchronization, noise, interference, coding, and receiver design still matter.
Honesty boundary.

The bridge keeps ideal chirp geometry separate from receiver evidence.

Computed
State count, symbol time, slope, bin spacing, offset, and channel share.
Specified
Spreading factor, bandwidth, symbol value, sample clock, and receiver configuration.
Observed
Frequency error, detected bin, SNR, interference, packet delivery, and error rate.

Correct, not complete: this page does not certify a waveform, receiver, radio, regulation, or deployment.

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