SDR and Digital Modes on Air
Software Defined Radio (SDR)
In a traditional radio, the signal processing is done by physical components — crystal filters, analogue mixers, discrete amplifiers. In an SDR, most of this is done in software. The hardware is simple: an antenna, a few amplifiers, and an analogue-to-digital converter. Everything else is code.
SDR Architectures
- Direct sampling: The ADC samples the RF signal directly. Requires very fast ADCs but is the simplest architecture.
- Direct conversion (zero-IF): The RF is mixed down to baseband (0 Hz) and then sampled. Most common in amateur SDRs.
I/Q Processing
SDRs use I (In-phase) and Q (Quadrature) signals — two versions of the signal with a 90° phase difference. Together, they capture both the amplitude and phase of the signal, allowing any modulation type to be processed digitally.
Think of I and Q as capturing "what" the signal is doing and "how" it's changing — enough information to reconstruct everything.
Digital Modulation Types
Different ways to encode data onto a radio signal:
| Type | What Changes | Example |
|---|---|---|
| FSK | Frequency | RTTY (170 Hz shift between two tones) |
| PSK | Phase | PSK31 (31 Hz bandwidth, very narrow) |
| QAM | Both amplitude AND phase | High-speed data modes |
Popular Amateur Digital Modes
| Mode | Bandwidth | Can decode at | What it's for |
|---|---|---|---|
| FT8 | ~50 Hz | −24 dB SNR | Weak signal DX — the most popular digital mode |
| FT4 | ~80 Hz | −17 dB SNR | Faster version for contests |
| PSK31 | ~31 Hz | −10 dB SNR | Keyboard-to-keyboard chat |
| RTTY | ~300 Hz | −5 dB SNR | Classic digital, contesting |
| JS8Call | ~50 Hz | −22 dB SNR | Keyboard messaging, store-and-forward |
| Winlink/VARA | Adaptive | Varies | Email over radio |
FT8's secret: FT8 uses 15-second transmit/receive cycles with LDPC error correction coding. Each message is only 77 bits, but it takes 12.6 seconds to send them. By spending a long time on very little data, it achieves incredible sensitivity. This is a practical application of Shannon's theorem — trade speed for reliability.
Error Correction
Two approaches to dealing with errors:
- FEC (Forward Error Correction): Add extra data so the receiver can fix errors without asking for retransmission. Used in FT8, satellite links — anywhere retransmission is impractical.
- ARQ (Automatic Repeat Request): If an error is detected (using CRC checks), ask the sender to retransmit. Used in Winlink, packet radio — works when a two-way link is available.