Modulation Modes
The SimTX Transmitter supports the following modulation modes. The Auto bandwidth option fills the typical bandwidth for the selected mode; the bandwidth field accepts any value down to the mode's minimum.
Choose a mode[edit | edit source]
Modulation is how the audio or data is carried. Set the receiver to the same kind of signal.
- For the first audio-file example, use AM Double Sideband Full Carrier and tick Auto bandwidth. This is ordinary AM.
- For narrow voice signals, use USB or LSB. These are single-sideband (SSB) modes; the receiver must use the correct sideband.
- For broadcast-style stereo from PipeWire Stereo, use WFM. It needs much more bandwidth than SSB.
- Use IQ only when another program already generates the waveform.
- Selecting FT8 or WSPR does not compose a message. The source must already contain the encoded signal.
The typical and minimum bandwidths below are reference values. Keep Auto enabled until there is a reason to select a different width.
Mode reference[edit | edit source]
| Mode | Full name | Typical bandwidth | Minimum bandwidth | Audio range |
|---|---|---|---|---|
| AM (full carrier) | Amplitude modulation, double sideband, full carrier | 10 kHz | 160 Hz | 80 to 4500 Hz |
| AM (suppressed carrier) | Double sideband, suppressed carrier | 6 kHz | 160 Hz | 100 to 2800 Hz |
| USB | Upper sideband | 2700 Hz | 300 Hz | 350 to 2700 Hz |
| LSB | Lower sideband | 2700 Hz | 300 Hz | 350 to 2700 Hz |
| CW | Continuous wave (Morse keying) | 150 Hz | 50 Hz | n/a |
| NFM | Narrowband FM | 12.5 kHz | 2.5 kHz | 200 to 3000 Hz |
| WFM | Wideband FM (broadcast) | 180 kHz | 50 kHz | 30 Hz to 15 kHz |
| IQ | External complex IQ input | 48 kHz | 100 Hz | n/a |
| WSPR | Weak Signal Propagation Reporter | 6 Hz | 6 Hz | 0.5 to 3 Hz |
| FT8 | Franke-Taylor FT8 | 50 Hz | 50 Hz | 20 to 80 Hz |
Mode notes[edit | edit source]
- USB and LSB are suppressed-carrier modes. The carrier leak and opposite-sideband suppression settings in Advanced Transmitter Settings control how imperfect the suppression is. By convention LSB is used below 10 MHz and USB above, but SimTX does not enforce this.
- AM modes expose the AM modulation index setting. An index above 1.0 is overmodulation and produces audible distortion and splatter.
- CW exposes the key rise time setting. Very short rise times produce key clicks that widen the occupied bandwidth. CW is also the most sensitive mode: its narrow bandwidth lets it remain readable on paths where voice modes fail, see Propagation Basics.
- NFM and WFM are constant-envelope modes; they are less affected by amplifier compression but need considerably more signal at the receiver than SSB. WFM supports stereo when fed from the PipeWire Stereo source.
- IQ passes externally generated complex baseband (for example from GNU Radio) straight to the channel. Selecting the GNU Radio IQ source forces this mode.
- WSPR and FT8 provide narrow digital-mode signal paths. The selected transmission source must already contain the corresponding encoded audio signal.
Extension modes[edit | edit source]
Installed Lua modulation extensions appear in the same selector. An extension may produce IQ directly or produce audio for a selectable built-in RF mode. Its declared controls and media inputs appear below the standard modulation fields.
Field interactions[edit | edit source]
- The low-cut field is editable for USB, LSB, both AM modes, NFM and WFM.
- Opposite-sideband suppression applies to USB and LSB only.
- Carrier leak applies to USB, LSB and suppressed-carrier AM.
- AM index applies to the two AM modes.
- CW rise time applies to CW only.
- PipeWire Stereo forces WFM; GNU Radio IQ forces IQ.