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.
| 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
- 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
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
- 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.