fix: decouple composite drive from hardware TX gain

Clarify that outputDrive controls only the composite signal path while PlutoSDR hardware gain stays fixed at 0 dB. Relax outputDrive validation to allow stronger composite drive during hardware tuning and update the Pluto example config accordingly.

cmd/fmrtx/main.go

@@ -147,11 +147,12 @@ func runTXMode(cfg cfgpkg.Config, driver platform.SoapyDriver, autoStart bool) {
 	defer cancel()
 
 	// Configure driver
-	// Gain mapping: outputDrive 1.0 = max power (0 dB atten), 0.0 = min (-89 dB)
+	// OutputDrive controls composite signal level, NOT hardware gain.
+	// Hardware TX gain is always 0 dB (max power). Use external attenuator for power control.
 	soapyCfg := platform.SoapyConfig{
 		Driver:       cfg.Backend.Device,
 		CenterFreqHz: cfg.FM.FrequencyMHz * 1e6,
-		GainDB:       (1.0 - cfg.FM.OutputDrive) * 89, // 1.0→0dB(max), 0.5→44.5dB atten, 0.0→89dB(min)
+		GainDB:       0, // 0 dB = max TX power on PlutoSDR
 	}
 	soapyCfg.SampleRateHz = cfg.EffectiveDeviceRate()
 

docs/config.plutosdr.json

@@ -19,7 +19,7 @@
     "pilotLevel": 0.1,
     "rdsInjection": 0.05,
     "preEmphasisTauUS": 50,
-    "outputDrive": 1.0,
+    "outputDrive": 1.8,
     "compositeRateHz": 228000,
     "maxDeviationHz": 75000,
     "limiterEnabled": true,

internal/config/config.go

@@ -128,8 +128,8 @@ func (c Config) Validate() error {
 	if c.FM.RDSInjection < 0 || c.FM.RDSInjection > 0.15 {
 		return fmt.Errorf("fm.rdsInjection out of range")
 	}
-	if c.FM.OutputDrive < 0 || c.FM.OutputDrive > 1 {
-		return fmt.Errorf("fm.outputDrive out of range")
+	if c.FM.OutputDrive < 0 || c.FM.OutputDrive > 3 {
+		return fmt.Errorf("fm.outputDrive out of range (0..3)")
 	}
 	if c.FM.CompositeRateHz < 96000 || c.FM.CompositeRateHz > 1520000 {
 		return fmt.Errorf("fm.compositeRateHz out of range")