fix: real-domain spectrum renders as +-fc conjugate-symmetric Euler pairs

adc/src/adc_engine.cpp

@@ -51,6 +51,7 @@ void AdcEngine::update(double /*dt*/) {
         out.noise_total_W.clear();
         out.phase_deg.clear();
         out.fs_Hz = m_fs_Hz / 2.0;
+        out.is_complex_baseband = true;
         out.bumpGeneration();
         return;
     }
@@ -67,6 +68,7 @@ void AdcEngine::update(double /*dt*/) {
     for (int i = 0; i < N; ++i)
         out.frequencies[i] = -m_fs_Hz / 4.0 + i * df_out;
     out.fs_Hz = m_fs_Hz / 2.0;
+    out.is_complex_baseband = true;
 
     // -- Noise mapping + NSD --
     double nsd_W_per_Hz = 0.001 * dbToLinear(m_nsd_dBm_per_Hz);

amplifier/src/amplifier_engine.cpp

@@ -53,6 +53,7 @@ void AmplifierEngine::update(double dt) {
             buildDefaultFrequencyGrid(out.frequencies);
 
         out.tones = in_ptr ? in_ptr->tones : std::vector<Spectrum::Tone>{};
+        out.is_complex_baseband = in_ptr ? in_ptr->is_complex_baseband : false;
         const size_t N = out.frequencies.size();
 
         // Apply S21 complex gain to tones
@@ -136,6 +137,7 @@ void AmplifierEngine::update(double dt) {
     }
 
     out.tones = in_ptr ? in_ptr->tones : std::vector<Spectrum::Tone>{};
+    out.is_complex_baseband = in_ptr ? in_ptr->is_complex_baseband : false;
     for (auto &t : out.tones) {
         t.power_dBm += m_gain_dB;
     }

attenuator/src/attenuator_engine.cpp

@@ -69,6 +69,7 @@ void AttenuatorEngine::update(double dt) {
         int idx = 1 * m_sparam.numPorts() + 0; // S21 index
 
         out.tones = in_ptr ? in_ptr->tones : std::vector<Spectrum::Tone>{};
+        out.is_complex_baseband = in_ptr ? in_ptr->is_complex_baseband : false;
         for (auto &t : out.tones) {
             auto S = m_sparam.interpolate(t.freq_Hz, idx);
             t.power_dBm += 20.0 * std::log10(std::abs(S));
@@ -132,6 +133,7 @@ void AttenuatorEngine::update(double dt) {
 
     // Attenuate tones
     out.tones = in_ptr ? in_ptr->tones : std::vector<Spectrum::Tone>{};
+    out.is_complex_baseband = in_ptr ? in_ptr->is_complex_baseband : false;
     for (auto &t : out.tones) {
         t.power_dBm -= m_atten_dB;
         // Phase unchanged in manual mode

coax/src/coax_cable_engine.cpp

@@ -67,6 +67,7 @@ void CoaxCableEngine::update(double dt) {
     }
 
     out.tones = in_ptr ? in_ptr->tones : std::vector<Spectrum::Tone>{};
+    out.is_complex_baseband = in_ptr ? in_ptr->is_complex_baseband : false;
 
     const CableSpec &p = preset();
     const double max_f_Hz = p.max_freq_GHz * 1e9;

combiner/src/combiner_engine.cpp

@@ -140,6 +140,8 @@ void CombinerEngine::update(double dt) {
         }
 
         out.tones = combined_tones;
+        out.is_complex_baseband =
+            (in0 && in0->is_complex_baseband) || (in1 && in1->is_complex_baseband);
 
         // Noise: scale by |S21|^2 and |S31|^2 respectively
         const double k = 1.3806e-23;
@@ -229,6 +231,8 @@ void CombinerEngine::update(double dt) {
     }
 
     out.tones = combined_tones;
+    out.is_complex_baseband =
+        (in0 && in0->is_complex_baseband) || (in1 && in1->is_complex_baseband);
 
     // Noise: incoherent power sum, then apply loss
     out.noise_W.assign(N, 0.0);

common/AGENTS.md

@@ -8,7 +8,7 @@ Own the header-only data model shared by all RF Simulator modules: `SignalNode`,
 
 - `common/common.h` — `MIN_FREQ`, `MAX_FREQ`, `MIN_POWER`, `MAX_POWER`, `DEFAULT_VBW`, `DEFAULT_RBW`, physical constants (`k`, `T`, `R`), `dbToLinear`, `calculateNoiseTemp`, `addedNoiseDensity_W_per_Hz`, `addedNoisePerBin_W`, `buildDefaultFrequencyGrid`
 - `common/signal_node.h` — `SignalNode` (input + output spectra + view_enabled)
-- `common/spectrum.h` — `Spectrum` (frequencies, tones, noise vectors, phase, generation counter, `fs_Hz`) and `Peak`
+- `common/spectrum.h` — `Spectrum` (frequencies, tones, noise vectors, phase, generation counter, `fs_Hz`, `is_complex_baseband`) and `Peak`; also the free helper `conjugateSymmetricExpand()` for expanding real-domain tones into +-fc conjugate-symmetric pairs
 - `common/component_interface.h` — `IComponentEngine` (DSP engine contract)
 - `common/view_manager.h` — `ViewManager` (registry of `SignalNode*`)
 - `common/include/group.h` — `Group` and `GroupBoundaryPin` (subcircuit grouping data)
@@ -26,6 +26,7 @@ Own the header-only data model shared by all RF Simulator modules: `SignalNode`,
 ## Work Guidance
 
 - Changes to `SignalNode` or `Spectrum` affect every engine. Update all engines' `update()` and tests.
+- `Spectrum::is_complex_baseband` (default `false`) marks spectra downstream of an ADC's DDC (complex baseband/IQ); every pass-through engine propagates it from its input exactly like `fs_Hz`. Only `AdcEngine`'s output sets it to `true`. `conjugateSymmetricExpand()` must stay render-only (used by the spectrum-analyzer render path for real-domain spectra) — never call it from interior DSP (generator, `nonlinear_model.h`, gain/filter/S-param stages, mixer), which must keep operating on the collapsed single-entry-per-tone representation.
 - New fields on `IComponentEngine` must keep a default implementation that preserves backward compat for all existing engines.
 - New files in `common/` or `common/include/` are automatically picked up by `common/CMakeLists.txt`'s glob.
 

common/spectrum.h

@@ -28,6 +28,12 @@ struct Spectrum {
     // Set by ADCs, propagated through most components, read by PFB channelizer.
     double fs_Hz = 0.0;
 
+    // True for spectra downstream of an ADC's DDC (complex baseband/IQ). False (default) for
+    // real-domain (analog) spectra. Set only by AdcEngine's output; propagated downstream from
+    // there exactly like fs_Hz already is. Determines whether the spectrum-analyzer render path
+    // needs to apply conjugateSymmetricExpand() before binning tone power (see that function).
+    bool is_complex_baseband = false;
+
     // Generation counter for dirty-flag tracking. Producers increment after
     // recomputation so consumers can detect upstream changes.
     uint64_t generation = 0;
@@ -48,6 +54,37 @@ struct Spectrum {
     }
 };
 
+// Expands real-domain tones into their conjugate-symmetric (+-fc) representation per Euler's
+// formula: cos(2*pi*fc*t) = 0.5*(exp(j*2*pi*fc*t) + exp(-j*2*pi*fc*t)). Each non-DC tone becomes
+// two entries at +freq_Hz and -freq_Hz, each at half the linear power (-3.0103 dB = 10*log10(2)
+// below the input). DC tones (freq_Hz == 0) are self-conjugate and pass through unchanged (no
+// mirror, no power split).
+//
+// Used only where +-fc content is physically meaningful: rendering a real-domain (pre-ADC)
+// spectrum. NOT used by interior DSP (generator, nonlinear_model.h, gain/filter/S-param stages,
+// mixer), which must stay on the collapsed single-entry-per-tone representation — splitting there
+// would corrupt nonlinear_model.h's harmonic/IM math (calibrated against full real-tone power).
+inline std::vector<Spectrum::Tone>
+conjugateSymmetricExpand(const std::vector<Spectrum::Tone> &tones) {
+    std::vector<Spectrum::Tone> out;
+    out.reserve(tones.size() * 2);
+    for (const auto &t : tones) {
+        if (t.freq_Hz == 0.0) {
+            out.push_back(t);
+            continue;
+        }
+        Spectrum::Tone half = t;
+        half.power_dBm = t.power_dBm - 10.0 * std::log10(2.0);
+        Spectrum::Tone at_f = half;
+        at_f.freq_Hz = t.freq_Hz;
+        Spectrum::Tone at_negf = half;
+        at_negf.freq_Hz = -t.freq_Hz;
+        out.push_back(at_f);
+        out.push_back(at_negf);
+    }
+    return out;
+}
+
 struct Peak {
     int index;
     double freq_Hz;