diff --git a/probeflow/io/spectroscopy.py b/probeflow/io/spectroscopy.py
index d26055e..87887cc 100644
--- a/probeflow/io/spectroscopy.py
+++ b/probeflow/io/spectroscopy.py
@@ -299,7 +299,7 @@ def _metadata_from_vert_report(
         "filename": report.path.name,
         "bias_mv": float(bias_mv) if bias_mv is not None else None,
         "setpoint_a": setpoint_a,
-        "spec_freq_hz": _f(find_hdr(hdr, "SpecFreq", "1000"), 1000.0),
+        "spec_freq_hz": _spec_freq_hz(hdr),
         "gain_pre_exp": float(_f(find_hdr(hdr, "GainPre 10^", "9"), 9.0)),
         "fb_log": hdr.get("FBLog", "0").strip() == "1",
         "sweep_type": "time_trace" if is_time_trace else "bias_sweep",
@@ -339,6 +339,21 @@ def _metadata_from_vert_report(
 
 
 def _createc_setpoint_a(hdr: dict[str, str]) -> float | None:
+    """Return the tunnel-current setpoint in amps, or None.
+
+    These header keys only describe a tunnel current when the Z feedback
+    actually runs on the current channel. For constant-Δf qPlus AFM the loop
+    runs off the PLL frequency-shift channel and ``SetPoint`` is a Δf in
+    hertz — reading it as amps fed an absurd "current" into setpoint
+    normalization (``y / 3.2 A`` instead of ``y / 100 pA``), silently
+    rescaling displayed spectra by ~10 orders of magnitude. Same bug class
+    as the scan-metadata fix; the gate is shared so the two paths cannot
+    drift apart again.
+    """
+    from probeflow.core.metadata import _createc_feedback_is_current
+
+    if not _createc_feedback_is_current(hdr):
+        return None
     for key in ("Current[A]", "SetPoint"):
         value = _f(find_hdr(hdr, key, None))
         if value is not None and value > 0:
@@ -349,6 +364,25 @@ def _createc_setpoint_a(hdr: dict[str, str]) -> float | None:
     return None
 
 
+def _spec_freq_hz(hdr: dict[str, str]) -> float:
+    """Return the sampling frequency for the time axis, sanitised.
+
+    A missing ``SpecFreq`` has always defaulted to 1000 Hz; an explicit zero,
+    negative, or non-finite value would otherwise turn the time axis into
+    inf/garbage (``x = idx / spec_freq``), so it is treated the same way —
+    loudly.
+    """
+    freq = _f(find_hdr(hdr, "SpecFreq", "1000"), 1000.0)
+    if not np.isfinite(freq) or freq <= 0:
+        log.warning(
+            "SpecFreq=%r is not a positive frequency; using the 1000 Hz "
+            "default for the time axis",
+            freq,
+        )
+        return 1000.0
+    return float(freq)
+
+
 def _scaled_channels_from_vert_report(
     report: CreatecVertDecodeReport,
     measurement: dict[str, Any] | None = None,
diff --git a/probeflow/spectroscopy/transforms.py b/probeflow/spectroscopy/transforms.py
index f73da4a..b59eb89 100644
--- a/probeflow/spectroscopy/transforms.py
+++ b/probeflow/spectroscopy/transforms.py
@@ -64,7 +64,12 @@ def make_displayed_spectrum(
         if (opts.normalize_mode or "none").strip().lower() == "channel":
             raise ValueError("channel normalization cannot be combined with numerical derivative")
         x, y = numerical_derivative(x, y)
-        y_unit = f"{y_unit}/{trace.x_unit}".rstrip("/") if y_unit else ""
+        # Mirror SpectrumDeltaMeasurement.slope_unit: A/V for the usual
+        # case, bare y_unit when x is unitless, 1/x_unit when y is unitless.
+        if y_unit and trace.x_unit:
+            y_unit = f"{y_unit}/{trace.x_unit}"
+        elif trace.x_unit:
+            y_unit = f"1/{trace.x_unit}"
         if trace.x_unit == "V" and _looks_like_current(trace.y_channel, y_label):
             y_label = "numerical dI/dV"
         else:
diff --git a/tests/test_spectroscopy_physics.py b/tests/test_spectroscopy_physics.py
new file mode 100644
index 0000000..b6a014b
--- /dev/null
+++ b/tests/test_spectroscopy_physics.py
@@ -0,0 +1,184 @@
+"""Physics-correctness tests for the spectroscopy pipeline (2026-06-11 review).
+
+Covers the seams the kernel-level tests miss: the VERT-header → setpoint
+extraction that feeds setpoint normalization (the qPlus Δf-as-amps bug, fixed
+on the scan path earlier but never on the spectroscopy path), time-axis
+sanitisation, unit-label propagation through derivative/normalization, and
+the documented pipeline order.
+"""
+from __future__ import annotations
+
+import numpy as np
+import pytest
+
+from probeflow.io.spectroscopy import _createc_setpoint_a, _spec_freq_hz
+from probeflow.spectroscopy.models import SpectrumDisplayOptions, SpectrumTrace
+from probeflow.spectroscopy.transforms import make_displayed_spectrum
+
+
+def _trace(y, x=None, *, x_unit="V", y_unit="A", y_channel="I",
+           y_label="current", metadata=None) -> SpectrumTrace:
+    y = np.asarray(y, dtype=np.float64)
+    x = np.linspace(-1.0, 1.0, y.size) if x is None else np.asarray(x, float)
+    return SpectrumTrace(
+        source_file="t.VERT", spectrum_id="s1",
+        x_channel="V", y_channel=y_channel,
+        x_raw=x, y_raw=y,
+        x_label="Bias", y_label=y_label,
+        x_unit=x_unit, y_unit=y_unit,
+        metadata=dict(metadata or {}),
+    )
+
+
+# ── VERT setpoint extraction (qPlus Δf must never read as a current) ──────────
+
+class TestVertSetpointExtraction:
+    QPLUS_HDR = {
+        "FBChannel": "4",   # PLL frequency-shift channel
+        "PLLOn": "1",
+        "SetPoint": "3.2",  # Δf in Hz — NOT a current
+        "FBLogI": "700",    # stale leftover from the current loop
+    }
+
+    def test_qplus_header_yields_no_current_setpoint(self):
+        """A constant-Δf qPlus header must not produce a setpoint in amps:
+        before the fix this returned 3.2 (Hz read as A), and setpoint
+        normalization silently rescaled spectra by ~10 orders of magnitude."""
+        assert _createc_setpoint_a(self.QPLUS_HDR) is None
+
+    def test_stm_current_feedback_reads_amps(self):
+        hdr = {"FBChannel": "0", "PLLOn": "0", "Current[A]": "1e-10"}
+        assert _createc_setpoint_a(hdr) == pytest.approx(1e-10)
+
+    def test_stm_setpoint_key_reads_amps_when_feedback_is_current(self):
+        hdr = {"PLLOn": "0", "SetPoint": "2.5e-10"}
+        assert _createc_setpoint_a(hdr) == pytest.approx(2.5e-10)
+
+    def test_fblogi_fallback_is_picoamps(self):
+        hdr = {"FBChannel": "0", "FBLogI": "700"}
+        assert _createc_setpoint_a(hdr) == pytest.approx(700e-12)
+
+    def test_zero_and_negative_setpoints_are_unknown(self):
+        assert _createc_setpoint_a({"Current[A]": "0"}) is None
+        assert _createc_setpoint_a({"Current[A]": "-1e-10", "FBLogI": "0"}) is None
+
+    def test_setpoint_normalization_blocks_instead_of_silently_rescaling(self):
+        """End to end: with qPlus metadata the displayed-spectrum pipeline
+        must refuse setpoint normalization (no finite setpoint), not divide
+        by a frequency pretending to be a current."""
+        trace = _trace(np.linspace(-1e-10, 1e-10, 32),
+                       metadata={"setpoint_a": _createc_setpoint_a(self.QPLUS_HDR)})
+        with pytest.raises(ValueError, match="setpoint"):
+            make_displayed_spectrum(
+                trace, SpectrumDisplayOptions(normalize_mode="setpoint"))
+
+
+# ── Time-axis sanitisation ────────────────────────────────────────────────────
+
+class TestSpecFreqSanitisation:
+    def test_valid_frequency_passes_through(self):
+        assert _spec_freq_hz({"SpecFreq": "512"}) == pytest.approx(512.0)
+
+    def test_missing_uses_documented_default(self):
+        assert _spec_freq_hz({}) == pytest.approx(1000.0)
+
+    @pytest.mark.parametrize("bad", ["0", "-5", "nan", "inf", "garbage"])
+    def test_invalid_frequency_falls_back_instead_of_inf_time_axis(self, bad):
+        """x = idx / spec_freq: a zero/negative/non-finite header value would
+        turn the whole time axis into inf or negative seconds."""
+        freq = _spec_freq_hz({"SpecFreq": bad})
+        assert np.isfinite(freq) and freq > 0
+
+
+# ── Unit-label propagation (invariant 7) ──────────────────────────────────────
+
+class TestUnitPropagation:
+    def test_derivative_updates_current_units_and_label(self):
+        disp = make_displayed_spectrum(
+            _trace(np.linspace(0, 1e-9, 32)),
+            SpectrumDisplayOptions(derivative=True),
+        )
+        assert disp.y_unit == "A/V"
+        assert disp.y_label == "numerical dI/dV"
+
+    def test_derivative_of_unitless_y_reads_one_over_x_unit(self):
+        disp = make_displayed_spectrum(
+            _trace(np.linspace(0, 1, 32), y_unit="", y_channel="aux",
+                   y_label="ratio"),
+            SpectrumDisplayOptions(derivative=True),
+        )
+        assert disp.y_unit == "1/V"
+
+    def test_derivative_over_unitless_x_keeps_y_unit(self):
+        disp = make_displayed_spectrum(
+            _trace(np.linspace(0, 1e-9, 32), x_unit=""),
+            SpectrumDisplayOptions(derivative=True),
+        )
+        assert disp.y_unit == "A"
+
+    def test_normalization_makes_units_relative(self):
+        disp = make_displayed_spectrum(
+            _trace(np.linspace(1e-12, 1e-9, 32)),
+            SpectrumDisplayOptions(normalize_mode="max_abs"),
+        )
+        assert disp.y_unit == "relative"
+        assert "divided by max |y|" in disp.y_label
+
+
+# ── Pipeline order (invariant 2) ──────────────────────────────────────────────
+
+class TestPipelineOrder:
+    def test_normalization_applies_to_the_derivative_not_before_it(self):
+        """max-abs normalization after the derivative must make the
+        *derivative* peak 1. If normalization ran first, the derivative of a
+        rescaled trace would carry an arbitrary 1/V scale instead."""
+        x = np.linspace(-1.0, 1.0, 201)
+        y = x ** 2  # dy/dx = 2x → max |dy/dx| = 2
+        disp = make_displayed_spectrum(
+            _trace(y, x),
+            SpectrumDisplayOptions(derivative=True, normalize_mode="max_abs"),
+        )
+        assert float(np.nanmax(np.abs(disp.y_display))) == pytest.approx(1.0)
+
+    def test_display_pipeline_metadata_records_documented_order(self):
+        disp = make_displayed_spectrum(
+            _trace(np.linspace(0, 1, 16)), SpectrumDisplayOptions())
+        steps = disp.metadata["display_pipeline"]
+        names = [s.split("=")[0] for s in steps[1:]]
+        assert names == ["smoothing", "derivative", "normalization",
+                         "outlier_mask", "vertical_offset"]
+
+    def test_smoothing_runs_before_derivative(self):
+        """Differentiation amplifies noise; the pipeline must smooth first.
+        With heavy smoothing the derivative of noisy flat data stays small;
+        if the derivative ran first the noise would already be amplified."""
+        rng = np.random.default_rng(7)
+        x = np.linspace(-1.0, 1.0, 401)
+        y = 1e-10 * np.ones_like(x) + rng.normal(scale=1e-12, size=x.size)
+        raw = make_displayed_spectrum(
+            _trace(y, x), SpectrumDisplayOptions(derivative=True))
+        smoothed = make_displayed_spectrum(
+            _trace(y, x),
+            SpectrumDisplayOptions(derivative=True, smoothing_mode="gaussian",
+                                   smoothing_points=25),
+        )
+        assert (np.nanstd(smoothed.y_display)
+                < 0.25 * np.nanstd(raw.y_display))
+
+    def test_channel_normalization_with_derivative_is_rejected(self):
+        trace = _trace(np.linspace(0, 1e-9, 16))
+        with pytest.raises(ValueError, match="channel normalization"):
+            make_displayed_spectrum(
+                trace,
+                SpectrumDisplayOptions(derivative=True,
+                                       normalize_mode="channel",
+                                       normalize_channel="I"),
+                channel_lookup={"I": np.ones(16)},
+            )
+
+    def test_forward_backward_sweep_is_refused_not_differentiated(self):
+        x = np.concatenate([np.linspace(-1, 1, 16), np.linspace(1, -1, 16)])
+        y = np.linspace(0, 1e-9, 32)
+        with pytest.raises(ValueError, match="monotonic"):
+            make_displayed_spectrum(
+                _trace(y, x), SpectrumDisplayOptions(derivative=True))