diff --git a/src/Core/Core.fsproj b/src/Core/Core.fsproj
index 99c76640..d1fa53aa 100644
--- a/src/Core/Core.fsproj
+++ b/src/Core/Core.fsproj
@@ -43,6 +43,7 @@
     <Compile Include="TemporalCoordinationDetection.fs" />
     <Compile Include="Veridicality.fs" />
     <Compile Include="Graph.fs" />
+    <Compile Include="PhaseExtraction.fs" />
     <Compile Include="Window.fs" />
     <Compile Include="Advanced.fs" />
     <Compile Include="Fusion.fs" />
diff --git a/src/Core/PhaseExtraction.fs b/src/Core/PhaseExtraction.fs
new file mode 100644
index 00000000..1c9245ad
--- /dev/null
+++ b/src/Core/PhaseExtraction.fs
@@ -0,0 +1,105 @@
+namespace Zeta.Core
+
+open System
+
+
+/// **PhaseExtraction — event streams to phase series.**
+///
+/// Completes the input pipeline for `TemporalCoordination-
+/// Detection.phaseLockingValue` (PR #298): PLV expects phase
+/// arrays in radians but doesn't prescribe how events become
+/// phases. Addresses Amara 17th-ferry correction #5.
+///
+/// Two methods shipped here — a caller picks the one matching
+/// their event-stream semantics:
+///
+/// * **`epochPhase`** — periodic epoch phase. For a node with
+///   known period `T` (e.g., slot duration, heartbeat
+///   interval), phase is `φ(t) = 2π · (t mod T) / T`. Suited
+///   to consensus-protocol events with a fixed cadence.
+///
+/// * **`interEventPhase`** — circular phase between consecutive
+///   events. For a node whose events occur at irregular times
+///   `t_1 < t_2 < …`, the phase at sample time `t` is
+///   `φ(t) = 2π · (t − t_k) / (t_{k+1} − t_k)` where
+///   `t_k ≤ t < t_{k+1}`. Suited to event-driven streams
+///   without fixed periods.
+///
+/// Returns `double[]` with one phase per sample time (in
+/// radians, `[0, 2π)` range). Downstream PLV calls don't care
+/// about the wrapping convention — only phase differences matter.
+///
+/// Provenance: Amara 17th-ferry Part 2 correction #5 (event
+/// streams must define phase construction before PLV becomes
+/// meaningful). Otto 17th graduation.
+[<AutoOpen>]
+module PhaseExtraction =
+
+    let private twoPi = 2.0 * Math.PI
+
+    /// **Periodic-epoch phase.** For each sample time `t` in
+    /// `sampleTimes`, compute `φ(t) = 2π · (t mod period) / period`.
+    /// Returns a `double[]` of the same length as `sampleTimes`.
+    ///
+    /// Returns empty array when `period <= 0` or `sampleTimes`
+    /// is empty — degenerate inputs produce degenerate output,
+    /// not an exception.
+    ///
+    /// Useful when a node's events tie to a known cadence
+    /// (validator slots, heartbeat intervals, epoch boundaries).
+    let epochPhase (period: double) (sampleTimes: double[]) : double[] =
+        if period <= 0.0 || sampleTimes.Length = 0 then [||]
+        else
+            let out = Array.zeroCreate sampleTimes.Length
+            for i in 0 .. sampleTimes.Length - 1 do
+                let t = sampleTimes.[i]
+                let m = t - (floor (t / period)) * period
+                out.[i] <- twoPi * m / period
+            out
+
+    /// **Inter-event circular phase.** For each sample time `t`
+    /// in `sampleTimes`, find the bracketing events
+    /// `t_k ≤ t < t_{k+1}` in `eventTimes` and compute
+    /// `φ(t) = 2π · (t − t_k) / (t_{k+1} − t_k)`.
+    ///
+    /// `eventTimes` MUST be sorted ascending. Undefined
+    /// behavior if it isn't — we don't sort internally to keep
+    /// the function O(|sampleTimes| log |eventTimes|), and
+    /// callers typically produce sorted event logs.
+    ///
+    /// Samples BEFORE the first event get phase `0.0`
+    /// (we're "at the beginning" of the first implicit
+    /// interval). Samples AFTER the last event also get `0.0`
+    /// — there's no successor to extrapolate against. Callers
+    /// that care about these edge cases filter `sampleTimes`
+    /// to the interior range `[t_1, t_n)` first.
+    ///
+    /// Returns empty array when either input is empty or the
+    /// event series has fewer than 2 points (no interval to
+    /// measure phase within).
+    let interEventPhase
+            (eventTimes: double[])
+            (sampleTimes: double[])
+            : double[] =
+        if eventTimes.Length < 2 || sampleTimes.Length = 0 then [||]
+        else
+            let n = eventTimes.Length
+            let out = Array.zeroCreate sampleTimes.Length
+            for i in 0 .. sampleTimes.Length - 1 do
+                let t = sampleTimes.[i]
+                if t < eventTimes.[0] || t >= eventTimes.[n - 1] then
+                    out.[i] <- 0.0
+                else
+                    // Binary search for the bracketing interval
+                    let mutable lo = 0
+                    let mutable hi = n - 1
+                    while hi - lo > 1 do
+                        let mid = (lo + hi) / 2
+                        if eventTimes.[mid] <= t then lo <- mid
+                        else hi <- mid
+                    let tk = eventTimes.[lo]
+                    let tkNext = eventTimes.[lo + 1]
+                    let interval = tkNext - tk
+                    if interval <= 0.0 then out.[i] <- 0.0
+                    else out.[i] <- twoPi * (t - tk) / interval
+            out
diff --git a/tests/Tests.FSharp/Algebra/PhaseExtraction.Tests.fs b/tests/Tests.FSharp/Algebra/PhaseExtraction.Tests.fs
new file mode 100644
index 00000000..cca8c224
--- /dev/null
+++ b/tests/Tests.FSharp/Algebra/PhaseExtraction.Tests.fs
@@ -0,0 +1,103 @@
+module Zeta.Tests.Algebra.PhaseExtractionTests
+
+open System
+open FsUnit.Xunit
+open global.Xunit
+open Zeta.Core
+
+
+// ─── epochPhase ─────────
+
+[<Fact>]
+let ``epochPhase of sample at t=0 is 0`` () =
+    let phases = PhaseExtraction.epochPhase 10.0 [| 0.0 |]
+    abs phases.[0] |> should (be lessThan) 1e-9
+
+[<Fact>]
+let ``epochPhase at half-period is pi`` () =
+    // Period 10; t=5 → phase = 2π · 5/10 = π
+    let phases = PhaseExtraction.epochPhase 10.0 [| 5.0 |]
+    abs (phases.[0] - Math.PI) |> should (be lessThan) 1e-9
+
+[<Fact>]
+let ``epochPhase wraps at period boundary`` () =
+    // t = period returns to phase 0
+    let phases = PhaseExtraction.epochPhase 10.0 [| 10.0; 20.0 |]
+    abs phases.[0] |> should (be lessThan) 1e-9
+    abs phases.[1] |> should (be lessThan) 1e-9
+
+[<Fact>]
+let ``epochPhase handles negative sample times`` () =
+    // t = -5, period 10 → mod = 5 → phase = π
+    let phases = PhaseExtraction.epochPhase 10.0 [| -5.0 |]
+    abs (phases.[0] - Math.PI) |> should (be lessThan) 1e-9
+
+[<Fact>]
+let ``epochPhase returns empty on invalid period`` () =
+    PhaseExtraction.epochPhase 0.0 [| 1.0; 2.0 |] |> should equal ([||]: double[])
+    PhaseExtraction.epochPhase -1.0 [| 1.0 |] |> should equal ([||]: double[])
+
+
+// ─── interEventPhase ─────────
+
+[<Fact>]
+let ``interEventPhase returns empty on fewer than 2 events`` () =
+    PhaseExtraction.interEventPhase [| 5.0 |] [| 3.0 |] |> should equal ([||]: double[])
+    PhaseExtraction.interEventPhase [||] [| 3.0 |] |> should equal ([||]: double[])
+
+[<Fact>]
+let ``interEventPhase at start of interval is 0`` () =
+    // Events at t=0, 10. Sample at t=0 → phase 0 (start of first interval).
+    let phases = PhaseExtraction.interEventPhase [| 0.0; 10.0 |] [| 0.0 |]
+    abs phases.[0] |> should (be lessThan) 1e-9
+
+[<Fact>]
+let ``interEventPhase at midpoint is pi`` () =
+    // Events at 0, 10. Sample at 5 → phase = 2π · 5/10 = π
+    let phases = PhaseExtraction.interEventPhase [| 0.0; 10.0 |] [| 5.0 |]
+    abs (phases.[0] - Math.PI) |> should (be lessThan) 1e-9
+
+[<Fact>]
+let ``interEventPhase adapts to varying intervals`` () =
+    // Events at 0, 4, 10. Sample at 2 → phase in [0,4] interval
+    // = 2π · 2/4 = π. Sample at 7 → phase in [4,10] interval
+    // = 2π · 3/6 = π.
+    let phases = PhaseExtraction.interEventPhase [| 0.0; 4.0; 10.0 |] [| 2.0; 7.0 |]
+    abs (phases.[0] - Math.PI) |> should (be lessThan) 1e-9
+    abs (phases.[1] - Math.PI) |> should (be lessThan) 1e-9
+
+[<Fact>]
+let ``interEventPhase returns 0 before first and after last event`` () =
+    // Events at 10, 20. Sample at 5 (before first) and 25
+    // (after last) both return 0.
+    let phases = PhaseExtraction.interEventPhase [| 10.0; 20.0 |] [| 5.0; 25.0 |]
+    abs phases.[0] |> should (be lessThan) 1e-9
+    abs phases.[1] |> should (be lessThan) 1e-9
+
+
+// ─── composes with phaseLockingValue ─────────
+
+[<Fact>]
+let ``epochPhase output feeds phaseLockingValue for synchronized sources`` () =
+    // Two nodes with identical period-10 cadence, same sample
+    // times → identical phases → PLV = 1.
+    let samples = [| 1.0; 3.0; 5.0; 7.0; 9.0; 11.0; 13.0 |]
+    let phasesA = PhaseExtraction.epochPhase 10.0 samples
+    let phasesB = PhaseExtraction.epochPhase 10.0 samples
+    let plv =
+        TemporalCoordinationDetection.phaseLockingValue phasesA phasesB
+        |> Option.defaultValue 0.0
+    abs (plv - 1.0) |> should (be lessThan) 1e-9
+
+[<Fact>]
+let ``epochPhase output feeds phaseLockingValue for constant-offset sources`` () =
+    // Node A period 10; Node B same period, offset by +2 seconds.
+    // Phase difference is constant → PLV = 1 (perfect locking at
+    // a non-zero offset).
+    let samples = [| 1.0; 3.0; 5.0; 7.0; 9.0 |]
+    let phasesA = PhaseExtraction.epochPhase 10.0 samples
+    let phasesB = PhaseExtraction.epochPhase 10.0 (samples |> Array.map (fun t -> t + 2.0))
+    let plv =
+        TemporalCoordinationDetection.phaseLockingValue phasesA phasesB
+        |> Option.defaultValue 0.0
+    abs (plv - 1.0) |> should (be lessThan) 1e-9
diff --git a/tests/Tests.FSharp/Tests.FSharp.fsproj b/tests/Tests.FSharp/Tests.FSharp.fsproj
index 5799674c..33167a61 100644
--- a/tests/Tests.FSharp/Tests.FSharp.fsproj
+++ b/tests/Tests.FSharp/Tests.FSharp.fsproj
@@ -21,6 +21,7 @@
     <Compile Include="Algebra/TemporalCoordinationDetection.Tests.fs" />
     <Compile Include="Algebra/Veridicality.Tests.fs" />
     <Compile Include="Algebra/Graph.Tests.fs" />
+    <Compile Include="Algebra/PhaseExtraction.Tests.fs" />
 
     <!-- Circuit/ -->
     <Compile Include="Circuit/Circuit.Tests.fs" />