diff --git a/dsp/src/lib.rs b/dsp/src/lib.rs
index b2acf34..9b9e966 100644
--- a/dsp/src/lib.rs
+++ b/dsp/src/lib.rs
@@ -1,4 +1,5 @@
-#![no_std]
+#![cfg_attr(not(test), no_std)]
 #![cfg_attr(feature = "nightly", feature(asm, core_intrinsics))]
 
 pub mod iir;
+pub mod pll;
diff --git a/dsp/src/pll.rs b/dsp/src/pll.rs
new file mode 100644
index 0000000..8cef99b
--- /dev/null
+++ b/dsp/src/pll.rs
@@ -0,0 +1,77 @@
+use serde::{Deserialize, Serialize};
+
+/// Type-II, sampled phase, discrete time PLL
+///
+/// This PLL tracks the frequency and phase of an input signal with respect to the sampling clock.
+/// The transfer function is I^2,I from input phase to output phase and P,I from input phase to
+/// output frequency.
+///
+/// The PLL locks to any frequency (i.e. it locks to the alias in the first Nyquist zone) and is
+/// stable for any gain (1 <= shift <= 30). It has a single parameter that determines the loop
+/// bandwidth in octave steps. The gain can be changed freely between updates.
+///
+/// The frequency settling time constant for an (any) frequency jump is `1 << shift` update cycles.
+/// The phase settling time in response to a frequency jump is about twice that. The loop bandwidth
+/// is about `1/(2*pi*(1 << shift))` in units of the sample rate.
+///
+/// All math is naturally wrapping 32 bit integer. Phase and frequency are understood modulo that
+/// overflow in the first Nyquist zone. Expressing the IIR equations in other ways (e.g. single
+/// (T)-DF-{I,II} biquad/IIR) would break on overflow.
+///
+/// There are no floating point rounding errors here. But there is integer quantization/truncation
+/// error of the `shift` lowest bits leading to a phase offset for very low gains. Truncation
+/// bias is applied. Rounding is "half up".
+///
+/// This PLL does not unwrap phase slips during lock acquisition. This can and should be
+/// implemented elsewhere by (down) scaling and then unwrapping the input phase and (up) scaling
+/// and wrapping output phase and frequency. This affects dynamic range accordingly.
+///
+/// The extension to I^3,I^2,I behavior to track chirps phase-accurately or to i64 data to
+/// increase resolution for extremely narrowband applications is obvious.
+#[derive(Copy, Clone, Default, Deserialize, Serialize)]
+pub struct PLLState {
+    // last input phase
+    x: i32,
+    // filtered frequency
+    f: i32,
+    // filtered output phase
+    y: i32,
+}
+
+impl PLLState {
+    /// Update the PLL with a new phase sample.
+    ///
+    /// Args:
+    /// * `input`: New input phase sample.
+    /// * `shift`: Error scaling. The frequency gain per update is `1/(1 << shift)`. The phase gain
+    ///   is always twice the frequency gain.
+    ///
+    /// Returns:
+    /// A tuple of instantaneous phase and frequency (the current phase increment).
+    pub fn update(&mut self, x: i32, shift: u8) -> (i32, i32) {
+        debug_assert!(shift >= 1 && shift <= 31);
+        let bias = 1i32 << shift;
+        let e = x.wrapping_sub(self.f);
+        self.f = self.f.wrapping_add(
+            (bias >> 1).wrapping_add(e).wrapping_sub(self.x) >> shift,
+        );
+        self.x = x;
+        let f = self.f.wrapping_add(
+            bias.wrapping_add(e).wrapping_sub(self.y) >> shift - 1,
+        );
+        self.y = self.y.wrapping_add(f);
+        (self.y, f)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    #[test]
+    fn mini() {
+        let mut p = PLLState::default();
+        let (y, f) = p.update(0x10000, 10);
+        assert_eq!(y, 0xc2);
+        assert_eq!(f, y);
+    }
+}