Autocorrelation Periodogram

What is the Autocorrelation Periodogram?

The autocorrelation periodogram is John Ehlers' spectral estimator for finding the dominant cycle in price. Instead of Fourier-transforming price directly, it first computes the autocorrelation of a band-limited version of price at a range of lags, then measures the power at each candidate period by correlating that autocorrelation series against sine and cosine waves of that period. The theoretical license is the Wiener-Khinchin relation: the power spectrum of a stationary series is the Fourier transform of its autocorrelation function. Working through autocorrelation adds a practical benefit: correlation values are bounded between -1 and +1, so the spectrum is normalized for swing size and quiet and volatile stretches become comparable.

In Ehlers' presentation the input is conditioned with a roofing filter to strip trend and high-frequency noise, the scan covers a bounded band of tradeable periods (roughly 10 to 48 bars in his examples), and the result is drawn as a spectrogram heatmap: time across, period up, color for normalized power. The dominant cycle is read as the power-weighted center of the strong periods and typically feeds dominant cycle measurement workflows that adapt indicator lookbacks. Market cycles drift, fade, and sometimes vanish entirely, so a smeared or shifting ridge is itself information: it says there is no stable cycle to tune to.

How traders use it

• Adaptive lookbacks: the measured dominant cycle sets oscillator and moving-average lengths on the fly (for example, tuning a stochastic to a fraction of the dominant period), replacing one fixed setting with a cycle-aware one.
• Regime filtering: a sharp, persistent ridge on the spectrogram argues for cycle-based tactics such as fading swings, while weak or smeared power argues the market is trending or noisy and cycle tools should stand down.
• Auditing cycle claims: because the display shows the whole spectrum through time, it exposes whether an apparent cycle was stable across history or an artifact of one lucky stretch.