In this paper, we present a complete study of the pattern sequence length influence in the optical clock recovery (OCR) process using passive filtering. For this analysis, we use a commercial Fabry--Perot filter (FPF) in front of a semiconductor optical amplifier. A 40-GHz optical clock is then recovered from an optical data stream at 40 Gbit/s. The recovered clock (RCK) quality relies on an original technique based on the remodulation by the optical clock with electrical data. Then we perform two studies in order to explore the dependency of the RCK on patterning effects: first, we carry out a numerical simulation of the tandem filter and amplifier, in order to study its performance as an OCR process. The impact of each component of the clock recovery process is clearly observable via the quality of the remodulated clock. Second, we experimentally study the sensitivity of the OCR to the incoming pattern length using phase noise and bit-error-rate measurements in an optical communication system environment. This study allows an in-depth study of this simple and frequently used OCR configuration, and outlines some limitations of the technique, linked to the compatibility between the finesse of the FPF, the pattern length, and the general optical communication system properties.