  The digital correlator is a crucial element in a modern radio telescope. In
this paper we describe a scalable design of the correlator system for the
Tianlai pathfinder array, which is an experiment dedicated to test the key
technologies for conducting 21cm intensity mapping survey. The correlator is of
the FX design, which firstly performs Fast Fourier Transform (FFT) including
Polyphase Filter Bank (PFB) computation using a Collaboration for Astronomy
Signal Processing and Electronics Research (CASPER) Reconfigurable Open
Architecture Computing Hardware-2 (ROACH2) board, then computes
cross-correlations using Graphical Processing Units (GPUs). The design has been
tested both in laboratory and in actual observation.
