The RX-888 is a high-performance SDR, and has proven very useful in applications that can make use of the full HF-band and beyond coverage. Many of these applications will benefit from a sampling clock having higher-accuracy than is provided by the RX-888 internal oscillator. Fortunately, this SDR includes a small connector on the circuit board that can be connected to an external 27 MHz reference clock.
This connector, however, is tied more or less directly to the input of the RX-888s clock generator chip, and so is a very high impedance port. There are clamp diodes on this connection which will cause asymmetrical clipping if the input clock is not AC-coupled. The clamps and unterminated cable can cause distortion and ringing on the clock signal, which will cause improper operation of the RX-888 clock generator.
The Turn Island Systems RX-888 Clock Kit includes an interface board that provides the appropriate termination, attenuation, and isolation for an external reference clock, as generated by a Bodnar or similar GPSDO. The kit also includes a replacement back-panel and a 10 cm jumper cable.
This document provides all the details: RX-888 Clock Kit