RX888 Clock Kit and Thermal Pad

Description

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, designed by Paul Elliott, WB6CXC, 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. Please note that once this modification has been completed the RX-888 will not operate without the external clock. This modification can be easily reversed, restoring normal operation. The RX-888 can get pretty hot. To reduce any thermal problems, two changes are suggested:
• Reduce the sampling rate. This is a matter of software and is not covered in this document.
• Add a large thermal pad to the bottom of the board. This makes good contact to the chassis and heatsink.

Thermal Pad Installation
The RX-888 Clock Adaptor Kit includes a thermal pad assembly, which allows for a much improved transfer of heat from the RX-888 circuit board to the external heat sinks. The materials used are:
Thermal pad: Laird Technologies A51340-01
( https://www.digikey.com/en/products/detail/laird-technologies-thermal-materials/A15340-01/2496314 )
Copper foil tape: https://www.amazon.com/dp/B09Z6F9RFG
Both sides of the thermal pad have adhesive, and the copper foil allows the pad to slide into the RX-888 enclosure while providing good thermal conductivity. In the kit, the foil tape and pad are separate.

KA7OEI has a blog post on this topic. He suggests a 45mm by 65mm piece of the Laird thermal pad material.
http://ka7oei.blogspot.com/2024/03/using-external-clock-with-rx-888-mk2.html
http://ka7oei.blogspot.com/2023/08/improving-thermal-management-of-rx-888.html
and other articles on his blog.

The manual is available online at Turn Island Systems:
RX888 Clock Kit Manual

Buy this item, AND the HF-RX-ISO Kit, and we will ship them free in the USA via USPS Ground Advantage (equal numbers of each).  It’s rather than give a combined product discount, as we can ship anywhere in the mainland 48 states USA, USPS Ground Advantage, for under $6)