The QCX+ 25MHz TCXO module option provides frequency accuracy and stability to your QCX+, QCX-mini or other QRP Labs kits (including Ultimate3S). 

This is a tiny, ready-assembled PCB measuring 14.8 x 10.5mm, containing two capacitors and a Temperature Controlled Crystal Oscillator (TCXO). 

Installation in the QCX+ is very simple. It replaces the 27MHz crystal and 0.1uF capacitor C2. In the QCX-mini it is the same, but even easier as there is no 0.1uF capacitor C2 to replace. The TCXO module has a cut-out so that it fits over the Si5351A chip on the main QCX+ board. Three pads (Ground, VDD and Out) on the TCXO module are connected to the main QCX+/QCX-mini PCB by soldering off-cut wires through the corresponding pairs of holes. Remember to set the configuration menu for reference frequency, to 25,000,000 to suit the 25MHz TCXO. 

You can also install the TCXO module in the Si5351A Synth kit which is used in the Ultimate3S and other QRP Labs kits. 

The TCXO module was tested from 4.5C to 65.5C. The graph above shows the corresponding output frequency change, in parts-per-million (ppm). Basically from 4 to 65C it was within +/- 0.25 ppm.