The RX888MKII radio is impressively stable when it is warmed up, but we are going to need long term stability AND short term stability and preciseness, with low jitter. We will explain this later why we need that. For that reason we use a mini Precision GPS  Reference Clock unit. For details see here.

The reference clock is phase locked and has very low jitter. The unit needs a signal from a GPS receiver, and there are lots of them in the market. The unit is connected to a pc and set to 27.000000 MHz and low power output. The output connector is connected with a small thin high Q sma cable to a RF 50 Ohm stub and (we used a T-connector) also to the input jack of the RX888MKII unit for external clock. Because there is no entry for that in the case, we have to make a 5mm hole into the box (at the side where the usb connector is) and install an sma connector with a 10cm cable and MHF plug which fits into the connector on the pcb of the RX888MKII.  Set the input on the pcb to 27MHz instead of the local oscillator.

Making the hole in the box is described in the next page where we also discuss the cooling.

After  this we have now a precision freq radio. Much preciser than 1 Hz !, because of the phase locking.