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 accuracy, 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 to a little network in order to block DC and attenuate 10 dB. With a small high Q sma cable this network is connected to the  input jack of the RX888MKII unit for external clock.

The output level of the little network is 1V top/top, which can be easily checked with an oscilloscope.

Because there is no entry for that on the case, we have to make a 5mm hole into the box (at the side where the usb connector is) and install a 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.