In the original PlutoSDR RevC or higher, there's a BSS138 FET
that [optionally] pulls low the SD1/SD2 pins in order to activate
the external clock input.
In this circuit, we don't have GPIO control but permanently enable
the LTC.
Related: OS#5892
This is a small circuit board containing the "external clock ref input"
circuitry that was added to PlutoSDR RevD. It can be used to
upgrade/rework RevB boards with the same clock input as the later RevD.
Related: OS#5892
Harald Welte