PlutoSDR RevB external clock rework

The PlutoSDR RevA + RevB don't yet have an input for an external reference clock. This is rather sad, as it makes the PlutoSDR unusable for pretty much any applications in the context of cellular technology, where clock accuracy requirements are very high.

One can replace the on-board XO with a U.FL socket to feed an external clock. This has been described a number of times by various people, like this post on tbspace

However, this can be extended further by adding the identical circuit added in PlutoSDR RevC on an external circuit board, see #5892

Hardware parts

Replacing XO on PlutoSDR RevB with U.FL socket

The PlutoSDR board needs some kind of connector to feed the clock signal to it. In order to do so, we remove the XO and place a U.FL socket.

Step-by-step pictures following below:

Original PlutoSDR RevB


XO removed using hot air


VCC pad removed

We need to remove the VCC pad of the XO as the U.FL overlaps it and might short-circuit


GND soldermask removed for U.FL

We need to remove some solder-mask above the top layer GND copper in order to have a GND pad for the left-hand side of the U.FL


U.FL socket soldered

We can now solder the U.FL socket. Center pin goes to the pad that used to be the XO output. The right two pads can both be used: The top right pad is GND, and the bottom right pad is not connected and is soldered only to improve mechanical aspects.


Adding RevC/RevD circuit as external circuit board

The design files of a small circuit board containing the full clock input circuit of PlutoSDR RevC/RevD can be found "in the osmo-small-hardware.git repository:

This circuit board primarily adds the LTC6957-3 clock input logic converter IC, which converts nicely and with low jitter from e.g. a sine wave 10MHz reference to the CMOS level square wave required by the AD9363 of the PlutoSDR.

This has the advantage that the performance / spec of the clock input is identical to that of the later-revision PlutoSDR.

The external circuit board is inter-connected with the PlutoSDR by a U.FL to U.FL cable, and itself provides SMA jacks for clock input (and output). The general idea is to mount the PlutoSDR PCBA and the plutosdr-revb-clkin next to each other in some kind of larger assembly, for example a module for a 3U component carrier (but of course any kind of mounting/board will do).

Updated by laforge 5 months ago · 3 revisions

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