SDR (Software Defined Radio)

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h1. 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":https://tbspace.de/plutosdrclockinput.html

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

h2. Hardware parts

h3. 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:

h4. Original PlutoSDR RevB

{{thumbnail(pluto-revb-orig.jpg)}}

h4. XO removed using hot air

{{thumbnail(pluto-revb-xo_removed.jpg)}}

h4. VCC pad removed

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

{{thumbnail(pluto-revb-pad_removed.jpg)}}

h4. 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

{{thumbnail(pluto-revb-copper_scratched.jpg)}}

h4. 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.

{{thumbnail(pluto-revb-ufl.jpg)}}

h3. 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:https://gitea.osmocom.org/electronics/osmo-small-hardware/src/branch/master/plutosdr-revb-extclk

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.