[Hackrf-dev] HackRF: MAX5864 -> MAX2837 differential pair connections

[Dominic Spill]

On 22 October 2017 at 08:47, Frank Liu <frank.zijie at gmail.com> wrote:
>
> I was flipping through the HackRF schematic when I noticed that the TX
differential pairs between the MAX5864 and MAX2837 seem to be reversed,
i.e. TXBBI- of the MAX2837 is connected to ID+ of the MAX5864 and vice
versa. The same goes for the Q data stream (and for the receive path).
>
> I assume this was done on purpose. What is the reasoning behind this?

Yes, this was intentional.  If you look at the layout of the board, the
pins are in different orders on the two parts, so routing them would
involve crossing the signals, and using vias to move them on to different
board layers.  All of this is a negative when it comes to signal integrity.

As they are differential signals, we can switch them as long as we correct
the values in the digital domain, which we do in the CPLD here:
https://github.com/mossmann/hackrf/blob/master/firmware/cpld/sgpio_if/top.vhd#L117

I hope this explanation helps and was clear enough, I'm definitely not one
of the hardware people on this project, so this was pieced together from
what I've learned from Mike, Jared, and others.

Thanks,
  Dominic
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