[Xylo-SDR] Fundamental Atlas design decisions

["Ray Anderson" <ray.anderson@xilinx.com>]

A few thoughts regarding the Atlas design:

1) Here is one possible stackup which keeps the LVDS signal on a
separate layer from the other signals  (audio, clock, JTAG, I2C, other
TTL/CMOS signals/control, etc.). I'm showing the PWR layer on the
outside, but it could be swapped with the GND layer. Having GND on the
outside may be a bit better from a safety/shorting/EMI perspective, but
having it between the LVDS and signal layer is probably better from a SI
perspective since the reference path for the clock and others won't be
as dependent on the goodness of the decoupling.


-------------------------  PWR


-------------------------  LVDS (100 ohm differential)


-------------------------  GND


-------------------------  Other Signals


2) We need to decide on a substrate thickness for the stackup. If we go
for a total typical thickness of .062 (which will provide good
mechanical rigidity for the backplane) then the dielectric layers will
be about 18 mils which is reasonable. This decision will dictate the
width and spacing of the LVDS conductors that need to have a 100 ohm
differential impedance.


3) Designers of the Janus and Lionheart boards should endeavor to keep
the LVDS driver/receivers as close as possible to the connector as
possible in order to minimize stub lengths (important in a point to
multipoint system).


4) We'll need to optimize the signal to ground ratio for the LVDS
connector pinout to minimize SSO problems.


5) Backplane decoupling is important as is buss termination. If the
design were to only support Lionheart and Janus then termination would
only be required on those boards and the backplane would be
'transparent'. However since we will have a minimum of 4 slots then
backplane termination is required. If the add on 'buss extenders' as
envisioned by Eric materialize we will need to make provisions to
turn-off the backplane termination scheme and move it out to the
extenders.


There are lots of other issues to be dealt with, but these are some
basic but important ones that will effect the viability of the whole
design.


Ray   WB6TPU