On Mon, Jul 17, 2017 at 1:16 AM, Luke Kenneth Casson Leighton lkcl@lkcl.net wrote:
On Mon, Jul 17, 2017 at 7:26 AM, Richard Wilbur richard.wilbur@gmail.com wrote:
Wonderful! That's music to my ears. No major obstacles on return current path except the vias. So when we change signal layers, we'll need adjacent ground plane-to-ground plane vias to provide a nice low-impedance path for the return current in the ground planes.
i've added as many of these as i can fit. space is... very very tight.
For our differential pairs we should only need one return-current-path via per signal via (and hopefully relatively adjacent to it) since we are using ground planes for our reference planes. It is basically trying to provide a relatively low-impedance path for the RF (radio-frequency) return current in the reference planes (since when we switch signal layers between top and bottom, our reference plane changes).
PADS can calculate impedance based on board stack, track width and track-to-track spacing... i'm... relying on that, and the fact that the tracks are 50mm long.
From what I'm reading this looks easiest (and best) to tackle in
segments of no more than 15mm at a time. Can PADS work with you on one section of the path at a time? More details in next message.
1. Are you specifying the track width, track-to-track spacing, and board stack or is PADS determining that for you? 2. What are you using for the ... a. microstrip differential pair track widths, b. intra-pair track spacing, c. differential-pair-to-other track spacing? 3. What is your copper thickness on top and bottom layers? 4. What is the height of insulator between top layer and adjacent ground reference layer? 5. What is the height of insulator between bottom layer and adjacent ground reference layer?
-- Richard