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sinara-hw / Fastino

Fast 32-channel, 3MS/s per channel, 16bit DAC EEM card compatible with Zotino
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Link design #15

Closed jordens closed 4 years ago

jordens commented 4 years ago

I have a prototype for the high speed data link over EEM at https://github.com/quartiq/fastino combined with the Kasli prototype at https://github.com/m-labs/artiq/tree/fastino

This works well (<2e-15 BER so far) without an ultimate level of tuning on a typically EEM ribbon cable length of 0.5m and the boards strewn out across the table. With 3.5m ribbon there is about 1e-9 BER. Still, there are a couple of knobs to tweak that should improve this even for long ribbon cables.

The projected DAC sample rate is 2.55 MS/s each channel. I would leave the filter where it is as this improves the signal quality and puts clock leackage close to the "notch".

Also with this design (due to the SPI-data-word clock relation) there is currently a sawtooth f_sample/3, 14 ns pk-pk deterministic jitter on the DAC update timing. This is irrelevant for most applications but it would become visible as an aliased spur on periodic signals. If this becomes problematic, there are ways around it.

This supports the case for the ice40. Several change requests to the pinout to follow.

gkasprow commented 4 years ago

IDC ribbon cable has 120Ohm impedance of single wire in G-S-G configuration. We use it in G-S-S-G config which gives value closer to 100Ohm. DIfferent ribbon cables have different impedances. There are ones with 105Ohm which should be avoided.

hartytp commented 4 years ago

Very nice. Do we get a red LED or something on checksum errors?

jordens commented 4 years ago

Interesting. My go-to ribbon cable has 102 ohm in gsg config. http://multimedia.3m.com/mws/media/22052O/3mtm-round-conductor-flat-cable-050in-3365-series-ts0080.pdf

gkasprow commented 4 years ago

And this cable gives 170Ohm in a balanced configuration. I was wrong in the post above. We need SGS impedance as low as possible, probably 85Ohm which would be closer to 100Ohm of differential impedance. People are using 25cm .05 ribbon for PCIe so we should not observe issues at 250MHz.

gkasprow commented 4 years ago

for high speed one can use this kind of cable

jordens commented 4 years ago

You can get a led. Counters for bitslip and crc errors that can be cleared are already there.

hartytp commented 4 years ago

:)