For the time reference this one was suggested on an earlier discussion…. http://www.leobodnar.com/shop/index.php?main_page=index&cPath=107&zenid=468ab2632bba5b5ffe4d2fe7d8bb19e2
If you need to reduce cost even more you can find GPS based 1PPS sources on ebay. https://www.ebay.com/itm/UBLOX-LEA-5T-high-precision-timing-GPS-module-dev-board-1PPS-USB-RS232-ntp-ser/263546760982?hash=item3d5c9c6f16:g:hbgAAOSwbsBXopDl:rk:19:pf:0 If you have a failure you should have a good plan for how you can obtain another time reference for the project before or after the failure. Also their can be some phase noise in a 1PPS signal. This phase noise can contribute to error in your system just like noise degrades RF reception. I have not looked at the SNAP spec but it is stated here as 2.0V into a 50 ohm load or resistor. A LVTTL CMOS output would be the wrong voltage level and could not supply 40 mA. In this case you will need a buffer circuit to match the output conditions of the 1PPS reference selected and the input of SNAP. You could build this yourself. Typically there will be the need for many time reference lines. You must be sure not to create any phase delay as you fan out N time reference lines. So each circuit path and connecting cable will be the same type and length. You can see examples of this in other systems posted to this list. Your circuit would interface the output and input with the proper voltage level and currents. Take a look at this link for some ideas of buffer interfaces… http://www.ti.com/lit/an/scea035a/scea035a.pdf I am a retired engineer that follows this list and offer these suggestions to hopefully help you in your efforts. Bob Stricklin On Oct 19, 2018, at 2:04 AM, Nitish Ragoomundun <[email protected]<mailto:[email protected]>> wrote: Hi all, We are building a low-frequency array for the observation of the deuterium hyperfine line at 327.4 MHz with a bandwidth of 250 kHz. We intend to use SNAPs for acquisition. The boards will operate at full 12 channels input, thus the ADCs at 250 MSps. We will subsequently decimate the data rate, as our working bandwidth is narrow. Concerning the clock input for the SNAP, https://casper.berkeley.edu/wiki/SNAP states the following: Digital 1 PPS: 50 ohm single-ended LVTTL logic levels * SMATP13 * Vin-high 2.0 to 3.3 Volts. Low current drive sources, such as typical LVTTL or CMOS gates, probably can not supply the 40mA required to supply the 2.0V into the 50ohm load. * Vin-low 0.0 to 0.8 Volts Actually we considered buying the GPS-disciplined Meinberg M500 LANTIME (https://www.meinbergglobal.com/english/products/modular-railmount-ntp-server-ieee-1588-solution.htm), which is advertised to output 1 PPS TTL levels. Now, our first question is whether the SNAP can work with this? Also, the note about low current drive sources is a little confusing, can anyone shed some light here. Secondly, we would like suggestions from anyone who has experience with clocking the SNAP. You see, we run a very tight budget and the M500 LANTIME clock is expensive. We would like to know if there is a less costly way to clock the SNAP and synchronise several boards. Thanks. Best regards, Nitish Ragoomundun Department of Physics University of Mauritius -- You received this message because you are subscribed to the Google Groups "[email protected]<mailto:[email protected]>" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]<mailto:[email protected]>. To post to this group, send email to [email protected]<mailto:[email protected]>. -- You received this message because you are subscribed to the Google Groups "[email protected]" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To post to this group, send email to [email protected].
