Hi Khalid, all PPS inputs are 50-Ohm-terminated; you can s
And, as Marcus Leech said, 2.52V should probably be OK – If you go to the datasheet that he linked to, p.8, you'll see that for the used Vcc of 3.3V, the rising edge threshold voltage is less then 1.16V; which means that over the 1.2K/2K voltage divider (R590 / R592) a total voltage of a little less than 3/2 * 1.16V = 1.74V must exist. Your 2.52V is higher than that. You should be fine, though not by a huge amount. Best regards, Marcus On 05.05.2016 14:28, khalid.el-darymli wrote: > Hi, > > I will appreciate it if someone could please answer my question. > > The PPS signal output from the FireFly 1-A GPS is designed to drive a > high impedance load (not 50 ohms). Without 50 ohm terminator, my scope > placed at the output of our PPS fan-out reads 3.68 V (as shown in the > attachment). With 50 ohm terminator, my scope reads 2.52 V. > > Now our fan-out circuit is designed to be a high input impedance and a > 50 ohm output impedance. What is the input impedance for the PPS > receiver on the N200 unit? > > I understand that the PSS input for N200 is recommended to be in the > range [3.3 V, 5 V], > http://files.ettus.com/manual/page_usrp2.html#usrp2_hw_leds > > Maybe due to impedance mismatch, the actual input PPS voltage taken > from our fan-out to the N200 unit exceeds the 5V thus causing > stability issues inside the N200 unit. I am just guessing because > using the same set-up, same code, everything worked before just fine. > The only major change we made was in our fan-out unit. > > Khalid > > > On Tue, May 3, 2016 at 5:20 PM, <mle...@ripnet.com > <mailto:mle...@ripnet.com>> wrote: > > What you're going to need to do is provide the *simplest* > flow-graph that demonstrates the issue, rather than requiring the > community to debug your entire end-to-end setup. > > Also a diagram of your setup showing the RF and 1PPS and 10MHz paths. > > > > > > > > On 2016-05-03 15:39, khalid.el-darymli wrote: > >> Thanks again Marcus. I really appreciate your help. >> >> I am setting Ref Clock / PPS to external, etc. I get the syncing >> to work properly around a year ago. Since then, we made various >> changes and I think one or more change may be causing the issue. >> Among the changes are: buy N200 units with a new revision (could >> that be a problem when syncing?), upgrading GNU Radio / UHD, >> enabling Rs 232 echoing in the GPSDO, etc. >> >> Similar to my last year tests (I entirely unplugged the RS 232 >> cable), and now it is not detected as an Internal GPSDO. However, >> I am still having issue syncing the two motherboards. >> >> Attached are two figures for the phase drift between Rx1 vs. Rx2, >> and Rx1 vs. Rx3. This was generated through splitting the Tx and >> looping it back to the Rx's. Signal processing was done properly >> (for dechirping, downsamplng, etc). My application is LFMCW >> radar, so each 490 samples in the attachment represents one >> sweep, and sweeping was done continuously. >> >> angle(Rx1./Rx2) looks great. As to angle(Rx1./Rx3), I would >> expect a linear phase offset between different motherboards. >> However, as .shown in the attachment, Rx1 vs. Rx3 has weird phase >> spikes. I think this shows that, for some reason, the Tx is not >> properly synced with the second motherboard. Any ideas on what >> might be causing this issue? >> >> I am using Starttech Ethernet Adapter (ST1000SPEX4). The problem >> I have with this card is that it won't turn AUTONEG off, it is >> always on. Could this cause the problem? >> >> I tried this on two different Ubuntu machines, with similar >> results as shown in the attachment. >> >> For the first machine, >> >> UBUNTU 14.04 LTS >> linux; GNU C++ version 4.8.2; Boost_105400; >> UHD_003.008.002-80-ge28d7844 >> >> For the second machine, >> UBUNTU 14.04 LTS >> linux; GNU C++ version 4.8.2; Boost_105400; >> UHD_003.008.000-46-g5b706d29 >> >> >> I'll highly appreciate any suggestions to solve this problem. >> >> Thank you. >> >> Best wishes, >> khalid >> >> >> >> >> >> >> >> On Tue, May 3, 2016 at 1:45 PM, <mle...@ripnet.com >> <mailto:mle...@ripnet.com>> wrote: >> >> The only way the USRP knows that there's a GPSDO present is >> if the serial data from the GSPDO is validated. If it >> doesn't see that data, it concludes that there's no >> (internal) GPSDO present. >> >> There is no concept of "external GPSDO" -- only that >> *something* is providing 10MHz and 1PPS externally. The N2xx >> has no idea what that might be. >> >> You should set both 1PPS and 10MHz clock to "external" in >> your flow-graph. The only time "GPSDO" is used is when you >> have a properly-installed internally-mounted, GPSDO unit. >> >> >> >> >> >> >> >> On 2016-05-03 12:02, khalid.el-darymli wrote: >> >> Thanks Marcus. >> >> OK, I'm communicating with the external Firefly-1A GPSDO >> unit using an RS 232 cable. I am able to do so after >> enabling echoing on RS 232 from the GPSDO. Would that be >> a problem? Do I need to completely disable the RS 232 >> echoing, even while the RS 232 cable is completely unplugged? >> >> When the RS 232 cable is plugged-in, N200 DETECTs it as >> an INTERNAL GPSDO. When I completely unplug the RS 232 >> cable on both ends, I am not getting any messages for >> detecting neither external nor internal GPSDO. It only >> shows the following in the terminal for both N200 devices, >> >> (1) catch time transition at pps edge >> (2) set time next pps (synchronously). >> >> khalid >> >> >> >> >> On Tue, May 3, 2016 at 12:20 PM, <mle...@ripnet.com >> <mailto:mle...@ripnet.com>> wrote: >> >> The PPS input is conditioned with a: >> >> http://www.ti.com/product/SN74AUP1T57/description >> >> Looks to me like it should work just fine. >> >> >> >> >> >> >> >> On 2016-05-03 10:23, khalid.el-darymli via USRP-users >> wrote: >> >> Hi, >> >> I'll appreciate any help on the following. >> According to the link below [1], the PPS voltage >> for N200 should be in the range *[3.3, 5]* V p-p. >> [1] >> >> http://files.ettus.com/manual/page_usrp2.html#usrp2_hw_leds >> >> I am getting a PPS signal through fan-outs from >> an external Firefly-1A GPSDO. I measured the >> output PPS voltage using a scope, and it is *2.52 >> V p-p* (terminated with 50 ohms). >> >> Would this relatively low PPS voltage work for >> N200? I am having a problem syncing two N200 >> units (2 LFRX/ 1 LFTX ), and I am not sure if >> this would be the cause? >> >> >> Thank you. >> >> Best wishes, >> Khalid >> >> _______________________________________________ >> USRP-users mailing list >> usrp-us...@lists.ettus.com >> <mailto:usrp-us...@lists.ettus.com> >> >> http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com >> >> >> >> >> >> >> > > > > > > _______________________________________________ > Discuss-gnuradio mailing list > Discuss-gnuradio@gnu.org > https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
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