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Re: [Discuss-gnuradio] [USRP-users] minimum PPS voltage for N200

From: Marcus Müller
Subject: Re: [Discuss-gnuradio] [USRP-users] minimum PPS voltage for N200
Date: Thu, 5 May 2016 15:50:13 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.6.0

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,
On 05.05.2016 14:28, khalid.el-darymli wrote:

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],

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.


On Tue, May 3, 2016 at 5:20 PM, <address@hidden> 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,

linux; GNU C++ version 4.8.2; Boost_105400; UHD_003.008.002-80-ge28d7844

For the second machine,
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,


On Tue, May 3, 2016 at 1:45 PM, <address@hidden> 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).


On Tue, May 3, 2016 at 12:20 PM, <address@hidden> wrote:

The PPS input is conditioned with a:


Looks to me like it should work just fine.




On 2016-05-03 10:23, khalid.el-darymli via USRP-users wrote:


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,

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