Hello Community,
Like in the topic, I know that a stable 10 MHz source is needed as a
clock signal but why do we need 1 PPS signal? How is it used by the
USRP hardware? Can someone explain that to me?
Thanks
Marcin

USRP-users mailing list --usrp-users@lists.ettus.com
To unsubscribe send an email tousrp-users-leave@lists.ettus.com

On 2022-05-11 06:17, Marcin Puchlik wrote:

Hello Community,
Like in the topic, I know that a stable 10 MHz source is needed as a clock
signal but why do we need 1 PPS signal? How is it used by the USRP
hardware? Can someone explain that to me?
Thanks
Marcin

USRP-users mailing list -- usrp-users@lists.ettus.com
To unsubscribe send an email to usrp-users-leave@lists.ettus.com

1PPS is used to provide timestamp-clock synchronization across multiple
devices, typically.  This is important when your application requires this,
such as in MIMO or
multi-receiver TDOA schemes, etc.

Basically, when you have multiple devices you use set_time_unknown_pps()
or set_time_next_pps() to signal to all devices in your multi_usrp object
that at the next
1PPS, to set the timestamp clock to the value given in the the API call.

This turns out to be useful even in single devices that are "bicameral",
such as B210 and X310, where there are (for historic and architectural
reasons)
TWO timestamp clocks.  Use the 1PPS synchronization primitives causes
the internal timestamp clocks to become synchronized.

USRP-users mailing list -- usrp-users@lists.ettus.com
To unsubscribe send an email to usrp-users-leave@lists.ettus.com

Marcus,
Thank you very much for the answer. Does it mean that 1 PPS signal is
optional? Can I only provide an external 10 MHz clock without 1 PPS?
*Z poważaniem *
Marcin Puchlik

Will it be enough to clock USRP from the external 10 MHz signal
generator? When I run the flowgraph I cannot see the information that
is using the external clock. Here is the output from GNU Radio:
[INFO] [UHD] linux; GNU C++ version 9.4.0; Boost_107100;
UHD_3.15.0.HEAD-0-gaea0e2de
[INFO] [B200] Detected Device: B200
[INFO] [B200] Operating over USB 2.
[INFO] [B200] Initialize CODEC control...
[INFO] [B200] Initialize Radio control...
[INFO] [B200] Performing register loopback test...
[INFO] [B200] Register loopback test passed
[INFO] [B200] Setting master clock rate selection to 'automatic'.
[INFO] [B200] Asking for clock rate 16.000000 MHz...
[INFO] [B200] Actually got clock rate 16.000000 MHz.
[INFO] [B200] Asking for clock rate 51.200000 MHz...
[INFO] [B200] Actually got clock rate 51.200000 MHz.
[INFO] [MULTI_USRP]     1) catch time transition at pps edge
[INFO] [MULTI_USRP]     2) set times next pps (synchronously)
[INFO] [B200] Asking for clock rate 51.200000 MHz...
[INFO] [B200] OK
[INFO] [B200] Asking for clock rate 51.200000 MHz...
[INFO] [B200] OK
[WARNING] [AD936X] Selected Tx sample rate (0.2 MHz) is less than
analog frontend filter bandwidth (0.2 MHz).

image.png

But you know what I observed and what is weird? When I ask for an
external source and I intentionally turn off the external generator
providing a 10 MHz signal, USRP behaves as if it was still seeing a 10
MHz reference signal at its input. Doesn't matter if the generator is
switched on or off - USRP behaves the same way. Because of that I am 
not sure if USRP is being clocked from an internal or external clock
source. Is it the bug in the GNU radio or UHD or am I doing something
wrong? How can I get the feedback from the USRP hardware that it was
locked to the external clock? Is it even possible?

There is a motherboard sensor that you can query to check the
‘ref_locked’ status.

However, I had noted its behavior during clock loss is inconsistent
across models. In UHD 4.1 (and I’m working from memory here), some
models (e.g. E320) check a digital lock-detect signal and always
report the current lock status.  Other models (e.g. N320) query a
register which latches a bit when the external clock is lost, and the
latched bit is only cleared during initialization. On these models,
once the external clock is removed the ‘ref_locked’ sensor will
continue to return false even if the clock is later reapplied while
running.

I can’t specifically speak to the B200 or UHD 3.15.

Hope this helps,

David

From: Marcin Puchlik puchlikmarcin@gmail.com
Sent: Friday, May 13, 2022 10:38 AM
To: Marcus D. Leech patchvonbraun@gmail.com
Cc: usrp-users@lists.ettus.com
Subject: [USRP-users] Re: Why do we need 1 PPS and 10 MHz signal to
synchronize

But you know what I observed and what is weird? When I ask for an
external source and I intentionally turn off the external generator
providing a 10 MHz signal, USRP behaves as if it was still seeing a 10
MHz reference signal at its input. Doesn't matter if the generator is
switched on or off - USRP behaves the same way. Because of that I am 
not sure if USRP is being clocked from an internal or external clock
source. Is it the bug in the GNU radio or UHD or am I doing something
wrong? How can I get the feedback from the USRP hardware that it was
locked to the external clock? Is it even possible?

śr., 11 maj 2022 o 16:14 Marcus D. Leech patchvonbraun@gmail.com
napisał(a):

 On 2022-05-11 09:51, Marcin Puchlik wrote:

     Will it be enough to clock USRP from the external 10 MHz
     signal generator? When I run the flowgraph I cannot see the
     information that is using the external clock. Here is the
     output from GNU Radio:

     [INFO] [UHD] linux; GNU C++ version 9.4.0; Boost_107100;
     UHD_3.15.0.HEAD-0-gaea0e2de
     [INFO] [B200] Detected Device: B200
     [INFO] [B200] Operating over USB 2.
     [INFO] [B200] Initialize CODEC control...
     [INFO] [B200] Initialize Radio control...
     [INFO] [B200] Performing register loopback test...
     [INFO] [B200] Register loopback test passed
     [INFO] [B200] Setting master clock rate selection to 'automatic'.
     [INFO] [B200] Asking for clock rate 16.000000 MHz...
     [INFO] [B200] Actually got clock rate 16.000000 MHz.
     [INFO] [B200] Asking for clock rate 51.200000 MHz...
     [INFO] [B200] Actually got clock rate 51.200000 MHz.
     [INFO] [MULTI_USRP]     1) catch time transition at pps edge
     [INFO] [MULTI_USRP]     2) set times next pps (synchronously)
     [INFO] [B200] Asking for clock rate 51.200000 MHz...
     [INFO] [B200] OK
     [INFO] [B200] Asking for clock rate 51.200000 MHz...
     [INFO] [B200] OK
     [WARNING] [AD936X] Selected Tx sample rate (0.2 MHz) is less than
     analog frontend filter bandwidth (0.2 MHz).

 Yeah, I don't think it puts out an "i'm locked to the external
 reference" message.

 But you've asked for "external" clock source, so you should be
 good to go, assuming your external generator meets the requirements.



     śr., 11 maj 2022 o 15:24 Marcus D. Leech
     <patchvonbraun@gmail.com> napisał(a):

         On 2022-05-11 09:18, Marcin Puchlik wrote:

             Marcus,

             Thank you very much for the answer. Does it mean that
             1 PPS signal is optional? Can I only provide an
             external 10 MHz clock without 1 PPS?

             *Z poważaniem *

             *Marcin Puchlik*

         *Yes, absolutely.  If timestamp synchronization is not
         important to you, then you can just provide a 10MHz
         reference when you want better
           frequency accuracy and drift characteristics than are
         offered by the on-board clock and/or you want some type of
         phase-synchronization
           but don't care much about mutual phase offsets....






         *

             śr., 11 maj 2022 o 14:24 Marcus D. Leech
             <patchvonbraun@gmail.com> napisał(a):

                 On 2022-05-11 06:17, Marcin Puchlik wrote:

                     Hello Community,

                     Like in the topic, I know that a stable 10 MHz
                     source is needed as a clock signal but why do
                     we need 1 PPS signal? How is it used by the
                     USRP hardware? Can someone explain that to me?

                     Thanks

                     Marcin

                     _______________________________________________

                     USRP-users mailing list --usrp-users@lists.ettus.com

                     To unsubscribe send an email tousrp-users-leave@lists.ettus.com

                 1PPS is used to provide timestamp-clock
                 synchronization across multiple devices,
                 typically.  This is important when your
                 application requires this, such as in MIMO or
                   multi-receiver TDOA schemes, etc.

                 Basically, when you have multiple devices you use
                 set_time_unknown_pps() or set_time_next_pps() to
                 signal to all devices in your multi_usrp object
                 that at the next
                   1PPS, to set the timestamp clock to the value
                 given in the the API call.

                 This turns out to be useful even in single devices
                 that are "bicameral", such as B210 and X310, where
                 there are (for historic and architectural reasons)
                   TWO timestamp clocks.  Use the 1PPS
                 synchronization primitives causes the internal
                 timestamp clocks to become synchronized.

                 _______________________________________________
                 USRP-users mailing list -- usrp-users@lists.ettus.com
                 To unsubscribe send an email to
                 usrp-users-leave@lists.ettus.com