Re: Discuss-gnuradio Digest, Vol 259, Issue 8

Hi Moses et al - 

I spend a lot of time working with B210s on a shared 10MHz clock and PPS source:  

• Some cheap GPSDO products, e.g. from EBay, work - but it's not clear if they're using BeiDou (or other system) as opposed to (US) GPS, and 
• Many that advertise 'four clock channels' actually have two square-wave signals paired with two sine wave signals - effectively two, not four, clock signals.
• Given the price difference and value of my time, I broke down and got an NI Octoclock-G, which works great and has eight (each) PPS / Clock ports.   
• For your use case, the $220 GPSDO EBay PPS/clocks may be fine - Mfr also claims you can special order a four-square-wave version. 
• The main drawback with (my) $220 EBay GPSDO is having a single port for the (TTL) PPS signal: in practice,  providing the PPS signal to more than one radio requires creating a DIY ('distribute it yourself') circuit for the PPS - this can be sub-optimal, as discussed below.

Regarding 'lock':

• I'm 98% certain that any USRP  `pps_locked` / `ref_locked` register is only valid when using an internal GPSDO for the B210 or higher-end units that have internal GPSDO - see 'caveat'. 
• In this context 'lock' refers to lock-on-satellite, which is a property of the PPS/Clock unit, not the radio. 
• The clock and pps signals are square waves / pulses, and do not have a method of coding lock status in the signal. 
• Any external GPS unit will have LED indicators for 'lock' status.   Check them early and often  :-)
• My $220 EBay GPSDO will produce PPS and 10MHz signals when **not** locked to a GPS signal, hilarity ensues.   
• The Octoclock-G has a separate 10Gbps(?) ethernet port that can report NMEA data, which I presume would indicate loss-of-lock.  Since the B210 isn't network-enabled, taking advantage of the NMEA info requires setting up additional hardware / software.  I haven't tried it yet.

When using an external GPSDO/clock/pps with the B210, the [UHD: USRP Source (or Sink)] block settings should be:

              Sync   [[ Unknown PPS ]]   ** see CAVEAT: Internal GPSDO

 Mb0: Clock Source   [[ External    ]]       

  Mb0: Time Source   [[ External    ]]

• External reference signals are the 'source-of-truth' when attached - so 'attached' or 'not attached' is the only status your're likely to get from your B210.  
• I once forgot to connect one of the PPS/Clock cables while using the above settings, and got errors due to lack of signal.
• In general, when using clock / PPS source - and in particular when distributing to multiple B210s - it's important to use identical high-quality cables of equal length for both 10MHz and PPS to all radio units.  
• Having different transmission-delays for PPS and 10MHz signals reduces accuracy - this is why 'distribute it yourself' can be sub-optimal.

 

** CAVEAT Internal GPSDO: The use case for  "Sync  [[GPS Time on Next PPS]]"  applies *only* if you installed the internal B210 GPSDO board, which disables the B210 from using a shared external clock of any kind.   

Hope this helps.

.-- W

---------------

On May 17, 2024, at 07:19, Moses Browne Mwakyanjala <mbkitine@gmail.com> wrote:

Hi Marcus,

The external 10MHz did the trick. I have another related question which I think we can address here without opening a new ticket. The USRP B210 has a 1PPS port. However, I was not able to poll the status of the time source, "pps_locked". When I searched for a list of all onboard sensors, the only visible sensor was "ref_locked". Am I missing something? How can I use and poll the status of 1PPS?

Regards, Moses

On Wed, May 15, 2024 at 6:04 PM <discuss-gnuradio-request@gnu.org> wrote:

Send Discuss-gnuradio mailing list submissions to
        discuss-gnuradio@gnu.org

To subscribe or unsubscribe via the World Wide Web, visit
        https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
or, via email, send a message with subject or body 'help' to
        discuss-gnuradio-request@gnu.org

You can reach the person managing the list at
        discuss-gnuradio-owner@gnu.org

When replying, please edit your Subject line so it is more specific
than "Re: Contents of Discuss-gnuradio digest..."


Today's Topics:

   1. USRP B210 Frequency Offset (Moses Browne Mwakyanjala)
   2. Re: USRP B210 Frequency Offset (Marcus D. Leech)


----------------------------------------------------------------------

Message: 1
Date: Wed, 15 May 2024 11:52:27 +0200
From: Moses Browne Mwakyanjala <mbkitine@gmail.com>
To: GNURadio Discussion List <discuss-gnuradio@gnu.org>
Subject: USRP B210 Frequency Offset
Message-ID:
        <CABYsGdmPhq54WArSY--5-7renj3PFrAhTOWucE_RsS8YpQ-yVQ@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"

I've encountered a consistent frequency offset of around 2ppm with my new
B210. Operating at a sample rate of 4 MSPS with the "internal" clock, all
calibrations were performed using a sine wave from an Agilent signal
generator.

Though seemingly minor, the 800Hz offset at UHF poses challenges in
receiving low-rate data from orbiting satellites. Is there an automated
method to approximate and mitigate this offset? Currently, I manually
adjust the frequency by subtracting the offset in ppm. However, I'm curious
if there are more sophisticated solutions available, excluding reliance on
GPS or a 10MHz reference.

Best regards,
Moses

[image: image.png]
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.gnu.org/archive/html/discuss-gnuradio/attachments/20240515/3082954d/attachment.htm>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: image.png
Type: image/png
Size: 33653 bytes
Desc: not available
URL: <https://lists.gnu.org/archive/html/discuss-gnuradio/attachments/20240515/3082954d/attachment.png>