[USRP-users] TwinRX tuning timing

Marcus D. Leech mleech at ripnet.com
Sat Jul 15 22:42:52 EDT 2017


On 07/15/2017 10:33 PM, Jacob Gilbert wrote:
> >  The only guarantee is that samples that arrive after the tag will 
> be after the comman has been issued.
> This has not been my experience, particularly at lower sample rates. 
> As far as I can tell, the code path responsible for tagging never 
> actually goes to the USRP device at all, when a retune is issued, a 
> flag is set in the work function that causes a tag to be emitted on 
> the next call 
> (https://github.com/gnuradio/gnuradio/blob/master/gr-uhd/lib/usrp_source_impl.cc#L645).
_tag_now is set *after* the command is already "in flight".   Note I 
just said "issued", NOT "completed".

>
> > The control-plane an data-plane run asynchronously to one another.
>
> This does not appear to be strictly true, as timed commands exist. It 
> more seems like this is just a feature that is not implemented yet. It 
> should certainly be possible within the current UHD paradigm 
> since timing metadata certainly runs synchronous to the data-plane as 
> it propagates downstream.
For the most part, it is true.   With the exception of commands that 
directly alter streaming, control commands (tuning, gain setting, etc) 
are conceptually asynchronous to each other in the FPGA.   In fact, the 
FPGA has no *concept* of "gain", or "frequency", or any of a number of 
other radio-hardware
operating parameters.  The "model" is that the FPGA exposes I2C and SPI 
buses so that the host can manipulate registers on the radio hardware in 
use,
but the actual *meaning* of those commands is completely opaque to the 
FPGA (and hence things like frequency tags).   Timed commands make it seem
more "synchronous", but the architecture of the FPGA in general makes no 
linkage between the data stream, and the radio hardware control stream.


>
> Jacob
>
>
> On Fri, Jul 14, 2017 at 2:46 PM, Marcus D. Leech via USRP-users 
> <usrp-users at lists.ettus.com <mailto:usrp-users at lists.ettus.com>> wrote:
>
>     On 07/14/2017 04:22 PM, Eugene Grayver wrote:
>>
>>     Yes, I definitely agree that there’s no way to tell when the
>>     tuning is DONE.  However, my question was about the ‘shortly
>>     after’ part.  Should it not be inserted ‘shortly before’, or
>>     ‘exactly at’?  Since there’s typically no way to look into the
>>     future, (yes, of course it is doable), there’s no way for
>>     somebody to know that the samples that are before the tag are
>>     actually ‘invalid.’
>>
>>     _______________________
>>     Eugene Grayver, Ph.D.
>>     Aerospace Corp., Sr. Eng. Spec.
>>     Tel: 310.336.1274 <tel:%28310%29%20336-1274>
>>     ________________________
>>
>     The control-plane an data-plane run asynchronously to one
>     another.   The only guarantee is that samples that arrive after
>     the tag will be after the comman has been issued.
>
>
>>     *From:*mleech at ripnet.com <mailto:mleech at ripnet.com>
>>     [mailto:mleech at ripnet.com]
>>     *Sent:* Friday, July 14, 2017 12:34 PM
>>     *To:* Eugene Grayver <eugene.grayver at aero.org>
>>     <mailto:eugene.grayver at aero.org>
>>     *Cc:* usrp-users at lists.ettus.com <mailto:usrp-users at lists.ettus.com>
>>     *Subject:* Re: [USRP-users] TwinRX tuning timing
>>
>>     The frequency tag is inserted at some point shortly after the
>>     command-set is issued to the hardware.  There's no way for the
>>     various bits and pieces to tell when the underlying (mostly
>>     analog) hardware has converged to an "acceptable" steady-state
>>     operating mode.  PLL synthesizers don't instantly switch from one
>>     frequency to another, digital (and analog) filters have group
>>     delays, etc.
>>
>>     The time to achieve steady-state can vary from hardware-type to
>>     hardware-type, ambient temperature, size of the frequency step, etc.
>>
>>     On 2017-07-14 15:10, Eugene Grayver via USRP-users wrote:
>>
>>         I am running some experiments to understand the timing of
>>         TwinRX tuning.  A very simple experiment shows that the tag
>>         indicating a frequency change is not placed at the right
>>         sample.  Here’s a capture: The dashed line indicates the tag
>>         sample number.  However, there’s clearly something happening
>>         before the tag.  Note that I am not using timed commands for
>>         this – just a regular tune request.
>>
>>         _______________________
>>         Eugene Grayver, Ph.D.
>>         Aerospace Corp., Sr. Eng. Spec.
>>         Tel: 310.336.1274 <tel:%28310%29%20336-1274>
>>         ________________________
>>
>>         _______________________________________________
>>         USRP-users mailing list
>>         USRP-users at lists.ettus.com <mailto:USRP-users at lists.ettus.com>
>>         http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com
>>         <http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com>
>>
>
>
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