[USRP-users] Exact alignment between gnuradio sample stream and USRP time

Marcus D. Leech patchvonbraun at gmail.com
Thu Jan 23 18:50:48 EST 2020

On 01/23/2020 12:32 PM, Lukas Haase via USRP-users wrote:
> Hi,
> TO MY UNDERSTANDING, the USRP has an internal clock that is different from host clock when running gnuradio (which makes sense because there are buffers etc in between).
> Example: I transmit a CW at f=1001, receive it at f=1000 and then use gnuradio to downconvert the remaining 1 MHz I run into trouble (tried it...).
> For this reason, there exist timed commands and the tune_request object with which I can execute commands (LO tuning) at a precice time. For example, with these commands I can phase align tuning between TX/RX at different center frequencies:
>    tune_req_tx = uhd.tune_request(fcenter-1e6, 1e6)
>    tune_req_rx = uhd.tune_request(2*fcenter)
>    tune_req_rx.args=uhd.device_addr(','.join(["mode_n=integer", "int_n_step=1000e3",]))
>    tune_req_tx.args=uhd.device_addr(','.join(["mode_n=integer", "int_n_step=1000e3",]))
>    now = self.uhd_usrp_sink_0.get_time_now()
>    self.uhd_usrp_sink_0.set_command_time(now + uhd.time_spec(0.1))
>    self.uhd_usrp_source_0.set_command_time(now + uhd.time_spec(0.1))
>    self.uhd_usrp_sink_0.set_center_freq(  tune_req_tx, 0)
>    self.uhd_usrp_source_0.set_center_freq(tune_req_rx, 0)
>    self.uhd_usrp_source_0.clear_command_time()
>    self.uhd_usrp_sink_0.clear_command_time()
> The commands execute execatly at get_time_now() plus 100ms. As far as I understand, these 100ms are to ensure that the host computer has enough time until the USRP processes the clear_command_time function. But it does not relate the exact point in time with anything that exists in gnuradio.
> MY QUESTION: What I am unsure is how to align samples in gnuradio with the time on the USRP. For example, suppose I have an ideal clock signal in gnuradio and I want to perform a timed command EXACTLY at a particular sampling point (e.g. rising edge). How would I go about this?
> The actions I want to execute exactly time aligned with gnuradio include: tuning requests, reading out sensors (PLL sensor when it settled), switching IO pins through the GPIO interface.
> For example, I would like to switch a GPIO port exactly once per period of a signal in gnuradio and exactly at the same time (clearly there will be delays but that's OK as long as the delay is fixed).
> As another example, I would like to re-tune exactly once in every period of a gnuradio signal. Then I would like to read out when the PLL has settled and generate a binary indicator signal out of it. Plotting the original signal and the indicator signal should tell me exactly (at least sample accuracy) how long the PLL took to settle *relative* to the signal in gnuradio.
> Thank you very much,
> Luke
Whatever "dance" you're using to set the USRP time, (presumably 
something like set_time_unknown_pps), you need to have it derive the
   USRP time register from the host time.  The normal code that is 
emitted in GRC for "unknown_pps" just resets the USRP time to zero.
   But you can arrange for it to be the host time (+1 second or 
something) instead.

You haven't indicated whether you're using GRC, or "naked" Gnu Radio 

General synchronization "things" are discussed in the knowledge base, here:


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