[USRP-users] [Discuss-gnuradio] Late packet and phase issue

irene menicagli irene.menicagli at gmail.com
Tue Oct 6 05:01:00 EDT 2015


Hi everyone,

I have the same problem (late packet). I think that the problem is the time
on packet is in the past when the command arrives to device. The cause can
be latency between pc and usrp.

My pc is quite powerfull so this behaviour seems strange.

Maybe I have to change timestamp on packet but I don't know if it is
possible because I work at python level and use UHD functions (usrp_sink
and usrp_source).

I also use only one USRP loopbacking TX e RX.

Thanks,
Irene



2015-10-05 21:13 GMT+02:00 Marcus D. Leech <mleech at ripnet.com>:

> On 10/05/2015 10:58 AM, Leonard Foxes wrote:
>
> Hi list,
>
> It's me again.
>
> I use a USRP N210 with WBX daughterboards and loop back cable to connect
> TX e RX. I have two problems:
>
>
>    1. Late packet. I use set_time_now and set_start_time command and
>    after few seconds I see some Ls printed.
>    2. Phase. Phase offset changes between executions. (I'm trying to use
>    set_time_command but nothing changes).
>
> This is my python code about usrp:
>
> self.samp_rate = samp_rate = 500e3
> self.USRP_address = USRP_address = "addr=192.168.10.2"
> self.tun_freq = tun_freq = 1.45e9
> self.tun_gain = tun_gain = 19
> self.rx_freq_off = rx_freq_off = 0
>
>
> self.timeref1= timeref1=time.time()
>
> self.uhd_usrp_TX = uhd.usrp_sink(
>         device_addr=USRP_address,
>         stream_args=uhd.stream_args(
>         cpu_format="fc32",
>         channels=range(1),
>         ),
>         )
> self.uhd_usrp_TX.set_samp_rate(samp_rate)
> self.uhd_usrp_TX.set_time_now(uhd.time_spec_t(timeref1), 0)
> self.uhd_usrp_TX.set_command_time(uhd.time_spec_t(timeref1+0.1), 0)
>         self.uhd_usrp_TX.set_center_freq(uhd.tune_request_t(tun_freq), 0)
> self.uhd_usrp_TX.clear_command_time()
>         self.uhd_usrp_TX.set_gain(tun_gain, 0)
>         self.uhd_usrp_TX.set_start_time(uhd.time_spec_t(timeref1+1))
>
>
> self.uhd_usrp_RX = uhd.usrp_source(
>         device_addr=USRP_address,
>         stream_args=uhd.stream_args(
>         cpu_format="fc32",
>         channels=range(1),
>         ),
>         )
>
>
>         self.uhd_usrp_RX.set_samp_rate(samp_rate)
> self.uhd_usrp_RX.set_time_now(uhd.time_spec_t(timeref1), 0)
> self.uhd_usrp_RX.set_command_time(uhd.time_spec_t(timeref1+0.1), 0)
>         self.uhd_usrp_RX.set_center_freq(uhd.tune_request_t(tun_freq +
> rx_freq_off), 0)
> self.uhd_usrp_RX.clear_command_time()
> time.sleep(0.15)
>         self.uhd_usrp_RX.set_gain(tun_gain, 0)
>         self.uhd_usrp_RX.set_start_time(uhd.time_spec_t(timeref1+1))
>
>
> Any suggestion will be welcome
>
> Thanks in advance,
> Leo
>
> Something to realize is that if you call set_time_now() on two different
> USRPs, they'll not agree in detail about the time-of-day.  If you're
>   using an external 10Mhz and 1PPS reference, you should probably use
> set_time_unknown_pps(), and make sure that both sides
>   of this are setting the same time AT THE SAME TIME.
>
> Also, WBX has a 180deg phase-ambiguity.  Two WBX boards with the same
> 10MHz reference, on USRPs with the same notion of time,
>   will either be in-phase, or 180deg out-of-phase, due to hardware
> ambiguity in the 2XLO mixer used on WBX.
>
>
>
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> https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
>
>
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