Hi Marcus,
Thanks for the background. That helps greatly. Having said that,
I am unclear which commands specifically tune the radios, so I
did the following around the frequency tuning (after all of the
time source, gain, and antenna setting code):
addr_string = "addr0=192.168.10.3,addr1=192.168.10.4"
self.uhd_usrp_sink_0_0 = uhd.usrp_sink(
",".join((addr_string, "")),
uhd.stream_args(
cpu_format="fc32",
channels=range(2),
),
)
self.uhd_usrp_sink_0_0.set_clock_source("external", 0)
self.uhd_usrp_sink_0_0.set_time_source("external", 0)
self.uhd_usrp_sink_0_0.set_clock_source("mimo", 1)
self.uhd_usrp_sink_0_0.set_time_source("mimo", 1)
self.uhd_usrp_sink_0_0.set_samp_rate(samp_rate)
self.uhd_usrp_sink_0_0.set_gain(31.5, 0)
self.uhd_usrp_sink_0_0.set_gain(31.5, 1)
self.uhd_usrp_sink_0_0.set_antenna("TX/RX", 0)
self.uhd_usrp_sink_0_0.set_antenna("TX/RX", 1)
self.analog_sig_source_x_0_1 =
analog.sig_source_c(samp_rate, analog.GR_CONST_WAVE, 10000, 1, 0)
self.analog_sig_source_x_0_0_0 =
analog.sig_source_c(samp_rate, analog.GR_CONST_WAVE, 10000, 1, 0)
*self.uhd_usrp_sink_0_0.set_time_unknown_pps(uhd.time_spec())
now = self.uhd_usrp_sink_0_0.get_time_now()
start_time = now + uhd.time_spec(.5)
self.uhd_usrp_sink_0_0.set_command_time(start_time)
self.uhd_usrp_sink_0_0.set_center_freq(915000000, 0)
self.uhd_usrp_sink_0_0.set_center_freq(915000000, 1)
self.uhd_usrp_sink_0_0.clear_command_time()*
However, when running it, this does not appear to produce a
constant offset either, but I'm not sure whether this is the
correct code to wrap around. Please keep me posted. Thanks.
On Tue, Jul 5, 2016 at 12:49 PM, <mle...@ripnet.com
<mailto:mle...@ripnet.com>> wrote:
That is precisely what I'm saying, and precisely what
timed-commands for tuning were invented. On certain
hardware, after the tune is complete, a phase-reset pulse is
sent by the FPGA. The only way for THAT to have the desired
effect is to make sure that the phase-reset pulse happens at
the same instant.
Modern synthesizers use a technique called fractional-N
synthesis. One of the side effects of this is that you can't
predict where the LO will "lock" with respect to the
reference clock. So, any two PLL synthesizers, even when feed
an identical reference clock, will not have the same phase
offset with respect to one another. It's the "physics" of
fractional-N PLL synthesis.
SO, if you're using GRC to generate you flows, you'll have to
modify the generated code, and wrap
set_command_time()/clear_command_time() around the place in
the code where it tunes the radios.
Clearly, if this depends on TIME, then all radios involved
need to agree on the current time, to high precision, hence
the related requirement for set_time_unknown_pps(), which
uses the 1PPS signal to trigger loading of the time-of-day
clocks on each USRP in the multi_usrp object.
On 2016-07-05 15:41, Pavan Srikrishna Yedavalli wrote:
I am using USRP N210 with SBX daughterboards. All devices
are connected to the octoclock ref and octoclock PPS. It
would be nice to get phase alignment, but mere
coherence-with-an-offset is sufficient if that offset stays
constant across different runs of the file/flowgraph. Are
you saying that that offset cannot be constant due to the
randomness of the LO phase offset at each run? Thanks again.
_____________________________
From: mle...@ripnet.com <mailto:mle...@ripnet.com>
Sent: Tuesday, July 5, 2016 12:35 PM
Subject: Re: [Discuss-gnuradio] random phase offset
constantly changing with octoclock setup
To: Pavan Yedavalli <psy2...@columbia.edu
<mailto:psy2...@columbia.edu>>
Cc: Discuss-gnuradio
<discuss-gnuradio-bounces+mleech=ripnet....@gnu.org
<mailto:discuss-gnuradio-bounces+mleech=ripnet....@gnu.org>>, GNURadio
Discussion List <discuss-gnuradio@gnu.org
<mailto:discuss-gnuradio@gnu.org>>
WHat specific hardware line-up do you have?
You have to use set_time_unknown_pps(), but also, if you
want phase alignment (as opposed to mere
coherence-with-an-offset), you need to use timed tuning
commands across your systems. This will result in zero
relative phase offset between boards, if you're using SBX or
UBX (on the X310). Note that this is phase between the
boards, there's no way to make certain the the LO phase has
a predictable offset with respect to external received
signals, only that the two LO phases agree.
On 2016-07-05 15:26, Pavan Yedavalli wrote:
Hi,
Despite all of my boards being connected via the
Octoclock (ref and pps), I am constantly getting
different phase offsets every time I run a gnuradio
flowgraph (or python file).
I am not sure why this is happening, but I do know that
I need to call one of the functions, set_time_now()
and/or set_time_unknown_pps(), to make sure all of them
are synced. Which one am I supposed to call? I have been
calling set_time_unknown_pps(), but I am getting the
above/below problem with that, so maybe set_time_now()
could be correct?
In general, I don't understand why I continually get a
different random phase offset of all of the radios after
every new flowgraph run. Shouldn't this offset be the
same if I continue to transmit at the same frequency?
Would one of the above functions fix that?
Hopefully that is clear. Thank you so much for the help.
--
Pavan
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Pavan
