Hi Michael,
I think I see a bug in gr-uhd there!
So point is that the python code generated by this is (example):
[...]
##################################################
# Blocks
##################################################
self.uhd_usrp_sink_0 = uhd.usrp_sink(
",".join(("", "")),
uhd.stream_args(
cpu_format="fc32",
channels=range(1),
),
)
self.uhd_usrp_sink_0.set_time_source("external", 0)
self.uhd_usrp_sink_0.set_time_unknown_pps(uhd.time_spec())
self.uhd_usrp_sink_0.set_samp_rate(4e6)
self.uhd_usrp_sink_0.set_center_freq(1.5e9, 0)
[...]
Now, what GRC generates here means that you first set your time, then
you set your sampling rate – and on B2xx, that might have the side
effect of changing your master clock rate. The MCR is the speed at which
the internal time counter is incremented.
You can avoid that auto-MCR behaviour by explicitely setting the master
clock rate first – could you add "master_clock_rate=30e3" to your device
Address or device Arguments?
Best regards,
Marcus
On 04/25/2016 04:09 PM, Michael Skaggs wrote:
> Hey Marcus,
>
> In GRC, I have a USRP Source block with "Sync" set to "Unknown PPS"
> and "Timing Source" set to "External". I assumed the device would be
> setting time based on the external PPS using these parameters. The
> synchronize(S0) LED goes solid (after about half a second of
> recording, maybe less) and the other LED (S1) is blinking with the PPS.
>
> Thanks,
> Michael
>
> On Fri, Apr 22, 2016 at 11:56 AM, Marcus Müller
> <[email protected] <mailto:[email protected]>> wrote:
>
> Are you sure you're setting the device time correctly? 2.5ms
> offset is definitely much much worse than what to devices that
> should have the same device time should exhibit. So:
> How are you setting the device time?
>
> Best regards,
> Marcus
>
>
> On 04/21/2016 08:59 PM, Michael Skaggs wrote:
>> Aha! You were correct. I was parsing the header incorrectly.
>> However, even after parsing, and the more accurate values (now
>> accurate to 10e-6), it appears the recorded RF data sets are
>> still are offset from each other by an amount of nearly 2.5e-3
>> seconds. Any idea why this would be? It's strange, especially
>> considering that the are synchronized to the same PPS, and I
>> think the time is with reference to the pulse.
>>
>> Michael
>>
>> On Thu, Apr 21, 2016 at 11:57 AM, Marcus D. Leech
>> <[email protected] <mailto:[email protected]>> wrote:
>>
>> On 04/21/2016 11:41 AM, Michael Skaggs wrote:
>>
>> I'm trying to time/sample synchronize RF recordings with
>> two B200minis. I am using the detached Metadata File Sink
>> in GRC. Both recordings are at 30MSps and both B200mini
>> boards are synchronized to the same 1PPS signal.
>>
>> My issue is this, when I extract the data from the
>> Metadata header file, the "rx_time" value is only
>> accurate to 10e-4 seconds (0.0001s). Which, with a
>> recording at 30MSps, this will only give me a sample
>> alignment accuracy to 10e-4(s)*30(MS/s) = 30,000 samples.
>>
>> If I'm attempting to align the two recordings by samples
>> or time, this is not nearly accurate enough. Is there a
>> way that I can get more accuracy out of my metadata
>> header or a way that I can synchronize the recordings of
>> these B200minis?
>>
>> Thanks,
>> Michael
>>
>> The precision of the timestamps from UHD should have a
>> precision of whatever the master-clock is on the device--how
>> are you interpreting
>> the rx_time tag? It's two parts--a uint64 with the
>> full-seconds portion, and a double-precision float for the
>> fractional part.
>>
>>
>>
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