Re: [Discuss-gnuradio] Time Granularity of USRP N2XX

[Ben Hilburn]

Hi Harry -

Actually, the name `tick_rate` is somewhat confusing. The tick_rate refers to the clock rate that the DSP chain runs at. Here is a bit of code regarding the tick_rate from the USRP2 / N-Series implementation that might make things a little more clear:

        _tree->create<double>(mb_path / "tick_rate")

            .publish(boost::bind(&usrp2_clock_ctrl::get_master_clock_rate, _mbc[mb].clock))

            .subscribe(boost::bind(&usrp2_impl::update_tick_rate, this, _1));

Marcus explained things correctly in his e-mail. So, on an N-Series device, the time step between samples is 1/100e6 = 10 ns.

Cheers,

Ben

On Tue, Nov 26, 2013 at 5:49 AM, Harry Zhang <address@hidden> wrote:

Dear Marcus,
    Thank you for your reply.
    I'm confused about two concepts. Referring to the file time_spec.hpp in UHD, fractional sec is calculated according to the parameter ticks and tick_rate. So I think the time granularity is  '1/tick_rate'  which is 0.1us using 10Mhz clock. But for the timestamp of sample stream, the granularity is the  sample duration. Is it correct?

Best,
Harry

 2013/11/26 20:12, Marcus Müller wrote:

Hi Harry,

as far as I know, the granularity of a time stamp is the sample duration at the downsampled side.
As for the 10MHz: The 10MHz clock is used to derive different other clocks, especially the rx ADC and tx DAC clocks, which are an order of magnitude higher; bear in mind that when you request e.g. "5MHz samle rate" you still get the 100MHz* samples of the ADC, but downsampled by a factor of 20.

Please also bear in mind that due to physics, processing etc the timestamp of the rx side is delayed to the actual reception of "that sample", and that this delay is largely constant when receiving using the same settings, but my change based on sampling rate and other factors.

Greetings
Marcus

*for USRP2 arch, if I remember correctly.
On 26.11.2013 13:04, Harry Zhang wrote:

Hi,
Since we can get receive time of rx_stream's first sample via rx tags, I
have two questions about the time granularity. Given the internal clock
is 10Mhz, is the time granularity 0.1us? The fractional second is stored
double type. Does it mean the last digits of the fractional sec are invalid?

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